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.
=20
>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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