Acceptable Capacitor Range for Monitor Power Supply

Thread Starter

SpiderSpartanju

Joined Apr 10, 2009
82
I had a LCD monitor die on me earlier this week. It turns on and displays for 2 seconds then goes black. Usually this due to failing capacitors on the power supply. In the past I've been lucky enough to be able to see the bad, bulging caps. This time nothing looks out of place. So I got a Honeytek A6013L capacitor meter and started taking readings. All the capacitors give me measurements, but many are not exactly as marked. I have 470uF reading 450-455uF, 1000uF reading 930-950uF and 120uF reading 110uF. What kind of range should I be looking for? The only reference I've been able to find is for AC starting caps should be within -5% to +10%. Does the same apply to all capacitors? Thanks for any tips.
 

MrChips

Joined Oct 2, 2009
26,807
There are hundreds or thousands of components in a modern electronic device of which any component can go bad.

High value electrolytic capacitors, >100μF are commonly used for one purpose, and that is to hold charge. These commonly have manufactured specs of -5% to +10% in value. For practical purposes, ±10% or even ±20% are acceptable. Granted, modern switching supply applications demand tighter specifications and low ESR.

Yes, capacitors have shorter life expectancy and higher failure rates than other electronic components. I do not generally as a rule replace capacitors unless there is evidence that it needs replacing. Some people go ahead and blindly replace all electrolytic capacitors. To me this is a shot in the dark. I prefer to take a more intelligent approach when trouble shooting faulty units.
 

MisterBill2

Joined Jan 23, 2018
12,745
There are hundreds or thousands of components in a modern electronic device of which any component can go bad.

High value electrolytic capacitors, >100μF are commonly used for one purpose, and that is to hold charge. These commonly have manufactured specs of -5% to +10% in value. For practical purposes, ±10% or even ±20% are acceptable. Granted, modern switching supply applications demand tighter specifications and low ESR.

Yes, capacitors have shorter life expectancy and higher failure rates than other electronic components. I do not generally as a rule replace capacitors unless there is evidence that it needs replacing. Some people go ahead and blindly replace all electrolytic capacitors. To me this is a shot in the dark. I prefer to take a more intelligent approach when trouble shooting faulty units.
OK, what constitutes "evidence that it needs replacing.?? not every failed cap bulges enough to show. So just what do YOU look for to show a failed cap???
 

Thread Starter

SpiderSpartanju

Joined Apr 10, 2009
82
I ended up finding bad CFL tubes in my monitor, actually melted one end. Not sure how it was working at all. Based on Mr. Chips response is be looking for anything outside of +-10%.
 

MrChips

Joined Oct 2, 2009
26,807
I am sorry that I have to admit that in my experience there is no magic bullet to identify a bad capacitor in a circuit with 2 or 100 capacitors. I can only describe a few examples of what I have experienced.

Someone had a musical device whose symptom was "motor-boating". This is the sound that a device makes like a outboard motor going "putt-putt-putt...". I instantly recognized this as a bad capacitor in the power supply circuit. I replaced the capacitor and the problem was resolved.

In a much more difficult trouble-shooting situation, a SMPS would not start up. The only way I was able to pin-point the fault was to examine all the signals on the oscilloscope after having reversed engineer the circuitry. The problem resided in the feedback control circuitry where a certain DC voltage was expected. Replacing a suspect 47μF/50V capacitor did not solve the problem. Replaced the capacitor with 100μF and the problem went away. Go figure.
 

MisterBill2

Joined Jan 23, 2018
12,745
Switching power supplies are one of the most challenging circuits to diagnose because all of the parts are important, and quite a few are highly stressed. So really it is a power oscillator with a DC output that is closely regulated.
In LCD monitors the backlight driver is usually some sort of switcher, and for various reasons the big capacitors have developed a reputation for being the parts that fail.
 
Top