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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The meter is 1 mA full scale deflection.
If you use a 25 MOhm resistor in series then full scale deflection is equivalent to 25 kV (25 kV x 1mA = 25W which is much more than the 15W rating that you mentioned) Asuming that the resistor is exactly 25 MOhm and that full scale deflection is exactly 1.000 mA
The voltage across the meter movement will be 85 mV at fsd.
For current meters an additional resistor is often added in series to give a convenient voltage, e.g. 15 Ohms would give 100 mV total at 1 mA, often this resistor will compensate for coil resistance changes with tenperature.
For a voltmeter application it was very difficult to make a meter exactly 1.00 mA fsd, and the AlNiCo magnets would weaken over time so sensitivity reduced, so the actual movement would be more sensitive than 1 mA fsd, and a resistor was added across the movement to reduce the sensitivity / calibrate the meter.
... from unreliable memory
So, you may want to add a trimmable resistance in parallel with the movement, or a trimmable resistance in series with the meter, or be satisfied with the tolerances of the movement and resistor
If you can measure the fsd current sensitivity of the movement then you can add precisely the correct series resistance for accurate readings, but plus or minus a few percent is usually good enough for most practical purposes.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Along the lines of what Sulaiman said, it's good to design in some adjustability if you want an instrument accurate to 5% or better. Of course that depends on having access to a high voltage reference for calibration, or accurately measuring the HV resistor and knowing about its voltage coefficient of resistance. Same considerations apply with digital panel meters.
Dialing down the sensitivity with a parallel resistor is easy. It adds safety, because there's a place for HV current to go if the meter broke & became open circuit.
Dialing down sensitivity with a series resistor needs to reduce the HV current, so the adjustment resistor must handle a lot of voltage and power.
Your 1 mA HV meter is going to slow down the capacitor charging, and start bleeding the charge down as soon as you disconnect the charger. Why don't you find a nice pretty panel meter with a 100 microamp movement, or less, such as in analog HV measuring probes for CRT service? Then your HV resistor will draw a lot less power, and be physically easier to manage.
The 4.5 x 4 inch analog panel meter in my hand has a scale card saying D.C. KILOVOLTS. Arc is labeled 0 to 25 on one side and 0 to 10 on the other. Fine print says FS = 50 uA.
If pictures were working, I would show my cap-charging control box that includes a Nixie-tube panel DVM and a HV resistor of hundreds of megohms. Also a high voltage switch. Switch positions are 1) connect charger; 2) disconnect charger, so only the meter is connected; 3) ground the HV wire, to bleed capacitor down. High power HV charging resistor is in series at the capacitor end of wire, so no failure of charger or control box can cause an explosion.
Capacitor voltage is accurately indicated whenever switch is in middle position (ready to fire). It can be bumped upward or downward by briefly turning the switch one way or the other. With your "floating" capacitor, you could get the same functionality and safety with charging resistors on both sides of the capacitor, two HV wires, and a double pole switch in meter/control box.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
klugesmith wrote ...
Along the lines of what Sulaiman said, it's good to design in some adjustability if you want an instrument accurate to 5% or better. Of course that depends on having access to a high voltage reference for calibration, or accurately measuring the HV resistor and knowing about its voltage coefficient of resistance. Same considerations apply with digital panel meters.
Dialing down the sensitivity with a parallel resistor is easy. It adds safety, because there's a place for HV current to go if the meter broke & became open circuit.
Dialing down sensitivity with a series resistor needs to reduce the HV current, so the adjustment resistor must handle a lot of voltage and power.
Your 1 mA HV meter is going to slow down the capacitor charging, and start bleeding the charge down as soon as you disconnect the charger. Why don't you find a nice pretty panel meter with a 100 microamp movement, or less, such as in analog HV measuring probes for CRT service? Then your HV resistor will draw a lot less power, and be physically easier to manage.
The 4.5 x 4 inch analog panel meter in my hand has a scale card saying D.C. KILOVOLTS. Arc is labeled 0 to 25 on one side and 0 to 10 on the other. Fine print says FS = 50 uA.
If pictures were working, I would show my cap-charging control box that includes a Nixie-tube panel DVM and a HV resistor of hundreds of megohms. Also a high voltage switch. Switch positions are 1) connect charger; 2) disconnect charger, so only the meter is connected; 3) ground the HV wire, to bleed capacitor down. High power HV charging resistor is in series at the capacitor end of wire, so no failure of charger or control box can cause an explosion.
Capacitor voltage is accurately indicated whenever switch is in middle position (ready to fire). It can be bumped upward or downward by briefly turning the switch one way or the other. With your "floating" capacitor, you could get the same functionality and safety with charging resistors on both sides of the capacitor, two HV wires, and a double pole switch in meter/control box.
Here is a little more explanation of my design. There will be 2 volt meters. One will be for charging and the other will be for discharging into a bank of power resistors. The charging circuit and meter will be physically disconnected by a double-pole switch which will actually consist of pulling 2 charging clamps free via lanyards from a distance. The discharge meter will then be connected via a single-pole switch which will look like a mousetrap. The other side of the discharge voltmeter will be grounded. Everything will be done remotely via non-conductive lanyards. The charging circuit will be physically disconnected before firing.
Efficiency is not really a problem. I am using a 1 KVA NST (15 KV AC at 60 ma) to charge the capacitor. The charging resistor will consume less than 2% of the NST power. The maximum voltage that I will probably ever charge the capacitor with will be around 15 KV DC which corresponds to about 11,000 joules and 9 watts of power dissipation in the charging multiplier resistor.
