# How to measure how much output capacitance a big power supply has

#### MikeJacobs

Joined Dec 7, 2019
226
I would like to measure how much output capacitance a benchtop power supply has.
I believe it to be a linear power supply given its size and weight.

How can i measure this?
Thanks

#### Ian0

Joined Aug 7, 2020
9,497
Can't you just take the cover off and look?

#### Dodgydave

Joined Jun 22, 2012
11,236
Have a look inside it..

#### MikeJacobs

Joined Dec 7, 2019
226
i cant take it apart its not mine.
I have tried to scope it. If the output capacitance was still charged when its off, wouldnt i be able to read a voltage at the output terminals with the unit off with a high impedance meter, aka nothing pulling it down?

Whenever i connect this power supply to a piece of equipment i have, with the PSU off, it blows the fuse of my down stream hardware. The downstream hardware has some input capacitance behind a fuse. The only thing i can think of is the output caps of the psu are staying charged and when i connect it up, they discharge rapidly.

Any ideas?

#### Ian0

Joined Aug 7, 2020
9,497

#### Hymie

Joined Mar 30, 2018
1,272
It might be that this PSU has sufficiently high output current capability to blow your hardware fuse, as a result of charging the downstream capacitors - whereas another PSU might be limiting the peak current to a value which avoids blowing the fuse.

In a well designed PSU, in the off state the output voltage should be zero – I suggest you check this with a multimeter.

#### crutschow

Joined Mar 14, 2008
34,044
The only thing i can think of is the output caps of the psu are staying charged
If so, then connect a resistor across the output (e.g. 1kΩ) to discharge the output capacitance when off.

#### ronsimpson

Joined Oct 7, 2019
2,893
You could make/get a current source and charge the output capacitors to 1V and see how long that takes. There are parts on the output (resistor and maybe silicon) that will give some errors, that is why I was thinking of charging to a small voltage not full voltage. Some of my power supplies have a current mode where I can charge large caps at 100mA or 10mA.

You could use a resistor to charge or discharge the cap and get some idea. Most power supplies have a resistor on the output that will cause errors in your experiment.

#### WBahn

Joined Mar 31, 2012
29,857
The simplest test would be to just put a multimeter on the output and turn the power supply off and watch how quickly the voltage changes.

Use that information to guide further investigation, as warranted.

#### Dodgydave

Joined Jun 22, 2012
11,236
i cant take it apart its not mine.
I have tried to scope it. If the output capacitance was still charged when its off, wouldnt i be able to read a voltage at the output terminals with the unit off with a high impedance meter, aka nothing pulling it down?

Whenever i connect this power supply to a piece of equipment i have, with the PSU off, it blows the fuse of my down stream hardware. The downstream hardware has some input capacitance behind a fuse. The only thing i can think of is the output caps of the psu are staying charged and when i connect it up, they discharge rapidly.

Any ideas?
So connect the psu when it's powered on , does it blow the fuses then?

#### MisterBill2

Joined Jan 23, 2018
17,790
Why do you want to know the internal capacitance of the power supply? The problem leading to the fuse failure is excessive inrush current and an incorrectly sized fuse, given it's position in the circuit.
So now a question: Does this power supply ever operate the " piece of equipment i have, " ?? Next question: how much current does this piece of equipment require, and how much current is the fuse rated at? Third question: Does the power supply you are trying to use have an adjustable output voltage, or an adjustable current limit? The very best approach would be to set the current limit to the current that " a piece of equipment " requires, less than the fuse rating. Then, with the power supply on, connect that piece of equipment.
If the power supply is not adjustable, then make that initial connection with a series resistor to limit the current flowing into the capacitors.
And now a suggestion to do before anything else: Verify that the power supply voltage is what that " piece of equipment " should be having applied. There might possibly be an error in the voltage setting.