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Old 31st January 2013, 03:18 PM   #1
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Default Help with a power supply

I'm not a design engineer. I need help with a power supply. As you can see from the schematic, the output should be +47 volts. Mine puts out +53 volts under load. All the caps are rated for +50 volts and the pre amp is also getting too high a voltage. I need to bring it down 6 volts. Will changing the value of R16
lower the output voltage?
thanks for your input
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Old 31st January 2013, 04:36 PM   #2
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How old are your electrolytic caps? If they are undervalued the voltage will creep up towards 70, but will collapse when a big load is put on it. All those diodes and C14-15 make a voltage doubler, and transformer ratings are the full load value, not the peak which is 1.4 times higher.
You could change out R16 pretty cheaply, or parallel resistors on top of it. But how much heat sink does Q4 have? If it has enough I would be tempted to buy a 30 v zener and a 15 v zener (I keep those values in my semiconductor stock box) and stack them up line to not line between Q4 base and plus of C15. Line to + of C15. I'd put a resistor in series with the zeners, say 4.7k or something. Then R16 would be irrelevant. Probably the voltage would have have less temperature sensitivity, also.
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Last edited by indianajo; 31st January 2013 at 04:39 PM.
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Old 31st January 2013, 04:53 PM   #3
jaycee is offline jaycee  United Kingdom
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It could be that your mains voltage is slightly high, so your secondary voltages are slightly higher too. This is not a regulated supply. The transistor is only functioning as a capacitance multiplier. It's also a bit odd that a capacitance doubler is used in the first stage to step up the voltage.

What is the power supply being used for exactly?
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Old 31st January 2013, 04:56 PM   #4
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Deja Vous, Jaycee, your post is word for word perfect for what I was going to post.

If we know what you are trying to achieve then maybe we can come up with some suggestions.

The PSU as it stands is a simple Voltage Doubler, Capacitive Multiplier with no regulation. In a low current situation this can be improved upon, how much current are you asking to draw?

Last edited by KatieandDad; 31st January 2013 at 04:59 PM.
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Old 31st January 2013, 05:32 PM   #5
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Thanks for all the input.
This power supply is used to power a JFET RIAA preamp. The amp draws no more than 3 ma. My mains range from 119-121 vac. I built the PS and amp 3 days ago. I got the kit from oatley.com down under. Nice kit.
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Old 31st January 2013, 05:40 PM   #6
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If it's drawing such little current I would just add a simple zener and resistor at the output to drop the voltage.

You could go to a shunt reg if you feel the need.
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Old 31st January 2013, 06:22 PM   #7
jaycee is offline jaycee  United Kingdom
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Is this the kit ?
http://secure.oatleyelectronics.com/files/k303notes.pdf

I find their choice of PSU topology very strange.
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Old 31st January 2013, 07:11 PM   #8
sbrads is offline sbrads  United Kingdom
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100k resistor across C16 will drop the output volts by 10% roughly.
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Old 31st January 2013, 08:53 PM   #9
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The voltage is probably not critical, since there is used components that in their self would give a possible deviation of up to 20-25%.
I agree with jaycee that the topology is a bit strange, but no fear.
The amp will function godd anyway.
OR have You fallen for the temptation of replacing the original Caps nearest the transformer? Lo ERS High Ripple current types is not suitable replacements at that position.
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Old 31st January 2013, 10:18 PM   #10
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Yes Jaycee, thats the preamp I have. It works very well, I just think having 53vdc on a 50vdc cap is a bit much. I may just look up a schematic for a regulated adjustable PS and build it.
thanks all.
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