Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

Gunner wrote:
Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Why use a DC-DC converter on 24 volts *ac*? You need a little transformer.

GWE

"Gunner" wrote in message
...
Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

It would need a big resister to do that, at least 84 watts and it would make
allot of heat. I would look for a replacement surplus transformer that
output at 12v then use a bridge rectifier or build a power supply with the
24v transformer. The power supply is probably the least expensive.

It could be made with a 7812 voltage regulator and a few pass transistors.
MikeH

Mike H wrote:

"Gunner" wrote in message
...

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001


It would need a big resister to do that, at least 84 watts and it would make
allot of heat. I would look for a replacement surplus transformer that
output at 12v then use a bridge rectifier or build a power supply with the
24v transformer. The power supply is probably the least expensive.

It could be made with a 7812 voltage regulator and a few pass transistors.
MikeH


If the pump is the same type, the motor is presumably the same wattage.
This would rule out using any sort of liniar regulator to drop the
voltage as the original supply will probably only be rated for 1/2 the
current you need. You cant afford to throw away watts as heat.

Two approaches.
1. Bridge rectifier, ideally a choke input filter, resevoir cap and a
12V out DC-Dc converter module. Cost you a bit if you cant scab the
converter out of some scrap control system.

2. old mains transformer with a 24V centre tapped winding or two 12V
windings rigged as a 24-12 v autotransformer. Insulate and ignore the
mains primary. It doesnt even matter if the old primary is open circuit
although if its got shorted turns, you'd have to cut it off the bobbin.
If I remember my tranformer theory correctly, the load current in the
lower half of the winding is opposed by the supply current so you only
need a 3.5A rated winding. Say 4A for 'built like a brick s@@thouse'
and 3A if the duty cycle is low or for occasional use. Follow up with a
10A 50V bridge and about 1000uF of 25V rated electrolytic for the 12V
resevoir cap.

What ever you do, put 0.047uF (or thereabouts) disc ceramic 100V caps
across ALL 4 diodes in the bridge rectifier to bypass any spikes induced
from the welder so the bridge doesnt curl up its little toes and die.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- &
[dot]=.

In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.

--
Free men own guns, slaves don't
www.geocities.com/CapitolHill/5357/

On Wed, 24 Nov 2004 06:29:42 -0500, Nick Hull wrote:

In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.

That will work. Just check that the transformer does not overheat after
some use. If the transformer core was marginal to begin with it might be
entering saturation because of the unipolar current it is delivering.

Regards,
Boris Mohar
Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
void _-void-_ in the obvious place

"Gunner" wrote in message
...
Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...


Not sure how this thing needs to work, but if you have a small battery
charger collecting dust on a shelf in the garage, that would power the pump
purfectly. Most of the small charges are capable of 8 amps at 12 volts.
Doug






Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

The big issue is getting a transformer large enough for 7 amps. If you
can find a transformer that has a 2:1 ratio capabile of 7 amps, you are
in business. Even an old transformer that changes 220 to 110, or 440 to
200, would work fine, if it was large enough. Are you sure the 24V
circuit will provide 3.5 amps? (same wattage as 12V at 7 amps). Not
sure what gear you have lying around.

Once you have 12VAC, a bridge rectifier will convert it to 12VDC, or
probably close enoughto 12V to run a pump. Not sure about the best way
to protect the bridge rectifier, other than to get a bridge rectifier
with a higher voltage rating. I suspect that a 100 Volt rating would do
fine.

A 12 volt 7Amp or larger power supply could also be purchased from Radio
Shack or many other places.

Richard



Gunner wrote:
Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

On Wed, 24 Nov 2004 07:23:46 GMT, Gunner
wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

Gee, I would expect them to be available used cheap - probably
because every other Motorhome and Travel Trailer out on the road uses
a Shurflo Tri-A-Fram pump for the potable water supply... ;-0

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

It doesn't have to be *exactly* 12 volts, just somewhere in the
neighborhood. Remember, when installed in RV's those pumps have to
put up with 14 or 14.5 volts when the battery charger is connected,
and 9 volts when the battery is darned near flat.

Let's see, I'd start with a bridge rectifier. The pump may run on
AC, but it'll be much happier with DC. Which is why they stuck one
inside the motor of the purpose-built unit. The 10-amp ones will come
in a big epoxy cube package, and will need a heat sink of some sort to
not blow up early - a big chunk of AL plate can suffice.

