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Old 31st January 2004, 02:03 PM   #1
pete.a is offline pete.a  United Kingdom
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Default fixed reg lm7915

im having prob's with a diy power supply. Its a split rail 15 0 15 well it should be. Input to the 7915 is 30v output is 21.7 v
Iv'e even used the suggested circuit as per nat sems web site, as the regs are a few inches away from the filter caps.
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Old 31st January 2004, 06:27 PM   #2
dhaen is offline dhaen  Europe
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Make sure:

It's a 79
That the pins are correct (different from 78!)
It's got a ground
There's a small cap across the oputput
It's got a load (unusual, but known with some regs)

And it'll work
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Old 1st February 2004, 03:54 AM   #3
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Quote:
Originally posted by dhaen
Make sure:
That the pins are correct (different from 78!)

Yes, centre pin is output on these, not ground.

Cheers
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Old 1st February 2004, 05:29 AM   #4
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if the connections are good then check input on the neg. reg.....could be a bad cap or the reg is kiboshed....also try preloading the reg. with a 5k resistor for testing....it will also drain the cap between testing


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Old 1st February 2004, 07:45 AM   #5
pete.a is offline pete.a  United Kingdom
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Default faulty regs

Hi guy's

Iv'e tried the ideas mentioned and it still ain't working.
Could it be due to the main filter caps being 10'000uf @40v?
The say upper limit for the reg is 35v
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Old 1st February 2004, 09:44 AM   #6
pete.a is offline pete.a  United Kingdom
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Default regvprobs

Now explain this ?
the pos reg works fine 14.89v, changed the neg reg twice and still the same happens..21.7v
Im now thinking that it could be the transformer/rectifier, as ive got a dual secondary and iv'e tied two legs to ground.
Just so you know im a qualified electrician, so you can leave out the basic's if you get my drift.
What i may do but three week old Harry is taking our my sunday,
is to use two rectifer's instead of one.
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Old 1st February 2004, 11:06 AM   #7
Jax is offline Jax  Sweden
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The 40V rating of the capacitor has nothing with this to do but I'm not explaining why since you told us to leave out the basics.

If you are using the TO220 version of the regulators, make sure the 7915 mounting tab is not grounded, it's connected to the output (middle pin).

Other things to try, connect small 1 to 10 uF capacitors as close as possible to the input and output of the 7915. Negative regulators are more prone to oscillate.
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Old 1st February 2004, 12:22 PM   #8
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Quote:
Yes, centre pin is output on these, not ground.
Quote:
If you are using the TO220 version of the regulators, make sure the 7915 mounting tab is not grounded, it's connected to the output (middle pin).
I attached a picture that shows the middlepin as being the input. Pin 1 is ground and pin 3 is output.
Attached Images
File Type: jpg negative power regulator.jpg (27.6 KB, 205 views)
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Old 1st February 2004, 01:36 PM   #9
Jax is offline Jax  Sweden
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Ops!

My memory isn't what it used to be, middle pin is od course the input. Sorry for the confusion
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Old 1st February 2004, 04:20 PM   #10
pete.a is offline pete.a  United Kingdom
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Default faulty tx

Thanks for the images, iv'e got them written down in one of my books anyway.


Just split the tx secondaries and used two rectifers, one pos and one neg. Now getting 40v per rail under no load condition , bear in mind that its an 18v tx and one would expect around 22v before the rectifiers (output *1.414) if my memory is correct ?
And around what after the rectifiers ? 40v seems abit to high !!
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