FAQ - Build a high voltage "bias" meter for free!

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Richard Hull
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FAQ - Build a high voltage "bias" meter for free!

Post by Richard Hull » Wed May 15, 2019 8:26 pm

Bias voltages, PMT and GM supply voltages often need to be measured with accuracy. A bias supply is an inherently weak supply. Portable instruments that are battery powered typically have extremely weak bias supplies. (capable of just a few microamps due to their Hi-Z load characteristics. It is rare to require any nuclear detector bias much above 2kv.

***************************** SAFETY NOTE *********************************************************
The meter described and constructed, as below, cannot be expanded to higher voltages! This is due to the mounting of the divider chain sealed in shrink tubing and epoxied to the meter case. This meter is a bias meter and not to be used on high current 2.5kv voltage tests.
DO NOT hold the meter in your hands while making any high voltage measurement.
**********************************************************************************************************

In some repair and maintenance manuals, it is recommend measuring the high voltage with an electrostatic voltmeter!!! Do you have one? The average DVM has an input impedance of 1 megohm to a maximum of 10 megohms. This will load a bias supply to the point of having you set your voltage and the instant you remove the meter, the voltage will skyrocket. Your meter loaded it down, you adjusted to the recommended level and now it is 20% higher in reality!

I will give a simple way to make a free 100 megohm digital voltmeter. It is abysmally simple to any better than average electronics guy on how to do this, but here is a method that is cheap and easy for anyone.

1 Get a free Harbor Freight DVM.....These are the list priced $5.99 meters. They have several types. Get the low end model. There is no shame in shelling out $5.99, but this is about a free meter, not a $5.99 plus meter. They do often have coupons for these meters for FREE in their store flyers, local Sunday newspapers, American Rifleman and other manly men's magazines! Once you give them your name and address at the register, you will get "free stuff flyers" in the mail, too.

Note* these little Chi-Com meters can be stunningly accurate! Using a Weston "standard cell", I have found they are often accurate to within +/- .001 volt. WOW! (Weston is 1.019 or there abouts based on temp) One megohm is a rather heavy load on the cell. Every old Fluke 50s-70's, hyper accurate differential voltmeter had a standard cell in it. A well made "standard cell" is usually good for a couple of hundred years!! 1.019 volts forever!

2. It is hoped you have a small supply of high ohm resistors around in the 10 meg and above range.

I supply an image below that is more or less self explanatory for those capable of monkey see, monkey do.

The epitome of the process..........................

The Harbor Freight meter has an input impedance on all ranges of 1 megohm. We need a precision matched 99 megohm resistor hooked in series with the red positive meter lead. Once this is done, we put the meter on the 20 volt range and now have a 0-2kv 100megohm impedance meter that will not load even the weakest of bias supplies. The best thing we can do is to get our hands on a stiff, (low-Z), DC supply of about 300-900 volts. (you are on your own here. Like maybe make one up with a transformer variac and some diodes and a filter capacitor with suitable bleeder!)

Using the Harbor Freight meter, as purchased, on the 1000 volt range, adjust the supply or the variac controlling it to a voltage at or above 300 volts...Precisely!!!! The voltage is unimportant just make it an even voltage at hundreds. (300, 500, 700, whatever) Turn the supply off. For the following text, we assume you have set to 500 volts. Turn off the supply!!! (note edit may 2019)....The current spate of harbor freight meters no longer sport a 1000 volt dc top end range...too bad.
get you hands on any dvm with a 1000vdc top range.

Start assembling your 99 meg resistor. I used (5) 18 meg, 1/4 watt 5% film resistors in series to start with. Hook this partially done, series resistor in series with the positive probe of the Harbor Freight DVM and the other side of the resistor to the hot of the HV supply. Needless to say, you hook the black negative probe to the negative of the supply. Place the meter on the 20 volt range. Turn on the supply. You should read a good bit high on the voltage. If you set the supply for 500 volts, you might read 5.45 = 545 volts. Now, to avoid killing yourself, TURN OFF THE SUPPLY!! If high, You need more resistance. Try a 3 or 8 meg resistor added in series. Retry and add resistance as needed until you are between 4.98 and 5.02. You are done! If you read low like, say, 488 volts. you will need to take some resistance out of the series string. In the end, you will have an ultra high impedance (~100 megohm), 2kv meter capable of better than 1% accuracy.

You can fancy this thing up in any number of ways you like. As I have over 54 of these free meters collected over the last 6 years, I dedicated my meter to a bias meter. (see the photo) I think I wound up with 7 resistors in my series string. Since I opt'd for boring a hole in the meter and connecting one end of the resistor string to the positive input on the meter's circuit board and planned on epoxying the string to the meter's right side, I shrink wrapped the string to avoid shock if holding the meter during testing. (see photo) I fashioned a little clip loop at the top end of the string for an alligator clip lead when in need of bias adjustment. By doing this, the meter is still good for use as a normal DVM with the special lug on the string for the 2kv 100 megohm bias setting. Just remember to put the meter on the 20 volt range when doing bias adjustments or setting GM or PMT supply voltages.

I do a lot of development and custom work on battery operated weak, (hi-Z), supplies in this range and need to know my voltages under load without further loading the supply while testing or making in circuit adjustments.

Good luck if you plan on making this bias meter.

Note*.....Try and use the highest voltage under 1000 volts to build your string. I used 800 volts. If you use 200 volts and settle for a reading of 203 on your string, at 800 volts you will be at 812 volts and at 1600 volts you will be at 1624. In short, the error multiplies. I settled at 801-2 bobbling. As a rule, HV bias levels need not be precise to the volt, but 1% or 2% accuracy is desirable

Richard Hull

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