Then we go 14 volts X 7 Amps = 98 Watts. Gee, go find 100 watts
worth of 12V automotive light bulbs (two headlights in parallel should
do it), attach in series with the pump, hook up a voltmeter across the
pump motor so you can see what's going on, and go for it. ;-)

Or if that thought scares you, go get a Malibu Lights transformer
good for at least 100 watts at 12 VAC, attach the bridge rectifier,
and you'll have about 14 VDC, right where you need it.

The RV pump will have the pressure switch set up for a demand water
system, where the purpose-built pump was set for continuous run. You
could do the same thing if you have a bladder accumulator tank for the
cooling water - let the pump cycle. Otherwise, bypass that
microswitch on the nose of the pump.

Oh, one more thing... If there isn't a relief valve on the pump
output, be careful, you could blow a hose in the torch lead. The RV
pump may put out more pressure than the torch hoses can handle.
Either install a gauge, a pop-off valve (plumbing supply) or both.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

On Wed, 24 Nov 2004 07:23:46 GMT, the renowned Gunner
wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner

Well, I havn't tried this but I think it should work...

The cheapest way would be to hack an old (free) microwave oven
transformer. Angle-grind the welds that are holding the E and I lams
together. Cut off the high voltage winding and discard. Remove the
magnetic shunt between primary and secondary and discard. You might be
able to salvage the wire from the primary side (if not, use some
slightly fatter wire than in the original primary).
If so, unwind it carefully (there should be about 120 turns), counting
the turns, and put 1/3 the number back (about 40). That's your 24VAC
primary. Replace the high voltage winding with 1/6 the original turns
on the primary (about 20 turns). That's your 12VAC secondary winding
to go to the bridge rectifier. Tack the wlams back.

Here's what a typical microwave ovent transformer looks like:
http://www.zbr.co.jp/e_catalog/e_catalog_10.pdf

Note the E-I lams are not interleaved, so it's easy to get the core
apart.

Here's a suitable bridge rectifier:
http://semi.shindengen.co.jp/en/d20xb80.pdf

20A/800V available from Mouser for $1.46 ea. qty. 1. P/N: 627-D20XB80
plus a few bucks for UPS ground.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Tue, 23 Nov 2004 23:34:20 -0800, Grant Erwin
wrote:

Gunner wrote:
Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Why use a DC-DC converter on 24 volts *ac*? You need a little transformer.

GWE

It already has a transformer in it that drops 110 down to 24vts AC to
power the various sensors and the 7 amp pump motor. I was trying to
avoid sticking yet another largish transformer into what is already a
very crowded control section.

Gunner



"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

Gunner, are you sure the Transformer in your unit 24 volts?. A quick check
of Lincolns manuals has the transformers marked 18V. What model is your
unit?

Manual page:
http://content.lincolnelectric.com/p...r/im/IM439.pdf
http://content.lincolnelectric.com/p...r/im/IM438.PDF

Idex of manual page:
http://content.lincolnelectric.com/p.../im/immenu.pdf

To answer your original question - you would need a resistor of 1.7 ohms at
84 watts. This is assuming the circuit is feeding 24VAC to the pump.

If your transformer is 18 volts then:
you need a .85 ohm resistor at 42 watts

Jim Vrzal
Holiday,Fl.


Gunner wrote in
:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

Nick's idea of half-wave rectifiying is simple, small, and will work
well if the transformer doesn't saturate. (It probably won't).

Another way that would be very small would be to chop the rectified 24
volts at a high frequency, using a MOSFET driven by a 555 timer.
You'd need a snubbing diode as well. This circuit is simple, could
be made small even with commodity thru-hole parts, would not require
much if any heatsink on the MOSFET and would cost about 5 bucks in
parts from Mouser, Digi-Key, or Jameco. This would not saturate the
transformer and it would not run hot.

Email me if you'd like a schematic. Or, find me a teflon liner for a
Cobramatic MK-3A and I'll build the circuit, check it out and mail it
to you. It could even be made adjustable if you like. I made such a
control for a little heater for Karl Townsend. That one was smaller
than an ice cube but it was only a couple of amps. I'd use a 40-amp
MOSFET on yours.



