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Old 19th January 2013, 12:42 PM   #11
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Quote:
Originally Posted by hyperion007 View Post
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.
Vin is on the case of the 7905
See datasheet. www.jaycar.com.au/images_uploaded/LM7905.PDF
Pinout is Common (Gnd), In, Out, looking from the front. Carefully check your wiring, particularly gnd of 7905 as it appears like a component not in your diagram but on the board is lifting it higher. Your diagram shows 2 ground wires ?, perhaps you are providing paths for 7805 and 7905 yet they are joined on your diagram at the regulator pins. Can you see why two wires are unnecessary if already joined at regulators. Two wires to a common point is nice but circumvented if the regulators already share gnd.
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Old 19th January 2013, 12:49 PM   #12
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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.
The answer is simple, but i don't want to blame anyone.

If you can exclude any error in layout and wirig the IC is oscillating.
Check with a sope and try it with a resistive load and different cap
or caps on output.

For heavy loading the input caps are too small.
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Old 19th January 2013, 01:14 PM   #13
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but how come it works when using the ATX power supply? I have also read that the 7805 and 7905 do not need minimum current draw
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Old 19th January 2013, 01:20 PM   #14
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Quote:
Originally Posted by hyperion007 View Post
but how come it works when using the ATX power supply? I have also read that the 7805 and 7905 do not need minimum current draw
I don't know what this ATX is. So far you only describe your 7805, 7905 setup.

Please note that I don't talk about minimum current, but without load it may oscillate.
Your DVM can not read this. Again, did you check with your scope?

Please consider that you probably used a 7909 which in my eyes (without my
glasses) looks very similar to 7905 .
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Old 19th January 2013, 01:27 PM   #15
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ATX power supply, from a computer. it has +12V and -12V

but when I use the transformer the 7905 gives -8V

and no, I know it is 7905 ICs not 7909.

Sorry I don't have a scope.
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Old 19th January 2013, 01:35 PM   #16
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If you can show us a picture of your set-up
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Old 19th January 2013, 01:40 PM   #17
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You can achieve the minimum current either by fitting 470R resistor across each output and waste the little bit of power they will use or you can usefully employ it with 300R resistors in series with LEDs to also indicate power available.
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Old 19th January 2013, 01:42 PM   #18
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Quote:
Originally Posted by hyperion007 View Post
but how come it works when using the ATX power supply? I have also read that the 7805 and 7905 do not need minimum current draw
So then ask yourself: what exactly is the difference between the two situations?
Is there a difference in input/output caps? Difference in input voltage? Think!

The reg needs a cap both at the input and the output to be stable, close to the pins. Even 10uF is enough for stability.
You may also try to load it a bit to see if that makes a difference. Asking here is good, doing some research is better

jan
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Old 19th January 2013, 01:44 PM   #19
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does the ground connection go to the centre tap on the power transformer?
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Old 19th January 2013, 03:01 PM   #20
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I have tried to figure it out myself but didn't want to burn anything up so I thought I would ask.

I have tried connecting a 4.7K resistor and a LED between negative output from 7905 and GND to put some load on the 7905 but the only thing that happened was the voltage dropped from about -8V to -7.12V

I have attached a picture of the test setup.

I don't think I can put the caps any closer to the 7805/7905
Attached Images
File Type: jpg IMAG02742.jpg (630.6 KB, 54 views)
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