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Old 17th Apr 2017, 5:46 pm   #1
MrBungle
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Default Another FET voltmeter

Thanks to some inspiration from other forum members and the desire to actually build some gear rather than just buy it I built a FET voltmeter. This is adapted from the FET voltmeter from "Test Equipment for the Radio Amateur", the 1978 edition of the book. The following changes were made:

1. 2n3819 was used as a substitute for the proposed FETs as that's what I had lying around.
2. The ranges were changed to 1, 2, 10, 20, 100 volts which suits my use cases better (usually 13.8v and below!) and don't necessarily trust the switches I am using with more than 100v.
3. I added a protection zener (5.1v) across the G-S junction on the input side FET so that sticking it on the wrong range on a 100v DC line won't blow it up. Also it conducts at -0.6v preventing any nasty bias of the FET gate diode which is a little fragile. I tested it up to 350v DC and it didn't blow up.
4. I skipped the individual probes with 1M at the tip as I wanted standard 4mm jacks and no error at the unit itself.
5. Turned out 8-12v DC was perfectly fine to run it off so I used 8xAA batteries. This should give it a battery life of a few years at least. The pack is sealed in a ziplock bag with a hole for the terminals and stuck behind a baffle to stop it leaking all over everything just in case.
6. Protection diodes were added across the moving coil to protect it from overloads.

The divider ratios and resistors used were:

5M (10M||10M), 4M (4x1M), 500k (1M||1M), 400K (4x100K), 100K (1x100k).

I made a big mistake at first with the voltage divider and used 5% resistors that were hand selected. This turned out to be rubbish so I went and bought some nice Vishay MRS25 1% units.

It is a surprisingly accurate meter, fast responding and completely linear which was a nice surprise. I've been making an effort to use just this and the RF probe described here instead of my more professional kit:

http://vintage-radio.net/forum/showthread.php?t=132296

Panel labelling got lazy and out came the dymo. I'll redo this when I feel like it which might be never

The PCB was drawn out on paper first with a pencil, the holes drilled, nail varnish applied and it chucked in a very shallow tupperware box of ferric chloride and floated in hot water in the sink. This was then cleaned, populated and bolted to the meter terminals directly.

Pictures:

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So far it has been used for characterising a pile of old 1n34a's with a test rig, has been used to measure RF output on a couple of things and is currently aiding me building a relatively hefty 0-20v 10A power supply.

All the parts came from bitsbox.co.uk apart from the divider resistors which came from RS and the PCB and movement which came from Rapid.
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Old 17th Apr 2017, 9:48 pm   #2
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Default Re: Another FET voltmeter

Very nice! The real spirit of what we do here.
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Old 17th Apr 2017, 9:57 pm   #3
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Default Re: Another FET voltmeter

Lots of GQRP members in here I see as well. Another one here
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Old 17th Apr 2017, 10:10 pm   #4
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Default Re: Another FET voltmeter

Sounds good! How about sharing the circuit, as I don't have that book?

Regarding your protection diode, it shouldn't be necessary! The 2N3819 is a junction FET, so taking the gate positive, it will just conduct - and the current is limited by the resistors in your attenuators anyway. Taking it negative - no current will flow till you reach the gate-channel breakdown voltage, and then you do need to limit the current, just like a Zener diode. But again, the input attenuator will do this. MOSFETs would die, JFETs not! (Though to be fair, some people have reported an increase in noise after Zenering the gate junction).

A 5.1V Zener diode has a rather soggy turn-on and with some samples of the 2N3819 and Zener, you might find that input sensitivity is compromised, so, if you do wish to add extra protection, choose a diode not far below the breakdown voltage (25V for the 2N3819).
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Old 17th Apr 2017, 10:45 pm   #5
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Default Re: Another FET voltmeter

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Circuit above. Please excuse the skewed image - smartphone camera.

Looks like I picked the original FET specified and didn't notice. First thing I did was to scribble the circuit out on paper and toss the book back on the shelf.