On Wed, 24 Nov 2004 07:23:46 GMT, Gunner
wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

Gunner - half rectify it - Crude Ascii art below

24v CT xformer
-----------P || S-----|||---------- V+ Out
P || S A Diode C
P || S
120c AC P || S----CentreTap----- Becomes Ground
P || S
P || S
-----------P || S------||---------- V+ out
A Diode C

Make the centre tap your ground - and put a diode capable of handling
the current on each of the other ends - Anode to the transformer side
Cathode to the circuit side. In a 'normal' diode the stripe is the
Cathode.

Tie the two outputs (after the diodes) together and run it to the (+)
end of a filter Capacitor and put the centre tap on the (-) end of a
filter capacitor
Use about 1000uF for every amp required - although this is not
critical for a motor application

Usual warnings apply - note correct spelling of CENTRE ;-)

Tom


Gunner wrote in message . ..
Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

--

SVL

In article ,
Nick Hull wrote:


In article ,
Gunner wrote:

What is the best way to drop the 24vac down to 12v?



You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


Bravo! i like this

together with the Boris Mahar comment on transformer saturation
and the lil ascii schematic from Tom...

sidd

On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:


"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

"Spehro Pefhany" wrote in message
...
On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:


"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor,
else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Oh, then I guess that explains why virtually every large non-switching dc
power supply I own happens to have a large bank of them connected in
parallel with the load, excepting perhaps a battery charger or two.

--

SVL

Better yet: find an an old selenium rectifier , go half wave and connect a
12v lead-acid bike battery in parallel with the pumpmotor.

There is your big capacitor and the leakage on the selenium rectifier will
keep any transformer happy.

cheers
T.Alan

In article ,
PrecisionMachinisT wrote:

"Spehro Pefhany" wrote in message
.. .
On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:

[ ... ]

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Oh, then I guess that explains why virtually every large non-switching dc
power supply I own happens to have a large bank of them connected in
parallel with the load, excepting perhaps a battery charger or two.

Yep -- those power supplies use the capacitors to hold the
voltage near the peak voltage -- so you would be applying nearly 24V * to
the motor. Take out the capacitor, and the voltage is allowed to drop
to zero volts between cycles. So with an electrolytic, you would be
overvoltaging the motor, both peak and average.

* Note that if it is 24VAC, then the peak voltage will actually be
just under 34V, which is even worse. If it is an AC voltage
tailored to produce 24 VDC, then you would be back at the 24 VDC
level.

Whether the capacitor would overheat and explode would be a
function of how small the capacitance was, relative to the current
demand. If it was quite small, the voltage would be dropping a lot
more, so there would be more current in the capacitor most of the time,
and thus the overheating.

In any case, the capacitors would at least be doing just what we
don't want -- keeping the average voltage applied to the motor from
being closer to 12 VDC than 24 VDC.

For normal power supplies (linear or brute-force, but not
switching supplies), the idea is to produce as close to a pure DC
voltage as possible. Here, we are after a very ugly voltage, just to
keep the motor from being over-voltaged on the average. The motor
really won't care much about how ugly it is, but electronics circuits
would object seriously, and those would need the filter capacitors.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Wed, 24 Nov 2004 16:16:16 -0800, the renowned "PrecisionMachinisT"
wrote:


"Spehro Pefhany" wrote in message
.. .
On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:


"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor,
else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Oh, then I guess that explains why virtually every large non-switching dc
power supply I own happens to have a large bank of them connected in
parallel with the load, excepting perhaps a battery charger or two.

That's a different situation. You notice you don't ever see output
caps on SCR motor controls.

A small (like a few thousand uF) capacitor will likely have the
*ripple current* rating exceeded, causing it to overheat and possibly
vent. It won't smooth the output voltage much with 7A load. To get 10%
peak (2.4Vp-p) ripple would take C ~= (1/60) * 7A/2.4 ~= 48,000uF

A large capacitor will charge to close to the peak voltage, which is
1.4 * 24 ~= 33V. In the above case (48,000uF) the average output
voltage would be 32VDC. That is an awful lot for that poor 12V motor.

Half-wave rectifying it gives an average voltage of 12V, which is okay
(maybe a bit hard on the brushes, and the motor may be relatively
noisy but it's okay) for the 12V motor.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Buy a light dimmer for $4 ... hook the dimmer to the step down transformer
then hook your voltmeter to the motor while turning the dimmer up.
It's an old variable Power Supply trick ... .
It works at from 40 to100% of output so 24 to 12 vdc is 50% ..well within
the range.

