HOMEMADE Dual output adjustable psi (Lm317+ka7912 - diyAudio
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Old 22nd August 2012, 03:43 AM   #1
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Red face HOMEMADE Dual output adjustable psi (Lm317+ka7912

I'm looking to make (my first) bench top supply for testing random shenannegins , with NO extremely precise output expectations , I dont need a top of the line design --->>>>>> a very basic design (just something that works). I'm using a fairly small ct +-12v tranny, a 400v 10a full wave rectifier, two 4700 elna filter caps, lm317t, ka7912 and tons of bits and tips and odds and ends at my disposal AND I MEAN TONs!

Expectations
Adjustable: -12 - 24v
Dual outputs
Maybe a fixed 5v (i have maybe 200 7805s)

So far this is what I have

Tranny->BR->+12vto+of1ofTheFilterCaps->-12v of -to2ndcap->?????
HELP
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Old 22nd August 2012, 07:00 AM   #2
Mooly is offline Mooly  United Kingdom
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If you want really really simple then use an NPN darlington (or discrete darlington) series pass transistor and feed the base from a 10K pot across the unregulated supply. Add a 5k6 in series from the wiper to the base to protect the pot in case of output shorts. That will give a variable 0 to around 15 volts DC. Do a mirror image (PNP darlington) for the neg rail. You could use two pots so each output rail is set separately or a dual gang pot so they alter together.

It's very very basic but works well and can be refined a little such as adding a stable reference to the feed the pots. Overall though there is no error correction and the output will alter a little from no to full load which may not be a problem as your not after ultimate performance.

Addition of another transistor and current sensing resistor to each side will give basic current limiting.

You can still use your IC regs to provide fixed outputs as well.

You said very basic and that is what this is. And it works
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Old 22nd August 2012, 09:59 PM   #3
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Thanks, can I use ANY npn ?

Also I came across a bag of to-3 something's lol labeled F(brand) 571104 334
I assumed they're lm334s but I've yet to find a data sheet
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Old 22nd August 2012, 10:37 PM   #4
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Start here with the LM317 data sheet. They give many power supple examples.

http://www.ee.buffalo.edu/courses/elab/LM117.pdf
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Old 22nd August 2012, 10:53 PM   #5
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I searched around a little and here is a tested dual tracking design.

Simple Dual-voltage Tracking Power Supply for 3 to 15 volts joebrown.org.uk
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Old 23rd August 2012, 06:15 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by GHeTToNoMiKS View Post
Thanks, can I use ANY npn ?

Also I came across a bag of to-3 something's lol labeled F(brand) 571104 334
I assumed they're lm334s but I've yet to find a data sheet
Not exactly. They have to be suitably specified for the task
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Old 23rd August 2012, 08:25 AM   #7
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Quote:
Originally Posted by GHeTToNoMiKS View Post
I'm looking to make (my first) bench top supply for testing random shenannegins , with NO extremely precise output expectations , I dont need a top of the line design --->>>>>> a very basic design (just something that works). I'm using a fairly small ct +-12v tranny, a 400v 10a full wave rectifier, two 4700 elna filter caps, lm317t, ka7912 and tons of bits and tips and odds and ends at my disposal AND I MEAN TONs!

Expectations
Adjustable: -12 - 24v
Dual outputs
Maybe a fixed 5v (i have maybe 200 7805s)

So far this is what I have

Tranny->BR->+12vto+of1ofTheFilterCaps->-12v of -to2ndcap->?????
HELP
what do you need -12V for, without +12V?
get a 2*20V trafo and use LM317 and 337 to get dual regulated voltage and a 7805 for fixed 5V(if we are talking mA and not 1-1.5A)
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