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'[EE]: Power Transformer Ratings Question'
2002\01\18@191349 by Rick Mann

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Hi.

I've got a handful of small power transformers that I'm using to determine
what the bare minimum is I can use in my project. But I'm having trouble
reconciling what I see on the specification with what I see on my scope.

Say, for example, I have a transformer, Jameco part #102517. The only info I
have on it is: parallel output 8V @  0.30 A. Since I don't need more than
about 250 mA total in my circuit, this sounds like a good transformer.
However, I want to minimize heating in the 7805 I'm using, so I'd like the
output voltage, after the bridge, to be not more than 8 volts.

With only a 3 mA LED connected as a load (to the 7805), I see more like 12.5
V before the 7805.

So, what gives? Do I need to increase the load to 300 mA to see this output
voltage? I'm I worrying needlessly (I'd like to avoid a heatsink on the 7805
if I don't need it)?

TIA,

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Rick

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2002\01\18@192343 by David VanHorn

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>
>With only a 3 mA LED connected as a load (to the 7805), I see more like 12.5
>V before the 7805.

An unloaded rectifier always gives you the peak value.
1% of rated qualifies as unloaded.

>So, what gives? Do I need to increase the load to 300 mA to see this
>output voltage? I'm I worrying needlessly (I'd like to avoid a heatsink on
>the 7805
>if I don't need it)?

Yes.

What I did, years ago, for testing power supplies, was to build up a bridge
rect, cap, and an 78XX regulator set as a constant current load.  For a 12V
supply, use a 10V reg, and 10 ohm load resistor.

With this load, you end up with a nice stable environment to make
measurements with. I added a DPM to the box, and put pass/fail limits on
the front, and we were able to fling those things through at about 10 per
minute, per tester.

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2002\01\18@193334 by Dave Dilatush

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Rick Mann wrote...

>I've got a handful of small power transformers that I'm using to determine
>what the bare minimum is I can use in my project. But I'm having trouble
>reconciling what I see on the specification with what I see on my scope.
>
>Say, for example, I have a transformer, Jameco part #102517. The only info I
>have on it is: parallel output 8V @  0.30 A. Since I don't need more than
>about 250 mA total in my circuit, this sounds like a good transformer.
>However, I want to minimize heating in the 7805 I'm using, so I'd like the
>output voltage, after the bridge, to be not more than 8 volts.
>
>With only a 3 mA LED connected as a load (to the 7805), I see more like 12.5
>V before the 7805.
>
>So, what gives? Do I need to increase the load to 300 mA to see this output
>voltage? I'm I worrying needlessly (I'd like to avoid a heatsink on the 7805
>if I don't need it)?

The output voltage rating of transformers, as well as DC wall-warts, is
usually the voltage they will deliver at their full rated load current.
The behavior you're observing seems about right.  Also, keep in mind
that the DC output from your bridge at light loads will typically be
about 1.4 times the AC output of the transformer, minus two diode
forward drops from the bridge rectifier.

Given the currents you're talking about, you may or may not need to use
a small heatsink on your 7805.  If it gets too hot to keep your
fingertip on it for a slow count of five without the heatsink, it needs
one.

Hope this helps a bit...

Dave

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2002\01\19@033336 by arpit

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Do normal transformers serve to regulate the voltage any? I brought from india a 12v  4 amp transformer with a fan connected to it from a pc, the output was 18 v. When I tried a 25 dollar transformer I bought in aust with exacty the same rating, the output was only 13 volts under the same load.

*********** REPLY SEPARATOR  ***********

On 1/19/2002 at 12:31 AM Dave Dilatush wrote:

{Quote hidden}

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2002\01\19@055839 by Jinx

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> Do normal transformers serve to regulate the voltage any?

No. Draw more current, the voltage goes down, and there's
nothing to stop you burning it out by drawing too much current.
The VA rating gives you some idea - as A goes up, V goes
down

> I brought from india a 12v  4 amp transformer with a fan
> connected to it from a pc, the output was 18 v. When I tried
> a 25 dollar transformer I bought in aust with exacty the same
> rating, the output was only 13 volts under the same load

4 amp load ? If so then, although the Indian transformer seems
to be better "value", I think I'd be happier with the Australian
one that at least got pretty close to the label rating. Quality, not
quantity

Never trust labels. There are some awfully over-rated
transformers around. Some manufacturers care, a lot
don't. Find one you like and stick with it

A very nasty trap is to believe the label on a wall wart. Apply
what claims to be a "5V 1A" DC supply to a PIC and you
can guess the rest. As above, a very light load may mean the
transformer has 50% or more above the label voltage. Pays
to use a regulator whenever possible

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2002\01\19@100426 by Roman Black

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Rick Mann wrote:
{Quote hidden}

The 7805 should only get hot at your 250mA
load, not your 3mA load. If the 250mA load
will never be exceeded you can put a resistor
between +ve and 7805 input, this will dissipate
some of the heat that the 7805 would have borne.
This works well if the load current has a
defined max and you allow just enough head
voltage to keep the 7805 working, about 1.5v
is enough.

With 9vdc in, 250mA load, you have about 4v
on the regulator total, so aim for 2.5v on the
resistor (0.625W), and 1.5v on the regulator
(0.375W). It should not need a heatsink. A
TO-220 part standing up is good for about 0.5W
before it needs a heatsink. You can get 0.75W
if you bolt it to the circuit board, 1W if it
is bolted to a copper PCB pad, or as little as
0.25W if it is jammed in with other parts.

Tips:
* The resistor as mentioned. :o)
* Those small transformers always have a thermal
fuse inside, so try to run them cool and ALWAYS
put a decent resistor betweens mains and transformer
or mains spikes will kill the transformer. I've seen
hundreds of clock radios and small hifis killed
like this. The good makers use the resistor, the
bad (smart?) ones go for product turnover.
Resistor drops about 5% to 10% of the mains when
the output load is 250mA.
* Only use one of the 2 8v windings if possible,
this won't heat the transformer any more than using
both, and won't hurt it in any way, but it will reduce
your regulator losses.
-Roman

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2002\01\19@113703 by mike

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On Sun, 20 Jan 2002 01:55:24 +1100, you wrote:

>Rick Mann wrote:
>>
>> Hi.
>>
>> I've got a handful of small power transformers that I'm using to determine
>> what the bare minimum is I can use in my project. But I'm having trouble
>> reconciling what I see on the specification with what I see on my scope.
>>
>> Say, for example, I have a transformer, Jameco part #102517. The only info I
>> have on it is: parallel output 8V @  0.30 A. Since I don't need more than
>> about 250 mA total in my circuit, this sounds like a good transformer.
>> However, I want to minimize heating in the 7805 I'm using, so I'd like the
>> output voltage, after the bridge, to be not more than 8 volts.
>>
>> With only a 3 mA LED connected as a load (to the 7805), I see more like 12.5
>> V before the 7805.
>>
>> So, what gives? Do I need to increase the load to 300 mA to see this output
>> voltage? I'm I worrying needlessly (I'd like to avoid a heatsink on the 7805
>> if I don't need it)?
Transformer ratings are usually quoted at full load. For small
transformers, the difference between this and the off-load voltage
will be significant. For heat dissipation you need to test at full
load - a 7805 passing 3mA with 12V in will not get warm!

Remember to take possible mains voltage variations into account when
determining minimum transformer voltage.

{Quote hidden}

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