"Spehro Pefhany" wrote in message
...
On Wed, 24 Nov 2004 16:16:16 -0800, the renowned "PrecisionMachinisT"
wrote:


"Spehro Pefhany" wrote in message
. ..
On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:


"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor,
else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of
a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the
higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Oh, then I guess that explains why virtually every large non-switching dc
power supply I own happens to have a large bank of them connected in
parallel with the load, excepting perhaps a battery charger or two.

That's a different situation. You notice you don't ever see output
caps on SCR motor controls.

A small (like a few thousand uF) capacitor will likely have the
*ripple current* rating exceeded, causing it to overheat and possibly
vent. It won't smooth the output voltage much with 7A load. To get 10%
peak (2.4Vp-p) ripple would take C ~= (1/60) * 7A/2.4 ~= 48,000uF

A large capacitor will charge to close to the peak voltage, which is
1.4 * 24 ~= 33V. In the above case (48,000uF) the average output
voltage would be 32VDC. That is an awful lot for that poor 12V motor.

Half-wave rectifying it gives an average voltage of 12V, which is okay
(maybe a bit hard on the brushes, and the motor may be relatively
noisy but it's okay) for the 12V motor.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com

In article ,
Richard Ferguson wrote:
The big issue is getting a transformer large enough for 7 amps. If you
can find a transformer that has a 2:1 ratio capabile of 7 amps, you are
in business. Even an old transformer that changes 220 to 110, or 440 to
200, would work fine, if it was large enough. Are you sure the 24V
circuit will provide 3.5 amps? (same wattage as 12V at 7 amps). Not
sure what gear you have lying around.

Why not check to see whether the transformer already there has a
center tap on the secondary? If so, just add another bridge rectifier
and filter capacitor to one half of the winding. (Or, to do a nicer
job, using the existing bridge rectifier, try a hookup like this):


| | |
| | | __________________________________________________ ___0 (~)
| | | E / \
| | | E (+24V)+--------------------------- /-+ \
| | | E | / | \
| | | E === / | \
| | | E | / | \
| | | E | / | \
| | | E----------------------+ (+12V) (-)+-0 +------0 (+)
| | | E | | \ /
| | | E | | \ Bridge /
| | | E === | \ Rect. /
| | | E | | \ /
| | | E (GND)+---------------------+ \ /
| | | E \ /
| | | E 0 (~)
| | | E_________________________________________________ ____|_
| | |
| | |
| | |

I can't make a good diode symbol in the bridge rectifier, so I
drew it without, just marking the terminals as '+', '-' and '~' (AC
input terminals).

So -- what is done above is to replace the filter capacitor with
two in series, with the common point also connected to the center tap of
the transformer's secondary. (I didn't bother drawing the primary, to
save some space.

The advantage of this over using an extra set of rectifiers to
develop the 12V from the center tap and one end is that the extra
current is shared between the two halves of the winding, so there is
less chance of overheating it. (This assumes that the transformer can
handle the extra load of a lower-voltaged pump.

Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"Spehro Pefhany" wrote in message
...
On Wed, 24 Nov 2004 16:16:16 -0800, the renowned "PrecisionMachinisT"
wrote:


"Spehro Pefhany" wrote in message
.. .
On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:


"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a
tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor,
else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead
of a
steady 12 volts you would be supplying a pulsed 24 volts, but I
doubt
the motor would notice. Make sure the rectifier can handle the
higher
pulses, bet even that is probably no big deal since it is the
average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Oh, then I guess that explains why virtually every large non-switching dc
power supply I own happens to have a large bank of them connected in
parallel with the load, excepting perhaps a battery charger or two.

That's a different situation. You notice you don't ever see output
caps on SCR motor controls.

A small (like a few thousand uF) capacitor will likely have the
*ripple current* rating exceeded, causing it to overheat and possibly
vent. It won't smooth the output voltage much with 7A load. To get 10%
peak (2.4Vp-p) ripple would take C ~= (1/60) * 7A/2.4 ~= 48,000uF

A large capacitor will charge to close to the peak voltage, which is
1.4 * 24 ~= 33V. In the above case (48,000uF) the average output
voltage would be 32VDC. That is an awful lot for that poor 12V motor.

Half-wave rectifying it gives an average voltage of 12V, which is okay
(maybe a bit hard on the brushes, and the motor may be relatively
noisy but it's okay) for the 12V motor.


