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Old 23rd Mar 2017, 10:04 pm   #1
dragonser
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Default High impedance DIY analogue voltmeter design

Hi,
while looking through some items I inherited over ten year ago, I found a metal box with a 1ma Shinonara panel meter and the beginnings of a simple voltmeter using a 741 op amp, zero set control, and range switch.
I thought that the box and meter could probably be reused as a high impedance voltmeter with some major modification.
I thought that there is probably a design on the web and after searching for a bit I found the following article
from December 1986 PW.
http://billingtonrepairs.files.wordp.../voltmeter.pdf

now there are possibly better op amps available, but the meter used in the PW design is a 100ua meter, but I have a 1m/a meter.
So I guess my questions are, would it be possible to modify the design to work with a 1 m/a meter ? and would it be worth using a more modern op amp ?

regards Peter B
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Old 24th Mar 2017, 12:17 am   #2
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Default Re: High impedance DIY analogue voltmeter design

You would need to change R19 to 100 Ohms.

The 1mA FSD current through the meter would then generate the same voltage at pin 2 of IC1 as the original design.
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Old 24th Mar 2017, 10:51 am   #3
David G4EBT
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Default Re: High impedance DIY analogue voltmeter design

I built one of these and it works well. I had a 4" square 100uA meter which gives better accuracy from a reading point of view than the 2" one that featured in the article. As the project uses the 1:5 decades, I re-scaled the dial accordingly. The meter works really well and is spot on when checked with a calibrated voltage source.

Enthusiasts of AVO 8s and other 20,000 Ohms per Volt meters, and AVO 7s (1,000 Ohms per volt), might find the PW article interesting in explaining how inaccurate they are when reading low voltages in some circumstances. In essence, they do read the actual Voltage, but that Voltage is much lower due to the parallel resistance of the AVO across a resistor in a potential divider.

In the example given in the article (to make the case for a high impedance Voltmeter), 12 Volts is applied across a potential divider consisting of two 100k resistors. We know from Ohms Law that there should be 6 Volts at the junction of the resistors. Using and AVO 8, it would show 4.8V, and using an AVO 7 would read just 1Volt. With the PW Mosfet meter (11 Meg Ohm input), it would read 5.97Volts. The article explains the calculation of the worked example.

I've attached a few pics of my PW meter and the AC & DC probes. I made a cob-jointed wooden box to suit the meter size. The AC probe is useful for measuring RF voltages, and for peaking up tuned circuits, with a Voltage limitation of 10V p-p. The PCB itself is quite small and is mounted on the meter connections.
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Old 24th Mar 2017, 11:35 am   #4
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Default Re: High impedance DIY analogue voltmeter design

Hi,
thank you for the replies, they both help.
regards Peter B
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Old 24th Mar 2017, 11:52 am   #5
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Default Re: High impedance DIY analogue voltmeter design

If you use a rail to rail op amp you won't need IC2. If you change the polarity switching method.
 
Old 24th Mar 2017, 3:50 pm   #6
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Default Re: High impedance DIY analogue voltmeter design

Quote:
Originally Posted by dragonser View Post
I thought that there is probably a design on the web and after searching for a bit I found the following article
from December 1986 PW.
http://billingtonrepairs.files.wordp.../voltmeter.pdf
As we have come to expect, there are some errors and omissions in the PW article. The "range" switch is describe as 1p, 12w in the "shopping list" but only 10 ways are used and how this is achieved is not mentioned (yes, I do know how). Also, on the full circuit diagram, positions 10V to 500V are misplaced and 5V is missing. Again in the "shopping list" the CA3140E is listed as just CA3140 and only one is actually listed. This is OK if you know what you are doing, I suppose, plus the device looks quite useful.

Merlin, what do you mean by "if you change the polarity switching method"?

Colin.

Last edited by ColinTheAmpMan1; 24th Mar 2017 at 3:55 pm. Reason: Dodgy memory
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Old 24th Mar 2017, 5:46 pm   #7
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Default Re: High impedance DIY analogue voltmeter design

Positive volts, reference meter and input to 0V, negative volts reference meter and input to +volts. And with a modern R2R opamp the current draw can be so small (when idle) as not to need an on/off switch. Of course it could be made as a positive only meter relying one to switch the probes manually, this is the method I prefer, I often miss the - sign on a DMM, no doubt at all on an AVO.
 
