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Old 7th September 2003, 09:19 PM   #1
JoeBob is offline JoeBob  Canada
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Default 7815 not regulating.

I have a 7815 fed 30V DC and it puts out 20V, not 15. Other parts do this, not just the first one I tried, so it's obviously something I am doing wrong. I have 2000uF before the regulator and 1000uF after. Is the capacitance too much after the regulator, or can I get away with that and the problem is something else altogether?
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Old 7th September 2003, 09:26 PM   #2
dhaen is offline dhaen  Europe
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Hi,

Have you got any load? With no resistive load at all, the voltage will float up.

Cheers,
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Old 7th September 2003, 09:33 PM   #3
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Do you have stable output voltage when you load?

Do you have stable output voltage if you variate the input voltage.

Have you measured the voltage between the gnd pin and out and gnd and Uin? The GND pin isn't disconnected?
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Old 7th September 2003, 09:49 PM   #4
JoeBob is offline JoeBob  Canada
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I have it feeding a OPA627 and I measured the voltage from the output of the regulator (which is tied to the +v of the opamp), oh, and this also aplies to the 7915, but I am gathering it was the same problem.

The GND pin is indeed connected to ground. And I did think it could have been the lack of loading, but I know not how much I have to load it to work. I don't want to fry my opamp with too much voltage if running it into a load isn't enough to bring down the voltage.

Could a simple resistive load connected across the capacitor after the regulator suffice? For the load from the opamp is going to be rather low. Or once I run the opamp into a load and it draws a little current you think I should be fine?

And I haven't tried to vary the input voltage
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Old 7th September 2003, 10:05 PM   #5
dhaen is offline dhaen  Europe
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Hi,

feeding an opamp should be suffficient load to stabilize the output voltage. There must be a mistake somewhere.
Have you taken into account that the 78 and 79 series have different pin-outs?

Cheers,
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Old 7th September 2003, 10:07 PM   #6
dhaen is offline dhaen  Europe
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BTW It is not a good idea to add large amouts of capacitance to the output.
Check the data sheet for the reccomendations.
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Old 7th September 2003, 10:09 PM   #7
JoeBob is offline JoeBob  Canada
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Ya, I checked and rechecked the pinout over and over again. I have them wired up correctly. Well, I'll keep pluging at it, I guess the load isn't the problem...
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Old 7th September 2003, 10:15 PM   #8
dhaen is offline dhaen  Europe
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Hi,

Well I'd take off the load and just put in 2 1K resistors (or similar). Use a 100n between the input and ground pin - close to the chip, and no cap on the output until you've prooved it.

Cheers,
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Old 7th September 2003, 10:17 PM   #9
dhaen is offline dhaen  Europe
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PS
The 78 is in the +ve rail
The 79 is in the -ve rail
And they are XX15?
?
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Old 7th September 2003, 10:21 PM   #10
JoeBob is offline JoeBob  Canada
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Yup, I didn't mix anything up. I'll try the resistor load...
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