Build a HV bias meter for free.

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Richard Hull
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Build a HV bias meter for free.

Post by Richard Hull » Tue Dec 27, 2016 3:44 am

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 bias much above 2kv.

In some repair and maintenance manuals they recommend measuring the high voltage with an electrostatic voltmeter!!! Do you have one? The average DVM has an input impedance of 1 to a maximum of 10 meg. 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 newpapers, Ameican 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 explanitory 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!!!

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 resitance 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

CLICK TO ENLARGE IMAGE
Attachments
HV METER pix.jpg
The finished meter - text in image should be enough to buidl from
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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Re: Build a HV bias meter for free.

Post by Richard Hull » Fri Dec 30, 2016 8:02 am

I actually had a chance to use the meter I made in the above posting today. I needed to design and build a 90 milliwatt 700 volt GM supply that could use the Arduino as the HF PWM driver in addition to running the entire GM counter. I had been using old CCFL supplies that were a bit large in physical size and used about 200 milliwatts. This is important when battery power is at a premium.

I attach an image of my test bench lash-up. The supply did not buckle from the 80CPM background to having the GM tube roaring at 27,000 CPM. The little HV meter kludge showed that under load the supply was in full output.

CLICK TO ENLARGE IMAGE

Richard Hull
Attachments
HV prototype.jpg
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.

Dan Knapp
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Re: Build a HV bias meter for free.

Post by Dan Knapp » Fri Dec 30, 2016 9:44 pm

From another fan of Harbor Freight freebe meters, a photo of my lab bench. They're great for monitoring multiple parameters, and when you zap one with an arc, you just pull another out of the drawer.
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Re: Build a HV bias meter for free.

Post by prestonbarrows » Sat Dec 31, 2016 2:55 am

Dan Knapp wrote:From another fan of Harbor Freight freebe meters, a photo of my lab bench. They're great for monitoring multiple parameters, and when you zap one with an arc, you just pull another out of the drawer.
Heavens to Betsy, where on earth do you make room to keep your slide rules handy on that desktop?! Rough and tumble ADC modules and FPGAs are so cheap nowadays they approach disposable.

A few examples of many.
http://www.terasic.com.tw/cgi-bin/page/ ... ish&No=593
https://labjack.com/products/u3

Throw a few TSVs on one of those and you got a dozen channels recorded in real time plus the option to roll your own code and control hardware in parallel.

Dan Knapp
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Re: Build a HV bias meter for free.

Post by Dan Knapp » Sat Dec 31, 2016 3:14 am

When you're working with apparatus prone to arcing, those $80+ ADC modules can do bad things to the budget.

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Dennis P Brown
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Re: Build a HV bias meter for free.

Post by Dennis P Brown » Sat Dec 31, 2016 7:55 pm

Just ran into this very problem with my newly made neutron detector tube power supply! Any load knocks downs the high voltage voltage output to a small fraction - even with a 1000 Mega-ohm resistor circuit! Fixed the problem using a cap feeding a HV diode (lucky the HV supply is AC so the cap allows direct voltage reading.) So, my battery (12.8 voltage DC with a charging ciruit built in, as well) powered supply that provides both 2.3 kV positive and 120 vAC out works nicely now and I can accurately read the 2.3 kV output.

Aside: the 120 v AC battery isolated section supplies my neutron counter circuit system and avoids ground loops.)

Hopefully, building this a bit complex supply system will cure my noise issues on the neutron counter (all coaxial and metal shielded - including the tube) from my fusor.

Aside, as well: for such a low current power supply (the 2.3 kV side without any cap yet), it had no trouble burning off some of my skin when I forgot it was turned on (lol.) Forgot how impressive the arc can be for such a trivial supply.

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Richard Hull
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Re: Build a HV bias meter for free.

Post by Richard Hull » Sat Dec 31, 2016 9:17 pm

You tell 'em Dan! Let 'em blow their stuff up.... Of course, if they are working on little cutsie stuff that never throws nasty crap back at their ADCs, then OK, But free is free and money is money. As long as those Chi-Coms are makin' 'em cheap enough for Harbor Freight to give 'em away, we'll keep takin' 'em off their shelves. I have 50 ++ of those little "red" DVMs and still counting. Got two more last week. (Note*** I pull the batteries out of the ones to be stored so they won't take a poo inside over time.) Remember, it is all about working and beating....
"th' system".

