FAQ - Measuring fusor voltage part II - Digital voltmeter

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Richard Hull
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FAQ - Measuring fusor voltage part II - Digital voltmeter

Post by Richard Hull » Thu Sep 12, 2013 1:49 pm

This FAQ rewritten 9/26/19 R.H.

Before reading this read part I at

http://www.fusor.net/board/viewtopic.ph ... 105#p27105



This effort uses a common digital panel mount voltmeter display that is getting rare now. These displays are typically 1 megohm input impedance. If you get one of a higher impedance, you will have to scale accordingly. Such meters are typically 1.999 volts. If you chose the LED display you will need a 5volt DC power supply to make it work. If you picked the backlit LCD display you will need a 8-11 volt DC supply with 9 volts being ideal. Make sure that the polarities of your DC supply is hooked up properly to power the meter.

Figure out what voltage you want to measure. if 20kv you want to lower the meter's input impedance to 20k ohms. For 40kv you want 40k ohms, etc.

The meter above with these resistance values placed across them, now have an input impedance of 20k ohms and 40k ohms respectively. It is still a 1.999 volt meter. To make the 20k impedance meter read 20,000 volts we need a series resistor of Sr = 2X10e4 X 1X10e4 = 2X10e8 or 200 megohms. This meter must have its decimal point turned on permanently to read 19.99 set it up with the meter using the spec sheet.

A problem exist with making a 40kv meter that reads 1.999 volts. Sadly, we will have to set it up as a 200kv meter! We must set the decimal point to read 199.9 so 40kv must read x39.9, leading zeros are normally blanked in all such metering automatically and we would read just 39.9 at or near 40kv.

We need a 2K ohm impedance meter to avoid a very high resistance series resistor. 2X10e3 X 1X10e5 =2X10e8 or 200 megohms

For a 20kv meter:

A 200 megohm resistor, be it manufactured or a homemade string, is not all that accurate. To make the meter Z=20kohms, we put a 20k ohm resistor right across the meter. However, this will not allow calibration!
The secret is to put in a 15k 1% fixed resistor in series with a 5K or 10k, 10 turn pot across the meter. Set the series combo for 20k ohms and then put the 200 megohm in series with this, now low Z meter.

Calibration: apply a know 2kv or 10kv across your 20kv meter and turn the 10 turn pot until the meter reads correctly x2.00 or 10.00.

The same goes for the 200kv meter only use 1.5K ohm 1% resistor and a 1K ohm ten turn pot across the meter and preset for 2K ohms total.

One image shows one of the many fully made up fusor voltmeter and ammeter systems that I use to offer for sale in my trading post. Never made any money at it so I gave it up. The ladder is made up of 1 watt flame proof resistors (10) 10meg resistors in series in each leg.

See images below

Richard Hull
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