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voltage drop at power source -what is happening?

C

catfarm

Jan 1, 1970
0
I have a circuit that is supposed to run on 9V. When I connect my 9V (well
regulated) power supply to the plug Im only reading ~2.2 V. What would cause
this? Is the implication that there is a short to ground somewhere?
 
L

Lord Garth

Jan 1, 1970
0
catfarm said:
I have a circuit that is supposed to run on 9V. When I connect my 9V (well
regulated) power supply to the plug Im only reading ~2.2 V. What would cause
this? Is the implication that there is a short to ground somewhere?

You're either not 'well regulated' or you're attempting to pull more current
than
the regulator can supply.

How about telling us what the device is that you are powering and the specs
of
the power supply.
 
B

Ban

Jan 1, 1970
0
catfarm said:
I have a circuit that is supposed to run on 9V. When I connect my 9V
(well regulated) power supply to the plug Im only reading ~2.2 V.
What would cause this? Is the implication that there is a short to
ground somewhere?

If the short is to gnd, you would be reading 0V. So probably an IC got its
supply pins reversed or a resistor has a wrong value. When you reversed the
supply before you might have killed your circuit.
Try to find out if the circuit works with another supply. When you have a
multimeter with a DC current range of 2A or 20A you can also measure the
current consumption and choose a wall-wart accordingly.
 
C

catfarm

Jan 1, 1970
0
Its an audio modulator. It uses 2x quad opamps (TL074) and some LED/CDs
pairs to modify the sound.

Interestingly enough when I pull the chips I read a nice 9V at the plug.
Putting in one chip drops the voltage to 3.7V. Bad chip? They aren't
getting warm when the circuit is running.

Im using a 9V source that comes from a 7809 and should be able to put out
500mA.
 
C

Chris

Jan 1, 1970
0
catfarm said:
Its an audio modulator. It uses 2x quad opamps (TL074) and some LED/CDs
pairs to modify the sound.

Interestingly enough when I pull the chips I read a nice 9V at the plug.
Putting in one chip drops the voltage to 3.7V. Bad chip? They aren't
getting warm when the circuit is running.

Im using a 9V source that comes from a 7809 and should be able to put out
500mA.

One thing you might try is removing the op amp and putting a resistor
across the 7809 output which simulates the load. Start with a 100 ohm,
1 watt or greater resistor and see what happens. Also, thake a look at
what's happening at the input voltage of the 7809.

I'm assuming you've got the pinout of the 7809 right. That's another
common newbie problem. If you've got a TO-220 package (square black
plastic with metal tab), holding the plastic side forward and pins
down, the pinout from left to right is

INPUT -- GND -- OUTPUT

Good luck
Chris


Good luck
Chris
 
L

Lord Garth

Jan 1, 1970
0
catfarm said:
Its an audio modulator. It uses 2x quad opamps (TL074) and some LED/CDs
pairs to modify the sound.

Interestingly enough when I pull the chips I read a nice 9V at the plug.
Putting in one chip drops the voltage to 3.7V. Bad chip? They aren't
getting warm when the circuit is running.

Im using a 9V source that comes from a 7809 and should be able to put out
500mA.

The LM7809 is good to 1 amp but you will need at least 11 volt input to the
regulator.
You can post a schematic on alt.binaries.schematics.electronic, that would
be
helpful. Off hand, it sounds like a power supply problem but you have no
details.

You might try connecting your power supply to a resistive load to see if it
hangs
in there. 9v/.5A= 18 ohms would load your supply half way and let you test
whether the voltage stays correct. The power dissipated in the load would
be
9v * .5 A = 4.5 Watts so use an appropriately rated resistor.
 
C

catfarm

Jan 1, 1970
0
Lord Garth said:
The LM7809 is good to 1 amp but you will need at least 11 volt input to
the
regulator.
You can post a schematic on alt.binaries.schematics.electronic, that would
be
helpful. Off hand, it sounds like a power supply problem but you have no
details.

You might try connecting your power supply to a resistive load to see if
it
hangs
in there. 9v/.5A= 18 ohms would load your supply half way and let you
test
whether the voltage stays correct. The power dissipated in the load would
be
9v * .5 A = 4.5 Watts so use an appropriately rated resistor.


Im not worried about the power supply. I use it for a variety of other
applications on a regular basis and it performs well. Its the circuit thats
problematic. Somewhere there is lots of power being dumped. I can't seem to
find out where though. It seems odd that the problem occurs when the opamp
chip(s) are inplace. I might try substituting in another batch of chips to
see what happens.
 
J

Jamie

Jan 1, 1970
0
catfarm said:
Im not worried about the power supply. I use it for a variety of other
applications on a regular basis and it performs well. Its the circuit thats
problematic. Somewhere there is lots of power being dumped. I can't seem to
find out where though. It seems odd that the problem occurs when the opamp
chip(s) are inplace. I might try substituting in another batch of chips to
see what happens.
did you scope it for oscillation?
 
M

Michael Redmann

Jan 1, 1970
0
Im not worried about the power supply. I use it for a variety of
otherseem to

If that amount of power were dumped something must get hot. It's very
unlikely that 7809 deliveres more current than possible and therefore
output voltage is reduced.
did you scope it for oscillation?

Maybe it is your specific load that makes the power supply oscillating.
Other loads could be more harmless. Do you have correct capacitors
across input and output if 7809 regulator?

Regards
 
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