Hi
I was wondering if someone could offer a little advice?
My office uses a large TFT display for presentations. The monitor powers up but there is only a very faint imagine displayed on the screen. I have removed the switch mode power supply from the case and I think the main reservoir cap may have gone. I don't have a capacitance meter but I have attached a DMM set to ohms, and it doesn't appear to charge. The top of the capacitor hasn't bulged but the plastic top has cracked, probably do to heat.
Is there anything else I can try?
Thanks
Richard
I was wondering if someone could offer a little advice?
My office uses a large TFT display for presentations. The monitor powers up but there is only a very faint imagine displayed on the screen. I have removed the switch mode power supply from the case and I think the main reservoir cap may have gone. I don't have a capacitance meter but I have attached a DMM set to ohms, and it doesn't appear to charge. The top of the capacitor hasn't bulged but the plastic top has cracked, probably do to heat.
Is there anything else I can try?
Thanks
Richard
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Starting from a discharged state, once your leads are attached, the ohmmeter should increase from zero ohms to infinity in a certain period of time, which would be a ballpark estimate of capacitance once you know the characteristic of your meter. If it doesn't behave like this at all, chances are it's smoked.
However, an ohmmeter doesn't even tickle a 450V capacitor, so you're better off, now that it's been removed, of just replacing it and be done. This capacitor runs between $3 and $5.
However, an ohmmeter doesn't even tickle a 450V capacitor, so you're better off, now that it's been removed, of just replacing it and be done. This capacitor runs between $3 and $5.
I hooked up a light bulb tester after changing the cap and the bulb glows brightly, and flickers in time with the switching power supply.
I didn't use the bulb tester before changing the cap so I am unsure if the bulb would have flashed in the same way.
Has anyone connected a light bulb tester to a SMPS, if so what are its characteristics? I'm use to the bulb glowing for a second due to inrush current, then going out completely if no short is found within the circuit.
Regards
Richard
I didn't use the bulb tester before changing the cap so I am unsure if the bulb would have flashed in the same way.
Has anyone connected a light bulb tester to a SMPS, if so what are its characteristics? I'm use to the bulb glowing for a second due to inrush current, then going out completely if no short is found within the circuit.
Regards
Richard
Is there any light coming from the screen? If not, it could be the backlighting inverter. Those fail. Try googling the model number and hopefully you're not the first to deal with this. If there's no help, check for a fuse near the high voltage inverters, or pull the driver transformers and check them for shorts or opens. It could also be due to bad power supply caps that feed the backlighting. Bulging tops usually means bad.
Hi Dangus
Thanks for your reply!
Yes the screen is black and I can just about make out a faint picture. I inspected all of the components and couldn't see anything untoward. I change the SMPS reservoir cap because the plastic top has started to break up and I thought it might not be supplying enough charge.
I don't have any experience with SMPS and was a little unsure whether to power the SMPS without a light bulb tester, just in case there's a short.
Do you have a bulb tester and SMPS handy?
For power supply capacitors, just charge the capacitor, leave a voltmeter connected to it, and connect a resistor across it (value such that the expected time constant is at least a few seconds) while timing for it to discharge to about 37% of the original voltage. Even some of the largest capacitors can be tested using this method.
For testing ESR, connect it through a resistor to a signal source (such as an audio amplifier) and use a frequency high enough that capacitive reactance is negligible. Then measure voltage across the resistor to find current and measure voltage across the capacitor to calculate ESR.
Thanks Mooly, that's interesting
I've been building an amp this past week but I'll have another look at the SMPS this weekend. I am just a little concerned with regards to the bulb tester glowing brightly, and pulsing in time with the switching supply.
Richard
Many SMPSU's are OK with a bulb but not all.
They need the full voltage applied correctly and quickly in order to self start. Some can latch (sometimes destructively) if the voltage is ramped up slowly.
I can't say either way on that without first hand experience of what you are working on.
A lot will depend on the current drawn from the rails on it.
Also many don't run properly with no load either.
They need the full voltage applied correctly and quickly in order to self start. Some can latch (sometimes destructively) if the voltage is ramped up slowly.
I can't say either way on that without first hand experience of what you are working on.
A lot will depend on the current drawn from the rails on it.
Also many don't run properly with no load either.
Your welcome 
Will just say that the majority of PSU problems (SMPSU) all come from capacitor failure... they are very highly stressed.
Unless you are used to working on them the best option is often just to replace ALL electroylitics... even the small ones as these too often fail.
You really need a fully isolated workshop so you can connect a scope across them and see what the ripple voltages are but PLEASE don't attempt that as the primary side is directly connected to the mains with no isolation.
Will just say that the majority of PSU problems (SMPSU) all come from capacitor failure... they are very highly stressed.
Unless you are used to working on them the best option is often just to replace ALL electroylitics... even the small ones as these too often fail.
You really need a fully isolated workshop so you can connect a scope across them and see what the ripple voltages are but PLEASE don't attempt that as the primary side is directly connected to the mains with no isolation.
Your welcome
Will just say that the majority of PSU problems (SMPSU) all come from capacitor failure... they are very highly stressed.
Unless you are used to working on them the best option is often just to replace ALL electroylitics... even the small ones as these too often fail.
You really need a fully isolated workshop so you can connect a scope across them and see what the ripple voltages are but PLEASE don't attempt that as the primary side is directly connected to the mains with no isolation.
I purchased some of the smaller caps, 25uf from memory at the same time as 400v reservoir cap. When I connected the DMM (ohms) two or three of them didn't increase in value.
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