High accuracy is not really a requirement. Within 1 KV is probably good enough for this application. If for no other reason is that I do not have access to a calibrated HV supply to set the meters more accurately with.
I was looking for 2 identical volt meters with 25 times some factor of 10 at full scale (25, 250, 2500, 25000) to make true voltage easy to read. It also makes the task of finding suitable HV power resistors simpler, at least for a reasonable price on E-Bay. 25M ohm, 15 watt , 30 KV Caddocks were readily available. Having a lower full scale current draw would have been nice but it was hard enough to find 2 identical volt meters without posing an additional requirement of low current at full scale. So I have to accommodate whatever full scale current draw my meters need which in this case is 1 ma.
I do have a question about voltmeter topology. Here are the 2 options:
1. Use a single 25M ohm multiplier resistor on each voltmeter with the other leg grounded. 2. Use 2 multiplier resistors per voltmeter, one per leg with the total adding up to 25M ohms, and no ground?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
MRMILSTAR wrote ... ... There will be 2 volt meters. One will be for charging and the other will be for discharging into a bank of power resistors. The charging circuit and meter will be physically disconnected by a double-pole switch which will actually consist of pulling 2 charging clamps free via lanyards from a distance. The discharge meter will then be connected via a single-pole switch which will look like a mousetrap. The other side of the discharge voltmeter will be grounded. Everything will be done remotely via non-conductive lanyards. The charging circuit will be physically disconnected before firing.
OK. So after a shot, you won't know how much charge is left in the capacitor, until you close the discharge switch and the voltage immediately begins to drop. If the quarter-shrinking circuit is underdamped, and doesn't go open circuit until the second or third quarter-cycle, the residual voltage on capacitor could even be negative. A popular time for arcs to go out is zero-current points, for example at the end of second quarter-cycle, with maximum voltage reversal on the capacitor. What do your quarter-shrinking references say about residual voltage?
Between disconnecting charger and firing a shot, you won't know how much charge has been lost through leakage or corona. If charging resistor is on capacitor side of the meter, meter will read high while charging current flows. (not a problem if you terminate charging by backing off on a variac that feeds the NST). If charging resistor is on charger side of the meter, there's risk of an explosive discharge from any sparking on capacitor side of the resistor.
Here is a quarter shrinker accident report I had no trouble re-finding today: Science/Quarter Shrinker/Accident/accident.htm
Here's one more idea, while I respect that it's _your_ project. Suppose you have just one meter, so no need to scrounge a matched pair. Set it up next to the capacitor, always connected. If it's too hard to read from a safe distance, use a tripod-mounted spotting scope or binoculars. Make it 0.10 mA or 0.05 mA, like most kV meters, to reduce the bleed rate after you disconnect the charger. Charge a bit higher than you want, disconnect charger, then wait for bleed down to desired target voltage.
Back to your question about one vs two HV metering resistors. If the meter(s) is at operator's end of HV cables, at what voltage will its case and terminals be with respect to earth ground? I had one experiment with a meter floating at some 5-figure voltage, where I think the misbehavior was related to electrostatic forces upon delicate moving parts.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
klugesmith -
Thanks for the replies. I was thinking that I shouldn't leave the meter connected during a discharge because I was afraid that the discharge would somehow damage the meter. If the meter will not be damaged during the discharge then perhaps I can use just a single meter permanently connected. I can find a lower current meter if I don't need to find two identical ones. Do you use a single meter permanently connected to the capacitor? Do you ground one side of the meter? Grounding seems like a good idea to protect the meter.
Just to clear up a misunderstanding, the intention was never to have a meter near the operator. It would be co-located with the rest of the apparatus on a cart. Binoculars will be used if needed to read the meter. The only thing near the operator will be the variac.
I was planning on using a remote variac to charge the capacitor. When the charge level has been reached, I was just going to quickly disconnect the charging cables at full voltage via lanyards without turning down the variac. No switch will be involved, just 2 jumper cables that can quickly be removed by lanyards. My reasoning is that if I turn down the variac to zero after charging, the only thing holding back the capacitor voltage from feeding into the NST output is the full wave bridge rectifier. If I have a diode failure at that time, the full massive current from the capacitor could potentially go into the NST output terminals and then, via an internal short in the NST, possibly into the power cord leading back to the variac where I will be located. Not a good scenario.
What is your procedure during charging on your quarter shrinker?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Isn't Signification a regular on this forum, who did some quarter shrinking? My own electroforming work never got past can crushing and can cutting-in-half, in about 2007, using up to 1 kJ in a capacitor whose ratings support 10+ kJ.
Your question inspired me to draw a schematic & try to post it here. Guess it needs clicking.
The HV switch poles are far enough apart, and the moving contact arm is long enough, that arcs don't persist when you open the charging or discharging path. The only way you could ever have more an ampere of current in the wires between switch and capacitor, is if both charging resistors shorted. They're ceramic tubes about 7 inches long and 1 inch in diameter.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Oops, that link I posted to a quarter shrinker accident report & failure analysis was broken. Let's try again. I think the root of all evil is URL's with white space in the middle, as if there were any good reason to allow it.
Again it didn't work. Before I edited this, I think the automatic link was OK in preview pane, and OK after hitting Post Reply button, and then went bad when I used "edit" for some unrelated wording change.
Cutting and pasting the follwing URL ought to work. "http://www.brianb.org/images/Backyard Science/Quarter Shrinker/Accident/accident.htm"
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Measuring HV using an analog voltmeter
