Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

TIA

Chris

It happens that formulated :
I seem to have run out of spare 5V supplies, but have a small box of 12V
switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

Yes, but it would be more efficient if you were to find the resistor
network or trimmer which sets the output voltage of the SMPSU and
adjust it to directly output 5v.

They all use some sort of network, by which the output voltage is used
as feedback to control the output.

I converted several ex-BT 50v (Farnell) SMPSU's so as to deliver 13.8v,
by simply tracing the correct resistors to change.

On Wed, 22 Mar 2017 10:03:44 -0700 (PDT), wrote:

Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

Depends on how much current you're planning to draw I think.

A 7805 is a linear regulator, so you could be disspating a lot of
power in it. At 12v in and 5v out, delta V is 7v. So at a current of
1A, you'd be dissipating 7w. That'll need a heat sink.

Obviously efficiency will be low. The 7805 will have an efficiency of
5/12 = 41%. And that's after the losses in the 12v PSU.

Yes but the disipation will be laess as you are in effect using the device
to dump the volts.
I'm not sure how these devices are made, but surely there has to be some
kind of chip in there that is multi standard to give out the range of
voltages one might need.
Brian

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wrote in message
...
Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V
switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

TIA

Chris

Caecilius wrote:

wrote:

have a small box of 12V switchmode PSUs which are redundant. Could
I convert them to 5V by the simple addition of a 7805 on the
output?

Depends on how much current you're planning to draw I think. A 7805
is a linear regulator, so you could be disspating a lot of power in
it.

You could use a 5V LDO (low drop out) regulator instead of a 7805, e.g.
if you need 1A, try this, or search for higher current ones ...

http://www.hobbytronics.co.uk/5v-voltage-regulator-ap1117

On Wed, 22 Mar 2017 10:03:44 -0700 (PDT), wrote:

Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

As others have mentioned a linear voltage regulator wouldn't be very
efficient and get quite warm.

So, whilst not very pretty ... and depending on the power required, a
'buck converter' (or converters) might be a cheap and easy way of
making use of some 'nice' SMPSU's?

That way you could also use them for other voltages at the same time
(and higher if you use a 'bang converter'). ;-)

Cheers, T i m

On 22/03/17 17:03, wrote:
Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

Adjustable Buck converters on eBay are about a quid a piece

10 PCS DC-DC Buck Step Down Converter Module LM2596 Voltage Regulator
TE097
http://www.ebay.co.uk/itm/10-PCS-DC-DC-/152045397355

Then again 5V 1A power supplies in Poundland are the same.

--
Adrian C

On 22/03/17 20:24, Adrian Caspersz wrote:

Then again 5V 1A power supplies in Poundland are the same.


Yes, and according to this video are very good.

https://www.youtube.com/watch?v=W1_PYVTTYkU

Another Dave
--
Change nospam to techie

Thanks for the replies everyone. I think I will invest in some buck converters.

On Wed, 22 Mar 2017 10:03:44 -0700, cpvh wrote:

Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V
switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the
output?

If you only need a 100mA 5v supply and you've already got a bunch of
7805 regulators to hand, then ok. Otherwise, if you need a 1A 5v smpsu,
you might as well buy a bunch of Poundland USB wallwarts and have done
with it.

As I've mentioned elsewhere, the no-load standby consumption of these
Poundland USB wallwarts is less than 100mW, quite possibly less than
50mW. Certainly more than low enough to make switching them off at the
socket when unused a total waste of calorific effort just to shave 10 or
15 pence a year off the electricity bill.

I couldn't detect the slightest hint of power draw using my Metrawatt
analogue watt meter (on the 100W scale -2 watts per division) with a
jeweler's loupe to closely observe the needle pointer's expected movement
from a gnat's dick's worth of displacement from its steady state reading
back down to the zero mark when switching the mains off[1]. IOW, the
power consumption was immeasurably small on a device not crippled by the
limitations of a digital readout limited to a resolution, at best, of a
tenth of a watt.

[1] Switching on results in the usual 3 or 4 watt 'kick' due to the HT
smoothing cap's inrush current so it has to be inferred from the switch
off transient.

--
Johnny B Good

On Wed, 22 Mar 2017 22:13:16 +0000, Johnny B Good wrote:

====snip====

Otherwise, if you need a 1A 5v smpsu,
you might as well buy a bunch of Poundland USB wallwarts and have done
with it.

As I've mentioned elsewhere, the no-load standby consumption of these
Poundland USB wallwarts is less than 100mW, quite possibly less than
50mW. Certainly more than low enough to make switching them off at the
socket when unused a total waste of calorific effort just to shave 10 or
15 pence a year off the electricity bill.

Oh yeah, I nearly forgot to mention that replacing just one of those 12v
wallwarts however modified (add in buck converter or change of resistor
in the volt sensing circuit) with a Poundland 5v1A USB wallwart is likely
to pay for itself after little more than 6 to 11 months use[1].

Most of those older 12v wallwarts typically drew anywhere from 1 to 2
watts from the mains just by being plugged in before being connected to a
load.

[1] If you're going to buy them a fiver or a tenner's worth at a time,
just make damn sure you get (and keep) the receipt. :-)

--
Johnny B Good

On 22/03/2017 17:03, wrote:
Hi All,

I seem to have run out of spare 5V supplies, but have a small box of 12V switchmode PSUs which are redundant.

As the output of these is (in theory) already smooth,

Could I convert them to 5V by the simple addition of a 7805 on the output?

TIA

Chris


You could but is it worth the hassle etc?

If you are an experimenter and want something for 'bench' use, rather
than general domestic use, better to get some variable supplies off Ebay.

I bought 5 or 6 for about £9. Wide range input, up to something like
30V, variable (lower) output. Smaller than a match box. Ideal for all
kinds of things. 'Clean' output, switchmode. Claimed 95% eff.

--

Suspect someone is claiming a benefit under false pretences? Incapacity
Benefit or Personal Independence Payment when they don't need it? They
are depriving those in real need!

https://www.gov.uk/report-benefit-fraud

En el artículo , Adrian Caspersz
escribió:

Then again 5V 1A power supplies in Poundland are the same.

Meant to grab a couple, but didn't spot them last time I was in. Clive
certainly seemed happy with them.

--
(\_/)
(='.'=) "Between two evils, I always pick
(")_(") the one I never tried before." - Mae West

En el artículo , Johnny B Good johnny-
escribió:

from a gnat's dick's worth of displacement



I thought the accepted term of measurement was a gnat's chuff.

--
(\_/)
(='.'=) "Between two evils, I always pick
(")_(") the one I never tried before." - Mae West

On Thursday, 23 March 2017 10:00:52 UTC, Mike Tomlinson wrote:
En el artÃ*culo , Johnny B Good johnny-
escribió:

from a gnat's dick's worth of displacement



I thought the accepted term of measurement was a gnat's chuff.

Maybe we have to wait until article 50 goes a head

On Thu, 23 Mar 2017 07:54:24 +0000, Brian Reay wrote:

Could I convert them to 5V by the simple addition of a 7805 on the

output?

I bought 5 or 6 for about £9. Wide range input, up to something like
30V, variable (lower) output. Smaller than a match box. Ideal for all
kinds of things. 'Clean' output, switchmode. Claimed 95% eff.

Sounds just like the 5 that arrived this morning, £5.99 free
delivery, ordered Monday midday. I'll be checking how much RF muck
they radiate and how clean the output is. The idea is buy meaty 12 V
PSU and hang these DC-DC converter of it for the devices that
normally live from wall warts
at 3.3 V, 5 V, 7.5 V, 9V etc.

--
Cheers
Dave.