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Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
In some cases damping of a meter movement is desired, and connecting a resistor across the movement accomplishes this. For a meter connected in series with a large series multiplier, as far as the meter is concerned, it is being fed from a constant current source.
If you tell me the Weston meter series number, I may have the catalog data, if it is a very old one. Since you have a low resistance movement , you might consider a MOV rated at a hundred or so volts. The resistance will not affect the meter, but if the meter movement opens, it will prevent arking that might ignite something.
I have no idea how many joules your capacitor will store, but if lethal, something more substantial than that meter should be in place to warn it is charged.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
I read about Brian's accident and have studied his design carefully. Three things made me uncomfortable with his implementation.
1. Everything seems too crowded for the voltage and power levels. 2. He used a metal cart. 3. The circuitry just seems overly complicated and relies too much on electronics for safety. Protection is good but I think there are simpler and more reliable ways to accomplish this.
All of these things just seems to invite flash-overs.
Before I started my design I studied all of the quarter shrinkers for which I could find information. I believe that I am incorporating the best features of these and adding some of my own. For example, in my design, all high voltage equipment and wiring is widely spaced and I am using a solid plastic cart to avoid flash-overs. I am also relying strictly on mechanical means activated by long non-conductive lanyards to trigger events. There will be no electronics to invite failure.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
radiotech wrote ...
In some cases damping of a meter movement is desired, and connecting a resistor across the movement accomplishes this. For a meter connected in series with a large series multiplier, as far as the meter is concerned, it is being fed from a constant current source.
If you tell me the Weston meter series number, I may have the catalog data, if it is a very old one. Since you have a low resistance movement , you might consider a MOV rated at a hundred or so volts. The resistance will not affect the meter, but if the meter movement opens, it will prevent arking that might ignite something.
I have no idea how many joules your capacitor will store, but if lethal, something more substantial than that meter should be in place to warn it is charged.
Because the Weston meter requires so much current (1 ma) and thus being quite inefficient, I have decided to use 2 Simpson microammeters instead that each require only 25 microamps full scale. A HV 1G ohm (two HV 500G ohm resistors in series) multiplier resistor will be used for each one. One meter will be used for charging. It reads 0-25 microamps, so I just multiply the scale by 1000 to get the charging voltage. It will be disconnected after charging and prior to firing. The other meter will be used for resistor bleed-off. Its scale is 25-0-25 microamps to account for a potential negative residual capacitor charge caused by voltage reversals during firing. It will be disconnected during charging and firing and connected during resistor bleed-off.
The ultimate final check for residual charge will be a chicken stick. I do not trust electronics for this.
If fully charged to its maximum rated voltage of 22 KV, the energy would be 24K joules. I never expect to charge it to that level because that is much more than is needed and would be quite stressful on the capacitor due to excessive voltage reversals. To maximize capacitor life, I will restrict the charge to about 15 KV which will result in 11,250 joules.
If you do have access to a spec sheet for the Weston voltmeter I would still like to have it for future reference. All that I know about it is what is printed on the face plate as follows:
* Made in 1958 (stamped on rear of case) * Model 741 * 0 - 2500 volts full scale * FS = 1 ma (presumably with external multiplier resistor #9492) * 1000 ohms/volt (presumably with external multiplier resistor #9492) * Use with external resistor #9492
The measured resistance with my Fluke DMM is 85 ohms. I don't know what value the external resistor is but I assume that it is 2.5M ohms based on the other information.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
The Weston info didn't show up. I sure wish we could get this image insertion problem with the web site fixed. The ability to attach pictures is one of the reasons that I really liked this forum.
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Measuring HV using an analog voltmeter