I checked leakage through the zener on the breadboard (with a marginally superior HP3478A ) and the FSD voltage at the FET is 1v which is way above the worrisome leakage point so it doesn't affect measurement as far as I can see. I think it resulted in a 0.005% error at FSD which is "meh" territory for a class 2.5 moving coil meter movement. I blew the front end FET up twice on the breadboard (at 100v in 1v range) and this stopped it from blowing up so I'm not 100% convinced it isn't necessary. This might be a hooky batch of 2n3819's though; they were from China off ebay

There's a version two being planned at the moment which has a FET op-amp in it (nice TI TLC27x series), ohms, current and RF power into 50 ohms ranges. It may end up with a power supply and frequency counter as well. I start with big ideas and remove the unreasonable bits
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Old 18th Apr 2017, 8:20 am   #6
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Default Re: Another FET voltmeter

Very nicely made unit, the PCB looks well made. Worth checking the alkaline cells every so often as they can leak.
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Old 18th Apr 2017, 11:55 am   #7
kalee20
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Default Re: Another FET voltmeter

Quote:
Originally Posted by MrBungle View Post
I checked leakage through the zener on the breadboard (with a marginally superior HP3478A ) and the FSD voltage at the FET is 1v... I blew the front end FET up twice on the breadboard (at 100v in 1v range) and this stopped it from blowing up so I'm not 100% convinced it isn't necessary.
You're right, 1V on the Zener shouldn't upset anything!

I'm surprised at the blowing up - after seeing your circuit, on the 1V range you still have 2M of series resistance in series with the gate! I'm tempted to find a few FETs and try and replicate your results. Thanks for sharing this saga, much appreciated! Learning a new failure mode is always worthwhile...
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Old 18th Apr 2017, 12:08 pm   #8
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Default Re: Another FET voltmeter

Indeed. I might reinvestigate. For ref I don't have the 1M at probe tip. I have two hypotheses:

1. Fudge up on breadboard means the 1M gate resistor was shorted. This is possible as the board is a hand carved matrix board in FR4 and the meter would still work if it was shorted.
2. Transients from my HV supply. It's a hefty boost converter that can generate 600v easily if the voltage feedback network has dodgy soldering so the peak inductor voltage may be higher (1kv+)
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Old 18th Apr 2017, 12:25 pm   #9
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Default Re: Another FET voltmeter

Nice meter and a simple design.

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Old 9th May 2017, 6:34 pm   #10
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Default Re: Another FET voltmeter

A minor, or perhaps major update for this.

Version 2 is in progress. I decided that FETs are monumentally boring so I've found a nice big old Sifam 100uA movement, ordered a pile of stuff from RS including an 12AU7 as the centre-piece of the new version, some high voltage capacitors and a couple of transformers to run back to back for a 115v HT winding. Oh and some chicken head knobs, some sheet aluminium, aluminium primer and black hammer finish paint

First valve project I intend to complete this one, before I try out something a little more complicated with a CRT. Scribblings so far:

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I bought 12VA transformers instead for ref.

Last edited by MrBungle; 9th May 2017 at 6:42 pm.
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Old 10th May 2017, 9:54 am   #11
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Default Re: Another FET voltmeter

If you're going the valve route, it's worth looking at the Heathkit V7-AU circuit. It was on sale for donkeys years, and works very well.

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Old 10th May 2017, 10:31 am   #12
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Default Re: Another FET voltmeter

Thanks for the tip. I have the Heathkit IM-18 schematic already here (the V7-AU successor) and am analysing it now.

This entire process was actually started after I failed to win 3 Heathkit VTVM's on ebay in a row
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Old 10th May 2017, 1:48 pm   #13
MrBungle
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Default Re: Another FET voltmeter

Brains of the outfit just arrived thanks to RS. Was surprised to find they sold ECC82/12AU7's! Chinese ones but should certainly be fine for this and don't command the Mullard+ebay price tag.
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