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Old 17th Apr 2017, 6:46 pm   #1
MrBungle
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Join Date: Jun 2016
Location: London, UK.
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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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