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Old 06-23-2013
garegin garegin is offline
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Default testing inverters with a multimeter

yes, yes. I know you can test the RF with a scope, noninvasively. However I want to straight up connect a multimeter to the output (AC leads) and read the output. I hear that you need a special multimeter for that. which one do I need?
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Old 06-23-2013
mariush mariush is offline
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If you mean power inverters (the ones converting 12-24v to 110-240v), then you need a multimeter with TrueRMS function/mode.
A regular multimeter will tell you the correct AC voltage only if the inverter outputs a pure sine wave AC. Most don't, they're simulated sinewave.. which confused multimeters without TrueRMS mode and makes them report an incorrect AC voltage.

If you mean LCD inverters, you can't really test them with multimeters, as the output is 700-1500 volts at a high frequency.
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Old 06-23-2013
garegin garegin is offline
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Quote:
If you mean LCD inverters, you can't really test them with multimeters, as the output is 700-1500 volts at a high frequency.
so how they do it in the industry? do they have some special multimeters for that or this is a brick wall?
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Old 06-23-2013
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I'm afraid you're SOL without a scope.
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Old 06-24-2013
mariush mariush is offline
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Quote:
Originally Posted by garegin View Post
so how they do it in the industry? do they have some special multimeters for that or this is a brick wall?
If you want to see that the controller works, it's often enough to get neon bulb very close to the connector that goes to the cfl tube.
Another cheap tester is to simply get a CFL bulb, open it up and remove the electronics and connect the wires going to the cfl tube to the connector.. if the controller works, it will light up the lightbulb. Just don't use your fingers on those uninsulated wires .. you have up to 1500v there, and even though there's only a handful of mA, it can still kill you.

Otherwise, when servicing monitors, the lcd monitor is powered from an isolation transformer and then an oscilloscope is used on the outputs of the pwm controller/driver which are low voltage to detect the pwm signal - the signal goes from there to other ics which send pulses of power to the transformers which raise the 10-30v up to 750-1500 volts.
Also, a service guy monitors other pins of the controller/driver to see if the controller raises an error flag.

For example, when lamps get old, they draw more current, more than what the transformers on the pcb could provide without overheating themselves so to protect the transformers the controller/driver may kill the backlight and raise an error flag on a pin.
Same if they don't detect the lamps starting within a few seconds - it can happen when the wires going to the CFL tubes get loose/desoldered to due lots of heat cycles.
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