You can jazz up a 4 meter array behind a panel with only the readout showing by cutting slots in the panel. Set the meters to the range needed, mount them to the back of the panel. You will have to bore two 1/16-inch holes at the plus and minus probe points and solder wires to the boards and run the thin wires to you app in the box. When the panel or box is no longer needed, you can reclaim the meters, intact, just plug the old probe leads back in and desolder the wires from the board.

ADCs!

The $4.95 Arduino pro-mini with 8 ADC channels, several PWM ports and up to 17 Digital I/O pins will do all of my ADC and digital work, combined! (When I need cutsie ADC work done, of course) Nothing I do needs more than 10 bits resolution. Higher resolution can lead to noise issues the higher you go in bits, depending on the environment your project is in.

The $4.95 is Marlin P. Jones price. Everyone else is more expensive, as a rule. I have stocked about 25 Pro-mini's over the last two years, 8 UNO rev3s, 9 Nanos and 3 Megas. I have yet to find a project so jinormous that it demands the power of the Mega......but I am ready. I mainly work on portable instrumentation apps. I do not consider myself a great programmer, just a capable one. I program and do my machine work like a soldier fires his weapon...........for effect.

Richard Hull

P.S. - UPDATE October 2017. I now have 81 free meters from Harbor Freight! I have two flyers for their November sale with free meters....I'll get two more for the pile in a week or two!
PPS - UPDATE #2 May 2018. I now have 106 of the free Harbor freight meters, but avoid all appearance of OCD!!
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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Re: Build a HV bias meter for free.

Post by Richard Hull » Sun May 27, 2018 8:48 pm

By the way, the little DVM voltmeter has a transistor checker, a diode checker, a continuity checker, ohm meter, ammeter just like a $200 Fluke and similar accuracy....Far more accuracy than you will need for simple projects. All this for free or $5.99, your choice.

I am up to 106 of 'em and have a coupon now for another free one. Tomorrow, memorial day, I will go grab one for free' and as you must buy something to get a free meter, I will get another meter with my 25% off any purchase coupon. $5.99 - $1.50 makes it $4.50 for two meters. After removing the batteries once at home, I will have 108 meters in storage.

Like Dan said above.....Blow one up? Grab another from your stash. I have blown up 3 in the past due to stupidity where harbor freight paid for my mistakes.

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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Re: Build a HV bias meter for free.

Post by prestonbarrows » Sat Jun 23, 2018 2:30 am

Once the failure rate of an insturment approaches (inevitably in your mind) unity, the effective accuracy (for all previous times) approaches zero.

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Richard Hull
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Re: Build a HV bias meter for free.

Post by Richard Hull » Sat Jun 23, 2018 7:26 am

As noted above the accuracy of these free or $5.99 meters tends to rival that of a good Fluke or Beckman DVM. Mistakes blow up meters like leaving the meter on ohms and connecting it to 2kv. Or placing the meter in the 20ma current position and then measuring a 12 volt power buss. This particular issue is obviated in my 2kv,100megohm HV kludge above as I have soldered the rotary switch, internally, in the HV measuring position. All of its other ranges are unavailable. You can do this sort of thing with 106 free DVM meters on hand.

All of the accuracy in all modern DVMs be they Fluke or Chicom meters is in their laser trimmed resistors. At one time 0.1% resistors were about $10.00each and ultra tight tempco 0.1 % could approach $25.00 each. 1% film resistors can now be had in units of 100 for what 100 old 5% resistors used to cost! In these mass produced high accuracy DVM assembly lines laser trimmed resistors are dirt cheap. the DVM only needs about 7 values for the voltage ranges. If you are making 50,000 DVM's, mass production can drive the OEM prices for laser trimmed to a penny or two. Fluke and Beckman have a great reputation for service and warranty follow up if you make a stupid mistake and you pay for your mistakes. The chicoms just make a good meter and sell it at a throw away price. Harbor Freight lets you make mistakes for free.

In addition there is no real viable difference in 99.999% of HV measuring situations between 1750 volts and 1768 volts, a 1% error. The average dunce will probably fail to take the highly accurate Chicom, Harbor Freight freebie and assemble the external composite range resistor to the level of 1% accuracy in the 2kv metering effort I describe. No matter, the important thing is the 100 megohm input impedance.

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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