EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: JakeC on October 20, 2013, 10:22:31 am
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Hi,
I am attempting to repair our old TV. When you turn it on the backlight only stays on for about a second. After some research I believe it is one of the caps, I have read about using the ohms range to charge the cap then measuring it with volts DC. Is this the best way?
Thanks
Jake
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Hi,
at first I would try a visual inspection, normally the Caps are visually damaged.
Charging the caps (any battery does the job, you don't have to use the Ohm-Range) and discharging them via the input-impedance of your DMM would do, then you have to calculate your time-constant (tau=RC) and so on... They have to be desoldered first!
Faulty caps normally have a big difference in Capacity (nF instead of µF), so this method may be accurate enough, but this is kinda annoying. Maybe it's time for a new DMM, even the cheaper ones have a C-Measurement build in.
Another type of diagnostic is the ESR, I'm not sure if the new DMMs can measure this (mine can't).
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In general ESR is a better diagnostic of failed caps in switching power supplies than capacitance. I don't know of any DMM that can measure ESR, you would generally look at LCR meters or dedicated ESR meters (e.g. the Bob Parker design). I don't see how you'd improvise an ESR measurement with just a DMM, in general you'd use a function generator and oscilloscope for that. Forum user krypton2035 has collected a bunch of DIY ESR meter schematics (http://kripton2035.free.fr/esr-repository.html) on his website.