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Old 28th April 2007, 05:07 PM   #1
owdi is offline owdi  United States
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Default Easy PSU question - help me make this 12v DC power supply better.

(disclaimer)electronics n00b(/disclaimer)

I have some car audio signal processors I want to use with my home stereo (Audiocontrol EQL equalizer and Audiocontrol 2XS crossover). Together they will draw somewhere between 500mA and 1A.

These were built to use the 9 - 15v DC "like" dirty power from a car, so I don't think I need a very complex power supply to power them.

The unit I have is a very simple 12V 2.5A power supply. It looks like it consists of nothing more than a transformer, bridge rectifier and 3300uF 25v capacitor.

The first obvious mod is to increase the size of the cap. I have an extra 12,000uF 63V capacitor which should greatly reduce ripple.

Is there anything else I can easily add to make this PSU better? I noticed the PCB has some missing components. There is a place for C4, C5 and R1. It looks like C4 and C5 are in parallel with two of the diodes in the rectifier. I don't know what the purpose is of R1.

My main concern is the small load on the power supply may result in a voltage so high, it will blow the signal processors I plug into it.

Thanks in advance for any advice or direction.

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Dan
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Old 28th April 2007, 10:28 PM   #2
owdi is offline owdi  United States
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I put a dummy 45ohm load on the power supply and measured voltage at 15.1V. On a hunch I switched my DMM to measure AC, and measured about 150mV, which I assume is the ripple. I didn't know if my DMM could even do that, but I have spare fuses for it, so I went for it :-)

I then added the 12,000uF capacitor. Voltage changed a little to 14.8V, and ripple fell to 18mV.

I'm pretty sure the equalizer and crossover can handle this voltage, so time to hook everything up and see how it sounds.

Dan
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Old 28th April 2007, 11:09 PM   #3
sandyK is offline sandyK  Australia
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owdi
Put a 3 Amp diode such as the 1N5404 between the 2 capacitors.This should give a small amount of extra filtering, but more importantly, reduce the voltage a bit more. Maximum car battery voltage under charge is normally 13.8 to 14.2V
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Old 29th April 2007, 05:08 AM   #4
owdi is offline owdi  United States
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Thanks for the tip sandyK. I'll pick up some diodes at my local electronics store. How would I go about reducing the voltage? In the past when I needed lower voltage I just added a series resistor, but I have a feeling that's not the best idea with this application.

Dan
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Old 29th April 2007, 06:11 AM   #5
sandyK is offline sandyK  Australia
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Default Easy PSU question

Dan
The series diode will drop the voltage about another 0.7 Volts (depending on the load) Going by your measurements that should give you about 14.4V. All car audio gear should be O.K. with that.
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Old 29th April 2007, 05:15 PM   #6
owdi is offline owdi  United States
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With everything plugged in, and levels set, voltage is now down to 13.88V. I will still buy the diode this week, and I want to find a way to hide the big capacitor inside the PSU case.

Dan
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Old 29th April 2007, 07:43 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
those small transformers usually run warm to hot, sometimes too hot to hold your fingers on.

The capacitors will get cooked if your's also runs hot.

A 25V cap of similar capacity will be much smaller and may fit inside.
You can also fit a capacitor inside the receive units as well.

Have you considered adding a 1A regulator (LM7809) to bring the 14V ish down to 9V or so? Opamps prefer a clean supply particularly if working with low level signals.
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Old 2nd May 2007, 04:08 PM   #8
owdi is offline owdi  United States
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Quote:
Originally posted by AndrewT
Hi,
those small transformers usually run warm to hot, sometimes too hot to hold your fingers on.

The capacitors will get cooked if your's also runs hot.

A 25V cap of similar capacity will be much smaller and may fit inside.
You can also fit a capacitor inside the receive units as well.

Have you considered adding a 1A regulator (LM7809) to bring the 14V ish down to 9V or so? Opamps prefer a clean supply particularly if working with low level signals.
Hello Andrew,

I'll check the temperature of the transformer after running it for a while. If it's hot, I'll keep the cap far away. I don't want to bring the voltage down to 9v, because the equalizer and crossover were designed to work in a car, which usually provides something like 12.5 +- 2v.

Dan
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Old 2nd May 2007, 04:42 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
if you do decide to regulate then more AC voltage is required.
For 12 to 14Vdc you will need about 13Vac to 15Vac.
Then choose 7812 or 317 and set to 13Vdc or 14Vdc.

If you stay unregulated then additional smoothing/decoupling inside the processor casing will work.
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