Bench PSU

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Re: Bench PSU

Post by Jalex2 on Sun Apr 12, 2015 2:28 pm

Hi
This is just an idea and probably not good for you 2 amp  as I very seldom draw that much current on my bench.
I use Harry's simple  PSU circuit but I power it from a switch-mode printer power supply. The supply puts out 5V and 42
volts the I just very the 42 volts. Of course this doesn't cure any of the heating problems mentioned above.

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Re: Bench PSU

Post by Ivan on Tue Apr 07, 2015 6:03 am

Hi CPM,
what has the output voltmeter to do with power dissipation ?
Consider the minimum and maximum output voltage and maximum current of the PSU. The maximum output volage is generally dictated by the secondary voltage of the power transformer. You can easily afford 30V or even 35V maximum voltage. Be careful, the rectified voltage Vsec*1,41 is valid at no load, but may drop under the full load.
Say you want the minimum voltage 3V. The dissipation worst case is at the minimum voltage and maximum current. In your case (46-3)*2 = 86W turned into heat on the regulating transistor(s)!
The possibilities to reduce this energy waste is either a transformer with tapped secondary, switched to match the output voltage, or a pulse preregulator.

BR from Ivan

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Re: Bench PSU

Post by Ruud on Tue Apr 07, 2015 5:58 am

A center tap full wave rectifier won't change the voltage, with a (2x) 33 volts transformer you will still get something like 46 volts...
It would be a different situation if the 33 volts transformer would have a center tap. That would give 23 volts DC.
Maybe I did not understand you correctly, but changing the meter won't make any difference at all!

What is usually done in variable power supplies, is switching the input (AC) voltage to the rectifier.
Say you have a power supply with a variable output between 0 and 30 volts.
Because a high input voltage and a low output voltage would mean a lot of heat, a lower secundary voltage of the transformer is used for 0 to 15 volts, and a higher secundary voltage for 15 to 30 volts. The switching is usually done with a relay.
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Bench PSu

Post by cpm on Tue Apr 07, 2015 3:35 am

Perhaps I should use a center tap full wave rectifier for the bench PSU circuit and replace the 25 volt analog meter with a lesser value. The heat dissipation would be much lower.

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Re: Bench PSU

Post by Ruud on Mon Apr 06, 2015 7:54 pm

If you use a 33 volts transformer, you will roughly get 1.4 * 33 = 46.2 volts DC, maybe even a bit more. You want 25 volts out, at 2 amps. This will mean you have to dissipate 2 * 21.2 = 42.4 Watts in the regulating transistor(s)! I suppose this will produce a lot of heat...
Wouldn't it be an idea to remove some turns from the transformer, so you would get a lower AC voltage going to the rectifier/stabilizer?
I suppose a 22 or 25 volts transformer would be better, that would give you roughly 31 or 35 volts DC.
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Bench PSU

Post by cpm on Sun Apr 05, 2015 4:15 am

Hello,
I would like to build the Bench PSU to provide 25 volts at a max of 2 amps.  My transformer is a hefty 33 volts ac with a center tap. Can anyone suggest modifications to the original circuit?  Perhaps higher voltage transistors?
Thanks.
C

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Re: Bench PSU

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