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Author Topic: 12 volt power supply transformer,diodes  (Read 4912 times)
AC2EU
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« Reply #15 on: December 27, 2012, 04:40:51 PM »

I haden't thought of test first,I would feel like crap if radio was toasted.schematic I'm looking at has a diode placed so fuse will blow if there's a rectifier failure.$65-75 isn't much more to spend,on the other hand for switching supply.

Doing the load test is no trivial matter either!  At 13.6V you need a really big  power resistor that is adjustable to .68 ohms. I have one, but they are hard to find and expensive unless you can get a used one or borrow one. Maybe an auto headlamp might work if you can find one that draws 20 amps.
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WB6BYU
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« Reply #16 on: December 27, 2012, 05:22:09 PM »

The headlamps I have draw something like 5A, so I use several.
 
If you turn them on one at a time you should get a good test.  If you try
to turn them all on at the same time the surge current might be too
high.
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KA4POL
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Posts: 1965




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« Reply #17 on: December 27, 2012, 09:52:50 PM »

Quote from: KA4POL
The answer to your question is called Ohms Law. Unless you got zero resistance you'll always find a voltage drop along a line.
If it was just resistance it would be much easier to build a supply with good regulation.

If you have a transformer rated at 12VAC (RMS) then the peak voltage will be ( 1.4 * 12 = ) 16.8V, and this is what the capacitor will charge to with no load. But at full load it will drop down closer to the nominal 12VAC, for a change of 4.8V.  This voltage change is in addition to any voltage drop due to resistance in the circuit.

First of all my intention was to keep the answer simple for easier understanding.

Of course you are right concerning the input capacitor, i.e. the one after the rectifier. To complete this, the transformer does have to be capable to deliver sufficient current as well and not only voltage. I usually use a transformer with an output of at least 1.4 times the desired DC output voltage. So 18V would be desirable here for 12V DC at whatever current it's planned for. Good transformers are hard ones, i.e. not much voltage change from no load to full. But now we are really getting into design details.
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