EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: joa.bulderby on February 16, 2015, 10:04:18 pm
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Hello all.
Does anyone know what the typical voltage output is from the transformer inside an lcd monitor? By the wires coming out it looks like it is powering the backlight. I have measured the input voltage on the board to 10 volts dc, but my multimeter cannot measure the output voltage, the multimeter has an upper limit of 500 volts dc.
The writing on the side of the transformer says: EFD20-0101 1430
I have attached 2 photos of the board
Thanks in advance
/Joachim
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Normal running voltage is a few hundred volts, but at startup, before the tube strikes, it outputs a couple of kV using resonance of the secondary with the output cap to strike the tube.
Not a good idea to connect a DMM to.
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Normal running voltage is a few hundred volts, but at startup, before the tube strikes, it outputs a couple of kV using resonance of the secondary with the output cap to strike the tube.
Not a good idea to connect a DMM to.
I have hooked up meters to these, meters complained but still work. Still not worth risking you meter.
One wonders about using these to make the magic smoke come out of components =-)
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My multimeter did make a strange noise when trying to measure it, which makes sense if the output is 4 times the limit |O I might have ruined it, I'll have to take it apart to see the damage.
Thanks for the super quick responses :)
BTW Mike, I am a big fan of your youtube channel, keep posting your videos.
/Joachim
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Th few kv will get clamped by the MOV in any decent meter, possibly glitching the MCU, but there is a risk that it will start a tracking process, that will make the PCB conductive enough for the main supply to burn.
These things can pull a good few mm of arc, though laptop ones tend to have more protection against overload so are often less fun than the older, simpler ones.
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I have at least one inverter laying round from an old scanner, I should have a play when I get time, it did not have the energy to make a Jacobs ladder :(
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I just saw this video, any other ideas for testing?
The inverter should be running about 35KHz, what he was picking up on his hands and near the laptop was the mains 60Hz and the case or metal frame around the screen and his body were acting as antennas (humans don't oscillate at 60Hz neither)
If you bring your probe near a metallic desk it will pick up the mains frequency as well. So that's not a good probing solution.
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DMMs and HV probes will have frequency limits - these will be listed in the manual.
The HV starting voltage will have quite a high source impedance, and stray capacitance in the probe & DMM will attenuate it significantly - PSU output caps are typically low tens of pF.
The best way to test these is with a tube ( of roughly comparable size).
Failiing that, a Dmm before the output cap will be more reliable.
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For a Tek TDS210 I repaired years ago there was specific instruction in the Service manual for backlight repairs.
It stated: use a 100:1 probe and expect a 1250V p-p output @ ~60 KHz.
If the voltage was present but still no display, replace the display.
I now use this MO for backlight investigations:
The switching frequency & primary voltage can be safely confirmed before the step-up transformer and HV presence can comfirmed using any scope probe as a "near feild" probe.
No contact needed, just a close presence of the probe will pick up the EMI.
Sure there are controllers that shut down the backlight if something is amiss, but thats another issue.
BTW the Tek as described above had NO controller, just a simple bipolar push pull inverter with a knackered timing cap. :-DD