FAQ - Analog metering of High voltage supplies.

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Richard Hull
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FAQ - Analog metering of High voltage supplies.

Post by Richard Hull » Sat May 02, 2020 7:12 pm

Analog metering refers to the old fashion, non digital, sweep needled meter with the glass of plastic face. Such metering has virtually disappeared in all electronic applications that demand a voltage or current reading to be displayed, usually in variable controlled electronic applications. Such displays are now handed over, almost totally, to glowing, easy to read, numerical digital displays.

Analog meters are all "interpretive in nature". That is, they require a calibrated eyeball that can resolve and interpret graduated readings on a printed scale between clearly numbered "tick marks" along a curved scale on the meter face. Their main advantage over digital instruments is that they are, by comparison, indestructible in digitally destructive environments. This includes very highly disruptive RF, EMP and pulsed environments to which they are connected. Being 100% magnetically functioning they are slow to respond, being magneto-mechanical engines designed to move a needle along a scale. Digital meters involve instant acting and transmitting sensitive electronic, semiconducting devices which are easily damaged in electrically nasty environments. Digital readouts and metering like smoothly varying input currents and voltages. Analog instruments like this, too, if they are to report accurately, but are just not as easily damaged by huge transient currents and voltages of short duration.

Analog metering suffers from its own limitations of being a low impedance device. (this is also their saving grace related to indestructibility.) Analog metering, in high voltage situations, require rather large high ohm strings of series connected resistors to limit the current to the more sensitive meter movements demanded in constructing high voltage analog readouts. For fusor supplies, analog readouts are ideal as the fusor is a nasty beast of high current pulse bursts, creating huge damaging EMP and RF energies not demanded of much more tame power supply designs and their metering.

The simple ohms law relationship applies in figuring out these complex metering strings. Most meters have their basic meter movement sensitivity well hidden in the lower right hand bottom corner of the printed meter face, often out of direct vision. The scale printed on the meter may or may not be correct. However, meters with a printed scale in the low milliamp and microamp range are usually what they say they are, but check to be sure. If a meter printed scale is 0-1000 volts it is usually never a 1000 volt meter but a 1ma or 100ua meter and will require a series dropping resistor.

Ohms law is easily consulted once you know the meter's core sensitivity. You know the voltage you wish to read, say, 10,000 volts full scale. For example, You have a meter sensitivity to go full scale at 200ua. So, in this case R= E/I. R= 10,000/.0002 = 1e4/2e-4 = 0.5e8 = 50,000,000 ohms. Using this resistance in series with your meter, the full scale now reads 0-10,000 volts

There are issues with the wattage of the dropping resistor. This rears its ugly head when high current, low sensitivity meters are used to read high voltage. Let us say you have a 1ma meter movement and want it to read 10,000 volts. R= 10,000/.001 = 1e4/1e-3 =1e7 = 10 megohms. The power handling of the resistor would be P=E x I = 10,000 x.001 = 1e4 x 1e-3 = 1e1 = 10 watts. Another way is P=I^2 x R = 1e-6 x 1e7 = 1e1 or 10 watts.

Few power supplies can waste 10 watts of 10,000 volt energy in just reading the voltage! In general you never want to use a dropping meter resistance that is the exact wattage needed as it will heat as its resistance changes. Thus, a 50 to 100 watt resistor would be needed in this latter case to remain cool. There are no real 10 megohm 100 watt resistors floating about. See what I mean?

Let us go back to the 200ua, 10,000 volt, 50 megohm resistor example, above. P = .0002 x 10,000 = 2e-4 x 1e4 = 2e0 or simply, 2 watts. There are no real 2 watt 50 meg resistors. By using a more normal series string of (10), 5 meg ohm 2 watt resistors you have a cool running 50 megohm 20 watt dropping resistor. This is still kind of big and cumbersome, but it has the advantage of putting only 1kv across each resistor. (no possibility of arc over)

From all of the above, you can see that you want the most sensitive, (low current) meter movement possible if you are reading voltages above 10kv with it. Regardless of meter sensitivity, you will always be using a series string of resistors and must be mindful of power dissipation in your dropping resistor chain and shoot for 10X the rated wattage at full scale, and then watch, carefully, voltage impressed across each resistor in the string.

In general, if you want an analog meter to read 10kv or more this usually means you do not have a 10kv power supply that can supply 1 amp. (a 10,000 watt supply). More likely it is rare to have a real 20kv supply that can supply even 10ma!!! (200 watt supply). You don't want to have to supply 1ma or 20 watts of wasted energy to a dropping resistor just to read the voltage! Wouldn't it be much better to use a 100 ua or a 50ua meter movement to read your 20kv. (this would waste only 2 watts or 1 watt respectively) It would also allow for much lower wattage resistors to be used in the dropping string.

Finally, rather heroic efforts must be brought to bear in any metering scenario over 25kv related to corona prevention and resistor string assembly. In any such string designed to read over 40kv, you must consider a possible "under oil" scenario. and consider no more than 1 kv across any one dropping resistor if using common electronic resistors that are not manufactured specifically for high voltage work.

Rules of thumb related to high voltage analog metering

1. Select a very high sensitivity meter movement of very low full scale current for all HV analog meter circuits. 20ua, 50ua, 100ua...... All metering circuits are a LOAD on your high voltage power supply!!!
2. Know the current capability of your HV supply at full measured voltage. Never allow the metering circuit to gobble up more than 5% of the supply current! All metering circuits use and waste power.
3. Use either a large number of common electronic resistors in your string of the proper wattage or use a smaller number of HV rated "special" resistors of the proper wattage.
4. Be prepared for rather complex construction of dropping strings if you require metering over 25kv as your are entering specialty High Voltage analog metering circuit work.
5. Always put your meter at the bottom of the resistor string! Ground one of the meter terminals. Apply high voltage to the top on the resistor string only!


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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Rich Feldman
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Re: FAQ - Analog metering of High voltage supplies.

Post by Rich Feldman » Tue May 05, 2020 5:48 am

I think it's good policy to have some overvoltage protection in parallel with the meter movement.
Even just silicon diodes, both ways, or a neon lamp.
Otherwise, if the meter or its connections failed open-circuit for any reason,
the high megohm resistor string would continue to deliver 50 uA or 100 uA or 200 uA, finding a path to ground by hook or by crook.
All models are wrong; some models are useful. -- George Box

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Richard Hull
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Re: FAQ - Analog metering of High voltage supplies.

Post by Richard Hull » Tue May 05, 2020 6:24 pm

Good advice for sure. Protect your meter with a path to ground. This will also keep the meter from becoming a source for high voltage in the meter movement in break down or flashover situations. Probably both protections are needed (belt and suspenders). See drawing.

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Dennis P Brown
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Re: FAQ - Analog metering of High voltage supplies.

Post by Dennis P Brown » Wed May 06, 2020 12:49 am

A lot of good info and ideas here.

For someone not very experienced with electronics, the last diagram has a neon light (if memory serves, these turn on at about 70 volts?) But what voltage diodes would be appropriate in that circuit?

I have a 45 kV probe style meter (it can read up to 55 kV.) I have a single 10 giga ohm resister and this works well (however, it was surplus since its orginal single long resistor was broken. I bought a chinese one cheap (same ohm and a cm shorter) and the meter works very well.) It, as you point out, uses a simple analog micro-amp meter and it would seem a very easy build considering how simple the whole assembly turned out to be.

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Re: FAQ - Analog metering of High voltage supplies.

Post by Richard Hull » Wed May 06, 2020 10:21 pm

I would use UF4007's they will conduct around .6 - .7 volts in both directions in a matter of 1us or less.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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