24Vdc Power supply

"Ian Tedridge"

** No need for capacitors.

** Really ?

So you don't imagine a 24 V ( ideally 26.5 V ) 20 amp tranny would do?

Just add one 40 amp bridge rectifier on a modest heatsink.

If there is any chance one of the motors could stall, then add a 20 amp breaker in series as well.

...... Phil

Thanks Phil,

My thoughts on the capacitors, was just to clean up and stabilise the output a little. Something I've seen used before.

Must admit, had not thought about a breaker for protection. Think I need to slow down a little and take stock of what I am trying to do.

Cheers

Hi, Ian. Ditto on everything Phil said. If you'd like some control of the motor speed, though, you can drive the primary of the transformer with the output of a Variac like this (view in fixed font or M$ Notepad):

| | _ T1 | o--o_/ \\o--. | FU1 ) | )0-240VAC .-----. | )

Some more to think about . . . if you filter it you raise the voltage because with raw RMS DC 24V=24V, with filtered DC you approach the peak voltage which is 1.4142 X the RMS value, or about 36 volts (unloaded - the transformer will drop a little voltage as will the rectifier diodes) Transformer ratings are for some specified current and at some nominal voltage - Voltage can be plus or minus 10%. off nominal.

And, you are using 20 amps. each diode in a four diode bridge will drop .6 volts for 1.2 volts in each half cycle at 20 amps that's 24 watts wasted power and requires a good heatsink. Use a center-tapped transformer and you cut the waste power in half - ditto the heatsink and size, and likely as not cost as well.

I use an adjustable 24 VDC 10 amp to run some fans - the SCR's doing the controlling also rectify the AC to save on wasted power and keep the heatsink small.

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"default"

** Shame how you then *waste* the output capacity of the transformer instead - by loading only half of the secondary copper at a time.

Very inefficient .

........ Phil

Correct.

John

Incorrect. A half bridge configuration wastes transfomer utilisation. The I^2R losses increase far more than anything you'll save in a bridge.

Graham

losses

No way.

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I see the errors of my thinking. Intrinsic lossses in the transformer cause the I sq R loss ot increase faster than the diode drop losses in that (24V) voltage range. At lower voltages the diodes waste more power than the transformer and a center tap makes more sense.

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I^2R losses

I'm afraid you're utterly wrong.

Graham

"default"

** Not really.

** Drivel.

The "full wave, center tap" ( two diode) arrangement REQUIRES that a transformer's VA be DE- RATED compared to using full wave bridge.The reason is as I posted earlier - only HALF the secondary copper is being used at any moment.

This applies to a transformer of any size.

....... Phil

--
I disagree.

For a resistive load and the same output voltage, current, and

"John Fields"

** Dig you own grave deep as you like if you wish .....

** Your SILLY false assumption is now plain.

The **context** here, is where the SAME transformer is used in one or the other rectifier mode.

Do try to improve your reading skills - John.

Unconscious context SHIFTING is such a GLARING indicator of your congenital AUTISM.

....... love, Phil

Why would a designer use the same size wire in the CT secondary? Each winding is working at a 50% duty cycle so the required ampacity of the wire is cut by half - things being what they are, the transformer maker would save copper by using smaller wire.

AND Looking at transformer ratings where they provide two independant secondaries the output will be rated 12V @ 2A , and 24V @ 1A (same game I would think)

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Well, let\'s see...

If we want to use the _same_ transformer to feed a full-wave bridge

--
And pay the penalty for the greater I²R losses in poorer regulation.

--
That is, by trying to get 2A out of each secondary for half the
time.

"John Fields" = desperately needs a remedial reading class

** Which is IDENTICAL to my original comments.

** English class at school ??

....... Phil

--
If you meant that one would have to run the transformer at a lower
VA rating than it was rated for if the center-tapped arrangement was