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High Voltage metering
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by KC2RLY on November 5, 2009
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Hello,
i have been thinking about trying to homebrew a small multi band amp for myself, perhaps with a surplus russian tube or maybe a 3-500zg. All I want is a clean 500 watts I would say and let the antennas do all the work. The builds in both cases seem pretty straight forward, chokes, capacitors and even transformers appear on ebay or can be home brewed but I am having a tough time with figuring how to accurately measure HV.
I purchased a panel meter which I figured i would just scale up. I had bought a 100vdc meter. I even called the manufacturer today to ask what the ohms/volt rating is and they had no idea. I bought a gme technologies pm 89 series meter. I figured I would buy the 100 volt dc meter and scale it to 10kv. I could always leave this on the supply case and if I ever change transformers or tubes etc i could always use my same 10kv meter. I already have a nice meter from them for the filament voltage and wanted to use this meter.
any ideas how to scale this meter?
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RE: High Voltage metering
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by W8JI on November 5, 2009
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You have to know the meter movement current rating or the "ohms per volt".
You can measure that with another meter.
Then when you know it you can determine the resistaors to use. The resistors must be high voltage rated resistors. Say for example you use ten 1 watt 1 meg ohm resistors for a 1 mA meter. Each resistor would have 1000 volts at full scale, so to make the meter safe at full scale you would need 1000 volt resistors. Normal small resistors are rated at about 250-350 volts.
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RE: High Voltage metering
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by WB2WIK on November 5, 2009
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From the initial post it "sounds" like maybe you don't realize the meter is actually measuring current, not voltage.
If it's calibrated in voltage, either it has an internal series (multiplier) resistor, or relied on an external one in some specific circuit.
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RE: High Voltage metering
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by KC2RLY on November 6, 2009
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WB2WIK:
Honestly I did not realize the volt meter was sampling current and not not the other way around. Thank you!
W8JI:
Also how can I measure the meter with another if it changes as the scale deflects? There must be a simple answer here that I am oblivious to! Thanks again for the help guys!
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RE: High Voltage metering
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by AA4PB on November 6, 2009
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If its a 100V meter (i.e. you can apply 100V across its terminals) then you can simply measure the internal resistance by placing an ohmmeter across the terminals and reading the resistance directly.
If its really a sensitive meter (like 1mA) that is calibrated to 100V but depends on an external multiplier resistor then the battery in the ohmmeter may slam the meter up against the peg.
All voltmeters are really sensitive current meters (often 1mA full scale) that measure the current in a multiplier resistor. According to Ohm's law, the current is dependent on the applied voltage. The current meter scale is then calibrated in volts. The multiplier resistor can either be internal within the meter case or it can be external.
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RE: High Voltage metering
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by AA4PB on November 6, 2009
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The meter's internal resistance does not change with the meter deflection. It is a constant. For example, if the resistance was 100 ohms and the full scale deflection was 1mA then the meter sensitivity is 100 ohms per mA. You could then use ohm's law to calculate what value of multiplier resistor you would need to make it read any voltage you wanted. Remember that the total current flow depends on the applied voltage and the TOTAL resistance (multiplier + internal meter resistance).
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RE: High Voltage metering
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by W8JI on November 6, 2009
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You better find out what the movement current is. This is also a time to learn all the applications of Ohm's law.
If the meter is an external multiplier it won't really be a 100 volt meter, it will be a current meter of some low value. Hopefully 1 mA full scale because that would be 1,000 ohms per volt.
If your meter is less than 1,000 ohms per volt it would be about useless as a high voltage meter. For example if it was 100 ohms per volt you would have big problems finding a suitable multiplier resistor.
Ohms per volt is the reciprocal of the meter full scale current. 1/.001 amp = 1000 ohms per volt
1/.01 amp = 100 ohms per volt.
A 1000 ohm per volt meter would take 1000*10000= 10 meg ohms of multiplier resistance. That would be a .001^2 * 10 million = 10 watts of multiplier power rating. You could use ten 1 meg ohm high voltage 2 watt resistors in series.
Now if it was 100 ohms per volt, or a 10 mA FS meter, the required resistance would be 100*10,000 = 1 meg ohm. The resistor dissipation would be .01^2 * 1 million= 100 watts. You will never buy a 200 watt precision high voltage resistor, or even a 20 watter, so you would have to use something like 100 2watt resistors in series. It would be a mess.
This is why you need to know the ohms-per-volt or the meter full scale current. You can ohm directly across the movement. But that won't always tell you the ohms per volt or meter current rating. It is best to measure the movement current.
Tom
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RE: High Voltage metering
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by KC2RLY on November 7, 2009
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ok, first thanks to all the posts helping me out! This is what ham radio is all about, learning and building and experimenting. Where would I be without elmers?
back to my meters. I measured the resistance of the meter itself today and it indeed does have a resistor of its own inside. the meters own resistance measures 1000 ohms.
i then placed my simpson 260 multimeter inline with 100volts dc and read 1ma full scale.
so I now know it is a 1milliampere full scale defelection meter. if I read the above posts correctly i should have a total multiplier of 10meg ohm but I also need to subtract the meter's own resistance? so would this be 9.9meg for a multiplier?
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RE: High Voltage metering
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by AA4PB on November 7, 2009
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You've got it. Just remember that most resistors are not rated to handle 10kV so you'll probably want a series string of smaller resistors adding up to 9.9Meg.
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