Yeah okay, thinking it through the vdc does rise to near peak, its been a
very long time since I built any power supplys, and when I did it was
generally for B+ supply in tube amps.

Also, its probly a brush motor and wouldnt give a hoot whether its running
off ac or dc anyways, so a 2/1 stepdown would probly work fine too.

But im not in the habit of jerry rigging things to work via use of scavenged
parts these days and also got no idea what he might have in his junk
box.......

Gunner, give us a complete inventory of all your odd bits and Im sure
someone will come up with a workable solution G

--

SVL

PrecisionMachinisT wrote:
"Nick Hull" wrote in message
...

In article ,
Gunner wrote:


Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead of a
steady 12 volts you would be supplying a pulsed 24 volts, but I doubt
the motor would notice. Make sure the rectifier can handle the higher
pulses, bet even that is probably no big deal since it is the average
current that limits diodes.



And add an electrolytic capacitor to smooth out the pulsing.

--

SVL



Fellas, unless the guy who designed the original unit went way overboard
on the transformer sizing, it's unlikely that the transformer which ran
that "same size" pump using 24 vac motor would have enough current
capacity to run a 7 amp 12vdc motor while throwing away half the power
in a voltage dropping resistor.

He might get by wusing half wave rectification with a big
inductor/capacitor L-filter to smooth things out, which would keep the
average load on the transformer about the same as it was with the 24vac
motor, but the uneven load might make its waveform funny enough to
bother some of the other stuff that's running off it, and the choke
would be about the same size as another transformer.

I'm in favor or using a separate transformer with a full wave rectifier.
He probably won't need a filter capacitor at all, most simple dc motors
will handle an unfiltered waveform.

HTH,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"

On Wed, 24 Nov 2004 19:04:44 -0800, the renowned "PrecisionMachinisT"
wrote:


"Spehro Pefhany" wrote in message
.. .
On Wed, 24 Nov 2004 16:16:16 -0800, the renowned "PrecisionMachinisT"
wrote:


Gunner, give us a complete inventory of all your odd bits and Im sure
someone will come up with a workable solution G

LOL


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Wed, 24 Nov 2004 17:02:31 GMT, Gunner
wrote:

On Tue, 23 Nov 2004 23:34:20 -0800, Grant Erwin
wrote:

Why use a DC-DC converter on 24 volts *ac*? You need a little transformer.


It already has a transformer in it that drops 110 down to 24vts AC to
power the various sensors and the 7 amp pump motor. I was trying to
avoid sticking yet another largish transformer into what is already a
very crowded control section.

So who says it has to be inside? Trying to do it on the cheap, and
now you're worried about the aesthetics all of a sudden? ;-)

One of our HOA's has an antique RainBird mechanical timer that still
worked fine, no rust, except for a dead control transformer and a very
small place to mount it inside the cabinet...

So rather than try to track down an exact replacement, or butcher
the insides making a generic uncased one fit, I got a generic .1 KVA
Type 3R raintight control transformer.

Mounted it on the outside, drilled a 7/8" hole into the wiring
chamber, placed a deep chase nipple and a few conduit nuts between the
KO on the back of the transformer and the side of the cabinet, wired
it all up. Done. Looks factory.

Oh, and before you give up, check that the internal transformer on
that chiller doesn't have a center tap on the 24 volt side - with a
bit of rewiring you could make it put out 12V DC for the new pump.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

In article ,
PrecisionMachinisT wrote:

[ ... ]

But im not in the habit of jerry rigging things to work via use of scavenged
parts these days and also got no idea what he might have in his junk
box.......

Gunner, give us a complete inventory of all your odd bits and Im sure
someone will come up with a workable solution G

Hmm ... you really want to crash the news severs around the
world, don't you? :-)

I'm not at all sure that I could even *lift* a printed copy of
that information.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Thu, 25 Nov 2004 02:52:27 GMT, "invntrr"
wrote:

Buy a light dimmer for $4 ... hook the dimmer to the step down transformer
then hook your voltmeter to the motor while turning the dimmer up.
It's an old variable Power Supply trick ... .
It works at from 40 to100% of output so 24 to 12 vdc is 50% ..well within
the range.

Sounds like an interesting dodge, have to remember that one...

