ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Switch mode power supply

I am thinking about many standard appliances used in the office and home environment that are fitted with switch mode power supply units and filtering circuits. Can anyone give me details of typical capacitor values and teir associated discharge resistors?

A computer PSU I have on hand at the moment uses two 330 microfarad caps in series, with a 30kohm resistor accross each.
I think that the resistors are there to help balance out any leakage and ensure the capacitors share the voltage, rather than to achieve a discharge.
A smaller PSU, from a fax machine, uses a 68 microfarad cap' and there is no obvious parallel resistor.

Lyle,
The filter capacitors can vary widely in value depending upon the operating frequency of the chopper, the design current, and so on.

Or did you mean the filter capacitors incorporated on the AC input to reduce interference being sent back down the mains?

You might find this link of interest. It contains many more links to articles on switched-mode supplies:
http://www.epanorama.net/links/psu_switching.html

[This message has been edited by pauluk (edited 08-19-2003).]

Many thanks guys. Paul there is enough here to keep me off the vino this week-end!
My concern is really in the safety testing of electrical equipment in-service, especially relating to insulation resistance testing using either the 500v applied voltage method or the touch current method.
I think it probably the filtering ccts that are causing problems. 500v can damage components but if a resistor is connected across capacitor to earth, a low reading would result. Similarly, with the touch current method, AC applied to a filter cct will show current flow and therefor suggest leakage.
I guess it mau be a case of consulting the manufacturer when in doubt!