Humm... I've been looking over some high voltage stuff
in the physics building at my school. Everything is homemade and rack mounted. They metered at the variac before the step up transformer with a huge resister and idiot light (neon) on the hv lead output. Is this an altermative to expensive power station type meters?
TV style probes?
The meter is an inexpensive 100v meter marked with the resultant voltage calculated by the turns ratio of the output xformer.
Larry Leins
Physics Teacher
hv metering cont.
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guest
Re: hv metering cont.
It's a cheap expedient to meter the supply at the transformer input . It will sort of work for a raw AC supply, but if there is any filtering on the output, the meter will be telling dangerous lies. I would feel safer in any case with the meter on the output side. HV resistors are readily available on Ebay.
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Richard Hull
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Re: hv metering cont.
I agree totally with our other Richard. Always strive to meter the "real thing". The effort will pay off in accurate data gathering once you move to the running data phase of the project.
Even though I use LED digital meters on Fusor III, I will try to move back to analog meters in the 100ua range in future. The kickback on pulsing fusors is of such a ferocity that the poor FET front ended, digital panelmeter suffers mightily even with reasonable attempts at bypassing and filtering.
Richard Hull
Even though I use LED digital meters on Fusor III, I will try to move back to analog meters in the 100ua range in future. The kickback on pulsing fusors is of such a ferocity that the poor FET front ended, digital panelmeter suffers mightily even with reasonable attempts at bypassing and filtering.
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
-
guest
Re: hv metering cont.
I recently managed to aquire an analog 50 KV meter on e-bay for about $10. I've been using a Digital Meter with a HV probe. Unfortunately, HV still leaks into the meter. I can occation hear discharge sounds through the meter, although the meter appears to be working fine. I certainly would not dare to touch the meter with HV voltage turned on!
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Richard Hull
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Re: hv metering cont.
I am sure that most folks here know that no analog meter movement is capable of reading into the kilovolt range as sold. Just because the scale reads 50 kilovolts has nothing to do with what it really measures at the terminals on the rear of the meter.
Most true 50 kilovolt analog meters made for industrial use, such as Fluke and the like, usually weigh in at about 50 lbs! All of their meter have the dividers in oil.
Common TV servicing HV probes are only accurate to about +/- 10% if one is lucky.
The common d'arsonval meter movement has a tiny print rating in the lower left or right hand side of the meter face. It is often tucked down so low that it is hard to read. The term FS=1ma or FS 100ua will indicate the full scale deflection of the meter. To make the scale printed under the needle read correctly an external resitor network is needed.
Herein comes the rub. The medium watt, high voltage, ultra high resistance resistors needed are rarely encountered and expensive.
Richard Hull
Most true 50 kilovolt analog meters made for industrial use, such as Fluke and the like, usually weigh in at about 50 lbs! All of their meter have the dividers in oil.
Common TV servicing HV probes are only accurate to about +/- 10% if one is lucky.
The common d'arsonval meter movement has a tiny print rating in the lower left or right hand side of the meter face. It is often tucked down so low that it is hard to read. The term FS=1ma or FS 100ua will indicate the full scale deflection of the meter. To make the scale printed under the needle read correctly an external resitor network is needed.
Herein comes the rub. The medium watt, high voltage, ultra high resistance resistors needed are rarely encountered and expensive.
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
-
guest
Re: hv metering cont.
I don't expect any low cost solution to provide accurate readings. Especially from an analog meter! I just want the meter for my Power supply so I have a rough estimate of the voltage. Am I at running 25 KV,35 KV or 50KV?
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Richard Hull
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- Real name: Richard Hull
Re: hv metering cont.
Analog meters can be damned accurate.
Needless to say for voltage it depends on the tolerance of the dropping resistors. However, for high voltage there is another factor. Leakage along the body of the resistor. This becomes significant when the value of the resistor approaches 100 megohms. It is extremely rare to find 100 meg + resistors ever offered in 5% tolerances for HV use.
Thus meters reading in the 20+kv range are notoriously innaccurate. Fortunately few applications exist where the value of the voltage is needed to be determined to more than 10% accuracy.
Finally, older D'arsonval movements have magnet structures which are old and the original flux value may have changed.
It is instructive to place an old 100ua meter on a Keithley 225 current source instrument and run it through its range 0-100ua. Most really old meters might be non-linear or linear but track low by 5-10%.
Richard Hull
Needless to say for voltage it depends on the tolerance of the dropping resistors. However, for high voltage there is another factor. Leakage along the body of the resistor. This becomes significant when the value of the resistor approaches 100 megohms. It is extremely rare to find 100 meg + resistors ever offered in 5% tolerances for HV use.
Thus meters reading in the 20+kv range are notoriously innaccurate. Fortunately few applications exist where the value of the voltage is needed to be determined to more than 10% accuracy.
Finally, older D'arsonval movements have magnet structures which are old and the original flux value may have changed.
It is instructive to place an old 100ua meter on a Keithley 225 current source instrument and run it through its range 0-100ua. Most really old meters might be non-linear or linear but track low by 5-10%.
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Richard Hester
- Posts: 1519
- Joined: Thu Jul 05, 2001 12:07 am
- Real name:
Re: hv metering cont.
There is one more factor that can limit the accuracy of high voltage measurements. Resistors can have a substantial voltage coefficient, that is, the resistance varies as a function of the voltage across it. This is actually a spec item for high voltage resistors. Metal glaze high voltage resistors are supposed to be pretty good in this respect, but it is a good idea to use a few extra resistors to keep the voltage drop per resistor relatively low. This will also help to limit leakage on the resistor surfaces as well. Before firing the voltage divider up, the resistors need a good wash in alcohol. They should be allowed to dry undisturbed. This step will help to remove fingerprints, which leak current like nobody's business.