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I'm planning to use a 1M resistor as the second one, for a ~ 1000:1 divider, maybe with an opamp attached to give a good output impedance (1M is rather too high, my digital vom would give me a wrong value) like 1Kohm.
this is a simple buildup schematic for the HV side only, with a 1M output impedance:
does anyone know a good OPamp that I can use to get a good output impedance?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Rather than dividing the voltage and using an op-amp as a voltage multiplier use the op-amp in inverting 'virtual earth' mode i.e. Input to one end of hv resistor, other end of hv resistor to Inv-I/P non-Inv_I/P to GND, feedback resistor from O/P to inv-I/P.
Put a couple of diodes (inverse parallel) from GND to inv-I/P for protection.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Another thing you can do is use the input independence of your dmm as the smaller resistor. Sometimes is is marked (like 1M or something), or you can use another meter to measure it (set it to volts, turn it on, then use another meter set for ohms on the voltage terminals). DMM's have a very high input independence (b/c the input is just an op-amp) so they put a fixed resistor across the input.
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
... wrote ...
Another thing you can do is use the input independence of your dmm as the smaller resistor. Sometimes is is marked (like 1M or something), or you can use another meter to measure it (set it to volts, turn it on, then use another meter set for ohms on the voltage terminals). DMM's have a very high input independence (b/c the input is just an op-amp) so they put a fixed resistor across the input.
yep... thats a good idea, I'll use my old 5€ multimeter as HV meter, gonna look at its imput resistance with the new one just bought at the last ham radio fair.
1GOhm will be enough as imput impedance for the hv side?
edit: Had another idea, a pot placed in series with the second resistor to "setup" the right ratio for the imput impedance of the DMM: Let's think my DMM does have a 10M impedance, if we suppose that we have 1µA in the 1M resistor. if we parallel the DMM we'll get about 0,909µA trough it, and a ~10% error. if the second resistor is 1,1M, when paralleled with the 10M impedance of the tester, the current will be 1µA, allowing a good measurement. But... how to set it to the right value? I was thinking to give to it 100V DC, and set the trimmer / pot to get out on the DMM 0,1V. Let's see if works in the next hours!
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High Voltage Probe
