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Old 6th August 2005, 07:39 AM   #1
Kenshin is offline Kenshin  China
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Default DIY HV Probe thisway possible?

Is it possible to DIY a high voltage probe (for oscilloscope) this way? "Hot" HV circuits are marked red in the attachment picture. The circuits at the left hand side of the HV capacitor is the circuit being measured (a model of mains operated SMPS)

The circuit use a high value, high voltage resistor to limit the current ( like screw driver with neon bulb for checking the mains ) and divide the voltage with another one. The capacitors act as an 2-order RC HPF to remove the 50Hz waveform / keep the switching waveforms.

The simulation waveforms worked well in distinguishing 5V 40KHz from 220V 50Hz, does this mean it's suitable for mains SMPS testing? IMHO the major problems is safty ratings of HV components (C and R marked red ).

About the input capacitor...use some 2KV ceramic one with safty gurantee. But which safty gurantee -- CE or UL ?

About the resistor...How to grantee its voltage rating? I havn't seen any resistors with CE or UL safty gurantee. It's said that 1/8 ohm resistor have a voltage rating of 250V DC (not enough); and a old physics textbook suggest DIY a "screw driver with neon bulb for checking the mains" with a single 2~5 M ohm, 1/2 Watt resistor. So THREE 1 M ohm, 1 watt ones in series is enough for 220V AC mains? and withstand how much transient spikes?

Is there any risk of breaking down due to crappy / fake products? Or say: am I depending my life on the morality of device suppliers? I have already met two accidents of soldering iron insulation breakdown.
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Old 6th August 2005, 11:12 AM   #2
SY is offline SY  United States
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Safety in HV probes is largely determined by the physical construction. Most of what you "know" about insulation will prove wrong when the commas start appearing in the voltage numbers. Unless you have extensive experience in engineering high voltage assemblies, this is probably not a safe thing to build yourself.

If HV is just a few hundred volts, go buy a good quality 10x probe and all will be well.
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Old 6th August 2005, 01:27 PM   #3
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DIY HV probe is a very, very bad idea -- some of the plasticizers used in fabricating plastic tubing are conductive -- this goes for engineering resins as well -- contaminants like grease, dust, moisture can also be conductive (yes, dust does conduct electricity.)

I have worked with ham radio linear amplifers with 3kV power supplies -- take my advice and purchase a Fluke HV Probe.
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Old 8th August 2005, 01:18 AM   #4
Kenshin is offline Kenshin  China
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3KV? not. I mean 220V AC mains, something running directly off the mains and produce isolated 30V DC, sealed in a plastic box, only with terminals / probe wire connected out. Some people call probe for measureing mains-operated things "HV probe".
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Originally posted by jackinnj
DIY HV probe is a very, very bad idea -- some of the plasticizers used in fabricating plastic tubing are conductive -- this goes for engineering resins as well -- contaminants like grease, dust, moisture can also be conductive (yes, dust does conduct electricity.)

I have worked with ham radio linear amplifers with 3kV power supplies -- take my advice and purchase a Fluke HV Probe.
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Old 8th August 2005, 01:26 AM   #5
Kenshin is offline Kenshin  China
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10x probe? How to deal with the grounding problems? The oscilloscope clip is connected with the shell, so it shouldn't be connected to the SMPS board.
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Originally posted by SY
:If HV is just a few hundred volts, go buy a good quality 10x probe and all will be well.
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Old 9th August 2005, 02:35 AM   #6
Eva is offline Eva  Spain
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Quote:
Some people call probe for measureing mains-operated things "HV probe".
The apparatus that allows for floating measurements is called a 'differential probe'. These are quite expensive.


Quote:
10x probe? How to deal with the grounding problems? The oscilloscope clip is connected with the shell, so it shouldn't be connected to the SMPS board.
You should never use live mains for SMPS testing and measurements, use your own isolated floating AC source instead. This is achieved by powering the SMPS under test through isolation transformers.
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