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Old 19th January 2013, 11:38 AM   #1
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Default wrong voltage 7905?

Hi,

I need some help figuring this out and YES I am a noob

The problem is that the voltage I am seeing after the 7905 is about -8.5V when it clearly should be -5V

I have tried several 7905's with the same results.

I do get +5V from the 7805.

and I get -12V and +12V directly after the rectifier

I have used the same 7805 and 7905 with a ATX power supply with +/-12V and that works great.

I have attached the schematic.


Any ideas would be great.

Thanks.
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File Type: jpg schematic.jpg (61.8 KB, 96 views)
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Old 19th January 2013, 11:40 AM   #2
AndrewT is offline AndrewT  Scotland
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minimum current stated in datasheet?

another unhelpful post?
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Old 19th January 2013, 12:10 PM   #3
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Quote:
Originally Posted by hyperion007 View Post
Hi,

I need some help figuring this out and YES I am a noob

The problem is that the voltage I am seeing after the 7905 is about -8.5V when it clearly should be -5V

I have tried several 7905's with the same results.

I do get +5V from the 7805.

and I get -12V and +12V directly after the rectifier

I have used the same 7805 and 7905 with a ATX power supply with +/-12V and that works great.

I have attached the schematic.


Any ideas would be great.

Thanks.
IIRC 7805 and 7905 have different pinouts. Did you verify?

jan
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Old 19th January 2013, 12:21 PM   #4
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Hi jan he seems to be aware of it .. That is if he has constructed to the posted scem..
Mark
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Old 19th January 2013, 12:26 PM   #5
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Hi,

Yes I am aware of the different pinouts and yes I did make the schematic.

please explain minimum current because I can't find it in the datasheet. The thing that puzzles me is that it worked with nothing connected except my multimeter when I use the ATX power supply.
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Old 19th January 2013, 12:27 PM   #6
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7905 TO220 case metal is Vin, is the case of the 7905 touching another part of circuit, or is it isolated ? Are you using TO92 TO220 or TO3 ?

Last edited by Chris Daly; 19th January 2013 at 12:33 PM.
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Old 19th January 2013, 01:08 PM   #7
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I am using TO-220

What is that about the case being Vin? See attachment.
And no, the 7905 is in a breadboard and no other component is touching the case.
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Old 19th January 2013, 01:13 PM   #8
Mooly is online now Mooly  United Kingdom
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Try two inverse connected diodes across the regulators and maybe two from each rail to ground. Sometimes different charge rates of the various caps can reverse bias a reg for a millisecond or so and cause a latch up.
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Old 19th January 2013, 01:18 PM   #9
impuls is offline impuls  Yugoslavia
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Quote:
Originally Posted by AndrewT View Post
minimum current stated in datasheet?
hyperion007, AndrewT gave You the answer. Read datasheet.
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Old 19th January 2013, 01:24 PM   #10
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
minimum current stated in datasheet?

another unhelpful post?
Not at all... I've come across "minimum currents" on regulators before.
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