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Old 16th July 2010, 11:07 PM   #1
semi932 is offline semi932  United States
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Default 7915 regulator troubles

I'm trying to drop a + and - 31.5v supply to+ and - 15v supply.

Not unusual. But I'm using the circuit reccomended by the manufacturer(Fairchild) in the manual that I will attempt to post. Good size PDF.

Problem is I'm getting -23v on the neg side and 15v +side.
Checked board work and all is ok.

Does this problem ring any bells offhand for anyone?
Easy solution?

Inputs are identical + and -.

Can't upload file. Too large and I don't know how to seperate.
Can't draw either.
Circuit is .33uf 2.2uf--------regulators----------.01 and 1.uf and two 1n4001 diodes from neg rail to Pos in series.
Hope that helps

Thanks
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Old 16th July 2010, 11:25 PM   #2
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Hi, I do not full understand how you connected the 2 diodes but I think it is easy to solve (if the wrong voltage appears in unloaded condition). A classic error I think. Connect a small load to the negative reg like a LED with a series resistor.

It will probably be solved. The 79xx negative regs do need a small load due to their internal construction. That is why it is good practice to connect the "power on" LED to the negative supply.

Second possibility could be the latch up phenomenon. To solve that you need a diode over the I and O pins on each reg. Of course connected "backwards" i.e. not conducting.

Third could be a defective reg...
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Old 16th July 2010, 11:25 PM   #3
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Are you sure you have the 7915 connected correctly? It has a different pinout than the 78xx series,and I know it's snagged me a couple times.
Other things to check are bypassing (Seems to be okay,though you might consider bumping up the 2.2uf cap a bit,maybe the 1uf cap too.) and possibly a bad regulator IC.
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Old 17th July 2010, 05:02 AM   #4
semi932 is offline semi932  United States
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Quote:
Originally Posted by jean-paul View Post
Hi, I do not full understand how you connected the 2 diodes but I think it is easy to solve (if the wrong voltage appears in unloaded condition). A classic error I think. Connect a small load to the negative reg like a LED with a series resistor.

It will probably be solved. The 79xx negative regs do need a small load due to their internal construction. That is why it is good practice to connect the "power on" LED to the negative supply.

Second possibility could be the latch up phenomenon. To solve that you need a diode over the I and O pins on each reg. Of course connected "backwards" i.e. not conducting.

Third could be a defective reg...
Good heads up. I think the load is probably it.
I've used this before but being powered by batteries and was working fine.

I'll try it in the AM as it's late here, and I'll let results known.

Thanks
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Old 17th July 2010, 05:05 AM   #5
semi932 is offline semi932  United States
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Quote:
Originally Posted by DigitalJunkie View Post
Are you sure you have the 7915 connected correctly? It has a different pinout than the 78xx series,and I know it's snagged me a couple times.
Other things to check are bypassing (Seems to be okay,though you might consider bumping up the 2.2uf cap a bit,maybe the 1uf cap too.) and possibly a bad regulator IC.
Appreciate the reply.
I have made that error about hooking 7915 up wrong before so I know what you are saying
Hadn't considered uping cap value but may help.

I'm hoping the load issue is right on.

Will post resullts.

Take care
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Old 17th July 2010, 07:55 PM   #6
semi932 is offline semi932  United States
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I do appreciate being pointed to the right direction.
Works %150 under a load.

A great learning experience for me personally.

Thanks a lot
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Old 17th July 2010, 08:04 PM   #7
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Hi, I hope you mean it works 100% otherwise the voltage will again be too much

Get accustomed to adding the power LED + series resistor to the negative supply and you won't experience this another time.

The value of the resistor can be determined like this: (Ubat - Ufled)/Iled. So with a 12 V reg it will be (12V - 1.8V)/0.015=6800 Ohm. This is for standard LEDs with 15 mA current.
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Last edited by jean-paul; 17th July 2010 at 08:09 PM.
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Old 18th July 2010, 12:11 AM   #8
semi932 is offline semi932  United States
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100%. Was using 150 for emphasis.
Supplies are just about dead on. Good idea with the led though.
I'll use that regularly.

Thanks again
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