Cap voltage without load

[notallbad]

Two Axial polarised electrolytic caps, 2700uf and 220uf 40v in parallel across a 12v DC supply (using a 12v car battery charger as supply voltage) with no load connected.

What would be the expected P.D voltage across the terminals be?

Trying to determine whether the caps are good or not.

[budm]

Re: Cap voltage without load

Devices in parallel circuit will see the same applied Voltage, so the Voltage across the terminals of BOTH caps will be 12VDC. You should draw out the diagram so you will see how it works.

http://www.bbc.co.uk/schools/gcsebit...uitsrev1.shtml
https://learn.sparkfun.com/tutorials...allel-circuits

[notallbad]

Re: Cap voltage without load

Thanks, shall take a look at those links.

DMM across terminals reads around 18v which does not make sense to me!

[budm]

Re: Cap voltage without load

What kind of 12VDC power supply? Linear unregulated power supply (Transformer, rectifiers, caps)? The no load Voltage of those type of power supply will be higher until fully load and the input Voltage applied is the same as printed on the unit.
For example, the label on the power supply shows 12VDC 1A, 120VAC input, it will put out 12V when the load is drawing 1A, but if the load is <1A then the Voltage will not be 12V but will be higher.

[R_J]

Re: Cap voltage without load

A car battery needs a min of 13.8 volts to start charging, and usually the charging voltage is around 14.2 volts. +18 unloaded sounds about right

[notallbad]

Re: Cap voltage without load

Thanks Budm, used an alternative 12v supply.

[notallbad]

Re: Cap voltage without load

Thanks R_J, that also helps.

[Th3_uN1Qu3]

Re: Cap voltage without load

Thanks Budm, used an alternative 12v supply.

I assume this was a mains-transformer powered supply, if you say alternative. Learn about the peak voltage of a sinewave, which is 1.414 times its RMS value (which is what's printed on your transformer). So 230v mains will have 325v peak, and a transformer outputting 12v RMS will have a peak voltage of 16.9v

With no load, a rectifier will charge a capacitor to the peak voltage of the AC supply. Under load the voltage drops and some AC will make its way on the output, this is called ripple. Ripple depends on the current being drawn, the power capability of the transformer and the size of the capacitor bank. Bigger capacitor, less ripple, higher average voltage. With ideal transformer, rectifiers and capacitor bank you would always have a DC level of the peak AC input, this does not happen in real life, of course. But with zero load, the voltage is extremely close to the peak.

12v times 1.414 is ~17v, but also, a transformer is rated for a certain voltage output at a certain load, with light or no load voltage will be higher. That explains your 18 volts.

[notallbad]

Re: Cap voltage without load

Thankyou for taking the time to explain, it is appreciated.

I am trying to repair an electronic (sensorless) power steering pump.
It was continuously running after ignition switched off and drained the car battery. Battery good-around 11.8v(now recharged, alternator good- outputting around 13.8v.
Replaced 2 of the 4 MOSFETs BUK7608-55A (one showed signs of 'blowing', the other shorted between gate,drain and source.
Now it just keeps stopping and starting, 3-4 sec intervals.
The sensorless i.c which drives the motor (Elmos 10203C 28-pin) is presumably the culprit for shutting down the motor but to be honest I am not really sure what to check now. other than a few smd caps and resistors there isn't anything to test.
The pump takes battery feed and grounds to chassis and alternator output which acts as an 'on/off switch', pump only runs when the car is running.

If anyone can help please let me know though I know this site isn't really for car repairs so understand if no replies of advice.

[notallbad]

Re: Cap voltage without load

Photo of the pcb (taken from the web).