I have a Central Machinery (HF) power feed on the X axis of my RF45
clone mill and it's acting weird . Works fine on the fast move button
but the speed settings are messed up . It doesn't start turning until I
get to "2" on the dial and by the time it's up to 3 it's going full
speed . It just started yhis , today is the firt=st time I've seen this
.. When it was working right it would start before I got up to one and
would ramp up perfectly as i turned it up .
Am I looking at a bad rheostat or maybe just dirty ? Should be clean
, that unit is sealed up tight - but ya never know .
--
Snag
Illegitimi non
carborundum

That sounds like a bad SCR.

On 7/13/2020 5:25 PM, Michael Terrell wrote:
That sounds like a bad SCR.


OK , I don't know a lot about SCR's , but I have some experience with
electronics . How do I test it ? I was planning on starting by checking
the resistance of both sides of the control pot ... but I have no idea
how to test an SCR .
--
Snag
Illegitimi non
carborundum

The easiest is to just replace it. Use the same part number. The sensitivity varies all over the map, with different types. They are usually cheap, and a lot cheaper than a test fixture.

"Snag" wrote in message ...

On 7/13/2020 5:25 PM, Michael Terrell wrote:
That sounds like a bad SCR.


OK , I don't know a lot about SCR's , but I have some experience with
electronics . How do I test it ? I was planning on starting by checking
the resistance of both sides of the control pot ... but I have no idea
how to test an SCR .
--
Snag
Illegitimi non
carborundum

===========================

A thorough test requires lab equipment to measure Off-state leakage current
at full working voltage and the minimum gate triggering current, but a
functional check is simple:
https://www.circuitstoday.com/how-to-test-an-scr

I'd suspect an intermittent pot wiper contact.

On Tuesday, July 14, 2020 at 5:57:09 AM UTC-4, Jim Wilkins wrote:
"Snag" wrote in message ...

On 7/13/2020 5:25 PM, Michael Terrell wrote:
That sounds like a bad SCR.


OK , I don't know a lot about SCR's , but I have some experience with
electronics . How do I test it ? I was planning on starting by checking
the resistance of both sides of the control pot ... but I have no idea
how to test an SCR .
--
Snag
Illegitimi non
carborundum

===========================

A thorough test requires lab equipment to measure Off-state leakage current
at full working voltage and the minimum gate triggering current, but a
functional check is simple:
https://www.circuitstoday.com/how-to-test-an-scr

I'd suspect an intermittent pot wiper contact.

+1

If turns out to be the SCR, let me know. I have some SCR's I salvaged from the scrap yard. I forget the specs, but they are good for about 1000 amps.

Dan

Jim Wilkins wrote:

OK , I don't know a lot about SCR's , but I have some experience with
electronics . How do I test it ? I was planning on starting by checking
the resistance of both sides of the control pot ... but I have no idea
how to test an SCR .

In principle, it's easy 8-). First, test it like a diode,
however if it conducts either way it's faulty.

Then test it forward biased and momentarily connect gate to anode.
It should start conducting and stay that way.

There are two traps in this: One is that an SCR generally needs at
least two forward diode drops, 1.4 volts, to work at all. The other
is that "latching" the SCR on requires some minimum current, maybe
1% or less of running current. Far more than a normal diode tester
provides. A car battery and taillight bulb would probably work.

HTH,

bob prohaska

On Mon, 13 Jul 2020 17:18:17 -0500, Snag wrote:


I have a Central Machinery (HF) power feed on the X axis of my RF45
clone mill and it's acting weird . Works fine on the fast move button
but the speed settings are messed up . It doesn't start turning until I
get to "2" on the dial and by the time it's up to 3 it's going full
speed . It just started yhis , today is the firt=st time I've seen this
. When it was working right it would start before I got up to one and
would ramp up perfectly as i turned it up .
Am I looking at a bad rheostat or maybe just dirty ? Should be clean
, that unit is sealed up tight - but ya never know .

Got a bad pot ..rheostat.

Open and spritz the pot with cleaner. However..it may be bad..so check
with any online sorce for new potentiometer.

Gunner
__

"Journalists are extremely rare and shouldn’t be harmed, but propagandists are everywhere and should be hunted for sport"

Yeah..with no bag limit.




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