ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Top Posters(30 Days)
Potseal 11
Admin 5
Recent Posts
Dimmable LED 2x4 lay in fixtures
by Potseal. 04/23/17 07:18 PM
Old decora style outlets
by Lostazhell. 04/22/17 07:59 PM
Permit Snafus...AHJs and Contractors Jump in
by gfretwell. 04/22/17 01:11 PM
How do you find a good employee?
by HotLine1. 04/22/17 10:44 AM
Electrode boilers question
by SteveFehr. 04/21/17 08:32 AM
New in the Gallery:
SE cable question
Popular Topics(Views)
234,650 Are you busy
169,334 Re: Forum
162,672 Need opinion
Who's Online Now
0 registered members (), 68 guests, and 11 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#138211 - 08/19/03 10:12 AM Switch mode power supply  
lyledunn  Offline
Joined: Jun 2002
Posts: 159
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?


lyle dunn

Test Equipment:
Large Selection of Test Equipment For Electrical, HVAC, Test & Measurement
Large Selection of Test Equipment For Electrical, HVAC, Test & Measurement

#138212 - 08/19/03 02:22 PM Re: Switch mode power supply  
geoff in UK  Offline
Joined: Dec 2002
Posts: 181
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.

#138213 - 08/19/03 06:17 PM Re: Switch mode power supply  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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:

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

#138214 - 08/21/03 06:20 AM Re: Switch mode power supply  
lyledunn  Offline
Joined: Jun 2002
Posts: 159
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!


lyle dunn

Member Spotlight
Atomic City, ID USA
Posts: 803
Joined: October 2004
Show All Member Profiles 

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.010s Queries: 14 (0.002s) Memory: 0.7583 MB (Peak: 0.8914 MB) Zlib enabled. Server Time: 2017-04-24 13:20:47 UTC