 voltage scaling question
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teslacoolguy
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Wed Feb 20 2008, 05:02AM
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Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
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I need to semi accurately measure voltages up to 5kvdc for a capacitor charger and i was wondering if anyone knows a formula or a place to find a formula to scale the high voltage down to something that i can read on a dmm. I need it to devide by 10 so lets say 1kv will indicate 100v on the dmm and 2kv would be 200v and so on i was wondering if anyone knows what kind of dropping resistor or resistor chain that i need to do this.
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CT2
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Registered Member #180
Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187
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When you add a series resistor you are creating a voltage divider between the impedance of your meter the resisance of the series resistor. If say your dmm had 10MOhms (10 million) or resistance and you used a 10MOhm series resistor then the voltage would divide evenly and you would get half the voltage on the dmm (in your case 2.5kv). In order to get the value for the resistor you need to know the value of meter, it should be on the back (and is usually 1 or 10MOhms). If you know the resistance of the meter and the voltage you want to read you calculate the current that must flow to accomplish that. So I=V/R = 50 uA. If you have 500 volts across your meter then you must have 4500 volts across the other resistor with 50 uA of current = 90MOhms. So for a divide by 10 on the 500 volt scale you'll need a 90MOhm series resistor. Which makes sense if the R of the meter is 1/10 the R of the total R then it will drop 1/10 of the voltage.
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Sulaiman
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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
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Take care when making your divider as normal resistors cannot handle high voltages,
e.g. a 1/4 W 10 MOhm resistor 'should' withstand 2kV using W=V^2/R ... but it will not (reliably)
You can use special (expensive) high voltage resistors OR many cheap resistors in series.
In either case I prefer not to rely on the meter input resistance as the 'bottom leg' of a divider
e.g. A 'typical 10 MOhm input DMM on a range suitable for measuring 5V needs a 1000:1 divider
which could be 10 x 10 MOhm and 1 x 100 kOhm across which you measure 1000th of the voltage.
The error caused by 10 MOhm across 100 kOhm is 1%, if you're fussy use a total of 101.010101010101 kOhm
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