A gradual short.

[RoboWanabe]

Hi

I need help on problem solving a circuit I have.

Everything looks fine and matches the schematic but when I power the circuit it the voltage gradually falls. I'm not sure how far it would fall because I am a bit wary on leaving it on just in case it causes damage.

Does anyone have any circuit analysis techniques, ive tried using a continuity tester on the supply sources and there is no direct short.

what could this be??

Thanks for your help

 

[kubeek]

I would start with measuring the power supply current. If it falls you are ok to keep it powered, if it rises you´d better turn it off soon. Care to post a schematic?

 

[panic mode]

if you don't have current limited PSU, try lowering voltage, at least until you find the problem. you can also connect some other load in series to limit the (power resistor or a light bulb)

 

[RoboWanabe]

Hi Cheers guys.

I managed to find the problem in the end, I had blown a capacitor which was shorting it. I had to bell out all the connection and found it in the end was a bit long winded but cheers for giving me stuff to test in future. Kubeek are you sure that the current wont be rising if the voltage is going down? i think i have a current limiting PSU so if its drawing to much current wold the unit then start turning down the voltage?

 

[kubeek]

I didnt say it WILL, but that it could happen. But really, I am now not quite sure which voltage you meant in the first post, I was thinking about voltage somewhere inside the circuit.

 

[RoboWanabe]

Hi again ive got a new problem.

Why when measuring resitance over a resistor would it read wrong value and slowly rise ?

 

[audioguru]

If the resistor is good then its resistance does not change.
You must measure its resistance when it is disconnected from a circuit. If it is connected to a capacitor then its resistance will be low when the capacitor is discharged but will rise as the ohm-meter charges the capacitor.
Your ohm-meter must also be good.

 

[RoboWanabe]

Cheers audioguru

thanks for your help. I didn't have a capacitor connected to this branch. It was just a basic potential divider circuit with two resistors in series from +12v to common. The reisitors were chose to give a 5v output. This is all that was on that branch? any ideas?

Thanks

 

[DragonForce]

Resistors can change value when they get warm.

If that resistor is ok, then something else in the circuit must be causing the problem. Lift out one leg of the resistor and then measure it. If the value still goes up then replace it. If not then there MUST be something shunting it.

A resistor divider across the psu is in series with a capacitor, assuming you're using a mains PSU I'd test the voltage output as well as that resistor.... You may have a leaky cap in the PSU

 

[RoboWanabe]

OK ill have a look that DragonForce and keep it in mind in the future.

I have another question. how will theses capacitors in diagram interact with each other? the reason i ask this is that the signal im getting in is abit distorted.

The signal comes from an analogue sensor whos voltage chagnes with position of sensor i.e 0 to 360 degrees. if i turn the sensor anti clockwise the output of the amplifier is ok but then if i turn to Clockwise the output of the sensor is distorted a little?

The amplifier im using is a differential one who is ment to be giveing me the velocity of the signal. I hope this is enough information, i cant put the schematic up due to this being a work project.

Thanks for your help guys

 

[audioguru]

The first capacitor to ground is shorting the signal which might cause oscillation, distortion or both. Why is it there?

 

[RoboWanabe]

Im not to sure i think its too clear the distortion from the sensor, the actual output of the sensor is not AC in a sense it dosent swing above and below ov, but it is a varying voltage from 0.6 to 5v? i think the idea was to reduce quantization noise from sensor to give a more smooth output.

 

[audioguru]

I do not know what is driving the capacitor to ground. Many opamps would oscillate when they try to drive a capacitor to ground.

 

[RoboWanabe]

Could you please describe what you mean by drive to ground? ive heard this term a lot and im not entirely sure what it is. is it the fact that im passing the ac signals to ground? i.e driving them to ground if so then the sensor is driving it to ground?

Thhsnks for your help

 

[audioguru]

"The first capacitor to ground" is connected to ground and maybe it is connected to the sensor, not to the output of an opamp. Then this capacitor is a filter.

 

[RoboWanabe]

Cheers audioguru I think my boss has made a mistake with the cap to ground. Do you rekon if I showed you an output from a differential op-amp tommrow when im at work, u could tell me the reason for distortion? Ige read a few of your posts and know this your forte as it were lol

 

[audioguru]

Do you rekon if I showed you an output from a differential op-amp tommrow when im at work, u could tell me the reason for distortion? Ige read a few of your posts and know this your forte as it were lol

It will rain here tomorrow so I should be home and I can help you. (I am retired)

 

[RoboWanabe]

Hi Audioguru
you lucky git being retierd, get to do what ever you want now

I'm not going to be able to show you the output I'm getting on the op-amp because my work doesn't have the facilities to save oscilloscope wave form (its an old SD card oscilloscope). But if you can help me with this question instead that would be great.

Ok so after we've done amplifying and rectifying this sensor signal I then need to read it with my pic MCU ADC. But the pic is on a different ground to the op-amp i.e. the op-amp has +- 12 v ether side and a common ground but the PIC is referenced to -12. My idea is to now use a subtractor op-amp to subtract 12v but i think op-amps struggle giving out voltages equal to there power rails. For e.g. if i do 12- 0v i should get -12 ( which is the potential my PIC is on) but instead im getting roughly -10 to -11v . basikly im trying to get an output for PIC to read between 0 and 3.3 v?

Any suggestions?

Thank you for your help again

 

[audioguru]

You should be able to make the grounds the same then there is no problem.
Most but not all opamps have inputs that do not work near the negative supply voltage and have outputs that do not go down that far.

 

[RoboWanabe]

Its on the negative becausebother parts of the circuit demand it, lie we have dome pwms for mosfets driving a solenoidbvalive on the minus rail, SPI speaking to a can network, and a set of thermistots and current sensors. I tjink thr idea is to limit thr amount of ground loops,? So most isvare on the negativ rail. Doed that make sense?