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Old 23rd October 2005, 04:06 AM   #1
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Default 7805 trouble

I've got a 7805 regulator to drive a character LCD and when I plug it in the 12v source I want, it just heats a lot and the LCD won't start working. There's a 470F cap @ the input and a 1000pF at the output. The 12v source is a computer(The fans are slowing down when this happens).
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Old 23rd October 2005, 04:10 AM   #2
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If the fans slow down it must be drawing far too much current. Either you have connected it wrong, or it is faulty. Check the pinout in the datasheet again. Have you measured the voltage at the output?
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Old 23rd October 2005, 04:14 AM   #3
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1.7v

Under no load there's no problem(5v) and the LCD works OK directly from 5v. (It was before I done this, I should try again)


EDIT: Oops I think I plugged it backwards.
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Old 23rd October 2005, 04:26 AM   #4
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And the slowing down fans occur only when the LCD is connected, not when you run the reg. without load? Sounds like it is current limiting, but that would mean that the LCD is faulty, or that you have connected it wrong in this case, but not when testing it separately. Could still be a faulty reg. too, I suppose, that seem to work under no load, but quickly drops when loaded.

Try loading the regulator with resistors and see if it can keep up the voltage at the output for reasonable load currents.

BTW, What type of 7805 are you using? The 78L05 for instance can only output 100 mA. How much is your LCD suppoed to draw?

Edit:

Just saw your edit.
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Old 23rd October 2005, 02:38 PM   #5
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Yup, I tried to invert 5v and GND and it works!
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