But if you want the light dimmer to have a long and healthy life in
that duty, make sure it's one rated for low voltage lighting like the
Lutron Skylark SLV-600P. (Of course, some people have access to a
deeper junk pile of "Good Used" light dimmers than others... ;-)

The LV rated ones have the extra circuitry inside to handle
transformer inductive and/or switching power supply loads common to
low-voltage lighting without a "Magic Smoke" incident. Regular
dimmers want to see a straight resistive Tungsten load only.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

BTW, if you use the approach I mention above, the average current
drawn from your 24 volt xfmr will be about half that delivered to the
12-volt load so the transformer will be grinnin'. It ain't
magic, it's just how these things work. The tranformer supplies
full current half of each high-freq cycle and zero current for the
other half. The inductance of the motor and the snubber diode fill in
the motor current during the "off" part of the cycle. It doesn't run
hot because very little power is wasted. It's just converted.

On Wed, 24 Nov 2004 11:52:21 -0600, Don Foreman
wrote:

Nick's idea of half-wave rectifiying is simple, small, and will work
well if the transformer doesn't saturate. (It probably won't).

Another way that would be very small would be to chop the rectified 24
volts at a high frequency, using a MOSFET driven by a 555 timer.
You'd need a snubbing diode as well. This circuit is simple, could
be made small even with commodity thru-hole parts, would not require
much if any heatsink on the MOSFET and would cost about 5 bucks in
parts from Mouser, Digi-Key, or Jameco. This would not saturate the
transformer and it would not run hot.

Email me if you'd like a schematic. Or, find me a teflon liner for a
Cobramatic MK-3A and I'll build the circuit, check it out and mail it
to you. It could even be made adjustable if you like. I made such a
control for a little heater for Karl Townsend. That one was smaller
than an ice cube but it was only a couple of amps. I'd use a 40-amp
MOSFET on yours.

Hey, Spehro, if Gunner finds me a teflon sleeve for my Cobramatic,
the PWM module I'd make for him would use your clever design with
the 555. I already have a board laid out for it. I used it
(with your permission and credited to you) in Kar'ls fingerwarmer
controller.

On Wed, 24 Nov 2004 11:26:47 -0500, Spehro Pefhany
wrote:



Well, I havn't tried this but I think it should work...

The cheapest way would be to hack an old (free) microwave oven
transformer. Angle-grind the welds that are holding the E and I lams
together. Cut off the high voltage winding and discard. Remove the
magnetic shunt between primary and secondary and discard. You might be
able to salvage the wire from the primary side (if not, use some
slightly fatter wire than in the original primary).
If so, unwind it carefully (there should be about 120 turns), counting
the turns, and put 1/3 the number back (about 40). That's your 24VAC
primary. Replace the high voltage winding with 1/6 the original turns
on the primary (about 20 turns). That's your 12VAC secondary winding
to go to the bridge rectifier. Tack the wlams back.

Here's what a typical microwave ovent transformer looks like:
http://www.zbr.co.jp/e_catalog/e_catalog_10.pdf

Note the E-I lams are not interleaved, so it's easy to get the core
apart.

Here's a suitable bridge rectifier:
http://semi.shindengen.co.jp/en/d20xb80.pdf

20A/800V available from Mouser for $1.46 ea. qty. 1. P/N: 627-D20XB80
plus a few bucks for UPS ground.


Best regards,
Spehro Pefhany

On Thu, 25 Nov 2004 01:20:13 -0600, the renowned Don Foreman
wrote:

Hey, Spehro, if Gunner finds me a teflon sleeve for my Cobramatic,
the PWM module I'd make for him would use your clever design with
the 555. I already have a board laid out for it. I used it
(with your permission and credited to you) in Kar'ls fingerwarmer
controller.

;-) Sounds good.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Don Foreman wrote:
... I used it
(with your permission and credited to you) in Kar'ls fingerwarmer
controller.

That was 2 years ago, how did it work out, as far as actually warming
Karl's fingers? Did you go with the curling iron heater wire, or what?

Bob

Good memory, Bob! I'll defer to Karl for the "field report" but I
think he's happy with it. He made a metal ring, embedded a couple of
resistors in it. I included a varistor current limiter in the
control so if something shorted it wouldn't burn him. Spehro's
simple pot-controlled PWM cct worked slicker 'n snot. Brilliant!

Geez, a lot changes in 2 years. If I were to make that controller
now it would certainly be done with surfacemount and be about the size
of a large sugar cube. Two years ago I would have said surfacemount
was well beyond my abilities. I've since learned that it's just a
matter of having a couple of "right" tools. Then it's easy!

