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Could someone check my math on this power supply design?

post #1 of 10
1/11/11 at 4:59pm
Thread Starter 

 

Alright, so I'm building a headphone amp to take with me for a long road job for work.  Just going to be a cmoy variation, So I'm looking to power it with a +/-5V supply, which I'm also going to build.

 

I'll be using LM7805/LM7905 chips for the voltage regulation, which would mean I'd need a minimum of 8 volts hitting the inputs.  I scavenged an AC wall wart at work that is basically a center tapped transformer putting out 19 volts with a center tap, so two 9.5V outputs.  If I hook it up to the circuit below (please forgive it's crudeness), I should have ~9.5V going to the regulators, yes?

 

 

500

It's been a while since I've built a power supply, and my memory is getting fuzzy.  Thanks.

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post #2 of 10
1/11/11 at 5:25pm

I hope my memory is serving me correctly. I think you will have closer to 12vdc

 

calculate peak voltage: 1.414 * 9.5v (rms) = 13.4

calculate rectifier losses: 13.4 - (0.7 * 2) = 12

 

You should reverse the polarity of the caps on the V- rail  wink.gif


Edited by balderon - 1/11/11 at 5:27pm
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post #3 of 10
1/11/11 at 5:36pm
Thread Starter 

yeah I realized I screwed them up after I posted this.. whoops!

 

Thanks, I believe you're right, looks real familiar from highschool.

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post #4 of 10
1/11/11 at 6:36pm

Actually, it depends on how big the filter caps are you have after the rectifiers, but a 0.8 corrective factor multiplied after balderon's results is usually customary.  It won't be quite as great as 12VDC, but you'll have plenty for your 8VDC requirement.wink.gif

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post #5 of 10
1/14/11 at 1:52pm
Thread Starter 

Ok so I got it built, ran some led's across the outputs to show that it each rail was providing voltage, and they lit up, SUCCESS!

 

Well not yet...

 

I built the amplifier circuit, got it all wired up go to test and there is almost no sound, I have to roll the volume pot to max to hear anything, and even that isn't much.  So I do some measurements, and find that I've got 5 VDC on the positive side, good, but I've got like -8 VDC on the negative, which is 3 volts too much.  This has really got me scratching my head.  Could a 3 volt offset cause there to be little output from the amp?

 

The amp is based off the tangent pimenta V2  (http://tangentsoft.com/audio/pimeta2/misc/sch-2.01.pdf), the only difference being that I don't have buffers (couldn't find any at the local electronics shop), so just replace it with a straight piece of wire.

 

 

I poked through an old radio shack power supply design book I got years upon years ago, and in one of their designs they show a diode going across the regulator in reverse bias to help protect the regulator when large capacitors are used, which I might have, 3300 uF.  So here's a revised drawing of the power supply, showing the diodes across the regulators:

PowersupplyV2.jpg

 

C1, and C2 are .33uF, and C3 and C4 are 3300uF.  I have not put the diodes in yet....

 

 

 

Any thoughts?

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post #6 of 10
1/14/11 at 2:42pm

The regulator could have blown up, what is the voltage on the input VS the voltage on the output side?

 

I would guess that the diodes are critical when the caps on the input of the regulator are significantly smaller than the caps on the output. When you turn the power off the caps on the input drain very quickly, where the big caps on the output stay pretty full and apply voltage backwards across the chip and poof. I have no idea why only one regulator is acting funny, maybe the other one was just luckier :)

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post #7 of 10
1/14/11 at 2:54pm
Thread Starter 

It is possible that the regulator is toast, they came in packs of 5, so I've got spares, hopefully I'll have some time tomorrow to pop back to my shop and swap it out.  I'll do some measurements then as well.  I don't know why, but I've never had luck building power supplies, some day.....

 

Something I also noticed was that the positive rail's LED would turn off almost immediately after I cut power, where as the negative rail's LED would fade out a second or so later.

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post #8 of 10
1/14/11 at 5:45pm

Be aware that the pin-out of the 7805 regulator is not the same as the 7905.  You may have wired it up wrongly if you assumed that they're the same.

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post #9 of 10
1/14/11 at 6:20pm
Thread Starter 

Thanks for the heads up, I did check their data sheets before installation, thankfully.  Danged pinouts.

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post #10 of 10
1/15/11 at 4:36pm
Thread Starter 

Failure smells like blown up capacitors.  Oh well, I guess this project will have to wait a month.

 

Thanks for the help guys.

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