On Thu, 25 Nov 2004 10:55:32 -0500, Bob Engelhardt
wrote:

Don Foreman wrote:
... I used it
(with your permission and credited to you) in Kar'ls fingerwarmer
controller.

That was 2 years ago, how did it work out, as far as actually warming
Karl's fingers? Did you go with the curling iron heater wire, or what?

Bob

Or a power MOSFET and suitable simple cheap elex at less than 1 watt.

On Wed, 24 Nov 2004 17:49:29 GMT, Mawdeeb wrote:
To answer your original question - you would need a resistor of 1.7 ohms at
84 watts. This is assuming the circuit is feeding 24VAC to the pump.

If your transformer is 18 volts then:
you need a .85 ohm resistor at 42 watts

Jim Vrzal
Holiday,Fl.


Gunner wrote in
:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor, else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

Dropping resistor? Ok....blush..what value and wattage do I use?

(sounds of derisive laughter in the backround...sigh)

DC-DC converter? Ok..where and how much?

Thanks...

Gunner





"If I'm going to reach out to the the Democrats then I need a third
hand.There's no way I'm letting go of my wallet or my gun while they're
around."

"Democrat. In the dictionary it's right after demobilize and right
before demode` (out of fashion).
-Buddy Jordan 2001

Mike H wrote:

It could be made with a 7812 voltage regulator and a few pass transistors.

and would still have to dissapate the best part of 100 watts. That
would require a serious heat sink or water cooling (considering the
application).

Ted

Ian Malcolm wrote:

Mike H wrote:

If I remember my tranformer theory correctly, the load current in the
lower half of the winding is opposed by the supply current so you only
need a 3.5A rated winding.

Sorry. No go. 7amps is 7amps. If you are getting 7A out of the
transformer, the wire of that half of the winding has to be carrying it.

Ted

Ted Edwards wrote:
Ian Malcolm wrote:


Mike H wrote:


If I remember my tranformer theory correctly, the load current in the
lower half of the winding is opposed by the supply current so you only
need a 3.5A rated winding.


Sorry. No go. 7amps is 7amps. If you are getting 7A out of the
transformer, the wire of that half of the winding has to be carrying it.

Ted


So riddle me this. Visulize the autotransformer. 24V AC Live at the
top. 0V at the bottom, 12V at the centre tap. Supply connected top to
bottom. 3.5A flows from the supply into the top terminal of the winding
and has to flow back to the supply from the bottom terminal. The 7A load
current flows out of the centre tap, through the load (Gunner's bridge
rectifier and pump motor) and back to the bottom terminal. Now remember
that the total current into any point must sum to zero so half of that 7
amps *MUST* go up the bottom half of the winding.

OK its a special case for 2:1 autotransformers but it *DOES* only need a
3.5A rating to supply 7A without overloading which would make a really
big difference to Gunner as it would be easier to source *and* to cram
in there.

Finally, this sort of stuff is not unrelated to my day job :-) You wont
see much from me about the best way to rebuild a lathe, make an
interrupted cut or whatever way of making chips is the topic of the
day.,because I have very little machine tool experiance, but if it is
electronics related (note: NOT US mains wiring) I try to return
something for all the good stuff I've read lurking here.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- &
[dot]=.
*Warning* SPAM TRAP set in header, Use email address in sig. if you must.

You're still talking 84VA.. One that comes close (80VA) from
Signal Transformer is 2.5" x 2.37" x 3". Don't know if that's
"small enough" or not. The $10 electronic solution could easily be
done in 1.5" x 1" x 0.5". A surfacemount version could be
considerably smaller.



On Sat, 27 Nov 2004 01:35:23 +0000, Ian Malcolm
wrote:


OK its a special case for 2:1 autotransformers but it *DOES* only need a
3.5A rating to supply 7A without overloading which would make a really
big difference to Gunner as it would be easier to source *and* to cram
in there.

I don't understand why a guy would post a question and then ignore
responses from others trying to help. Smacks of a "you watch my six
and I will too" attitude. Shrug, sigh and roger.

For others that may be interested, I've posted the outline of a
dirt-simple dirt-cheap "DC transformer" at
http://www.goldengate.net/~dforeman/...mple_24_to_12/



On Wed, 24 Nov 2004 07:23:46 GMT, Gunner
wrote:

DC-DC converter? Ok..where and how much?