Old 24th Mar 2017, 6:21 pm   #8
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Default Re: High impedance DIY analogue voltmeter design

I've built one of these bodged from the design in EMRFD and it works pretty well. Not quite as fancy but it does what I need and is startlingly linear and accurate! Excuse the absolutely terrible labelling job.

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I have modified this slightly but it's about right - the divider is different and the FSD is 200mV on the input and the opamp has gain. Uses a TLC272 instead of a CA3140 as that's all I had lying around. Meter movement was from Rapid. Enclosure and rest of contents from the junk box, bitsbox and Tayda.

Thing you have to watch out if for is that the input offset potentially 5-10% of the FSD of 200mV so you need to be able to trim this out so make sure you add the offset trim pot to the opamp (see datasheet), short the input and zero the meter with that trimpot inside before you set the range pot.

Does need an on/off switch or you risk leaky batteries when you chuck it in the drawer and just use your Fluke which is what happened to me

Edit: divider is: 9 Meg (4.3M+4.7M), 900k (1.8M||1.8M), 90K (180K||180k), 10K totalling 10M. Op-amp gain was picked experimentally. Didn't write the values down. The whole thing is constructed on the back of the meter and the range switch - there is no PCB or protoboard.

Just to add, I didn't want more than 200v on ranges as it uses an RCA jack and no shrouded leads. I would suggest everyone else takes that precaution when they build one of these things. It'll probably never see more than 24v or so to be honest. Also works with the RF probe I described here as I swapped the connector out for an RCA jack: http://vintage-radio.net/forum/showthread.php?t=132296

Built a matching transformer in a box too (2x 6v AC @ 6VA - regulators go on the target circuit!):

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Old 24th Mar 2017, 7:30 pm   #9
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Default Re: High impedance DIY analogue voltmeter design

Quote:
Originally Posted by David G4EBT View Post

Enthusiasts of AVO 8s and other 20,000 Ohms per Volt meters, and AVO 7s (1,000 Ohms per volt), might find the PW article interesting in explaining how inaccurate they are when reading low voltages in some circumstances. In essence, they do read the actual Voltage, but that Voltage is much lower due to the parallel resistance of the AVO across a resistor in a potential divider.
As one such enthusiast - and the Avo 7 is actually only 500 ohms per volt unless the 'divide by 2' button is pressed - the reason for using such an insensitive meter is not to measure the true voltage, but to replicate the test conditions used in the manufacturer's or Trader service sheet when comparing voltages during fault-finding in vintage radios with those expected, by ensuring identical meter loading. Using a higher sensitivity meter in some circuit positions, such as the local oscillator anode, will give an erroneously high reading, and send you on a wild goose chase looking for non-existent faults! Another benefit of meters of the Avo type is the long scale length, and accurate readings can easily be taken even when the pointer is within the bottom 5% of its travel, as long as the movement is properly zeroed beforehand. Hence using an Avo on the 400 volt or even 1,000 volt range to measure voltages round a valve set is perfectly feasible, and reduces the loading accordingly.

Some manufacturers' pre-war service manuals give two sets of valve electrode voltage readings, one with a 36-range Avometer (167 OPV, from memory) and a second set for the lucky few service engineers who were lucky enough to have access to a new Avo Model 7 after 1936!

I'm told that the reason the Avo 8 continued in production for so long (2008?) was that its use was stipulated in numerous MoD servicing manuals for avionics etc.
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Old 24th Mar 2017, 8:16 pm   #10
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Default Re: High impedance DIY analogue voltmeter design

Also it's possible to measure voltage with an analog meter without loading the circuit as such by using the meter as a null indicator along with a variable voltage source.

I seem to remember 50kohm per volt meters were available and possibly 100kohm per volt ones?

Lawrence.
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Old 24th Mar 2017, 9:19 pm   #11
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Default Re: High impedance DIY analogue voltmeter design

Yes you're right, I have 30k, 40k and 100k OPV meters in my collection. The 100k meter actually isn't as useful as you might think, as the 10uA movement is quite heavily damped (presumably for its own protection) and takes rather too long to settle at its final reading.
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Old 24th Mar 2017, 9:22 pm   #12
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Default Re: High impedance DIY analogue voltmeter design

Thanks Phil for the clarification regarding the 100kohm per volt meter.

Lawrence.
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Old 24th Mar 2017, 9:36 pm   #13
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Default Re: High impedance DIY analogue voltmeter design

Quote:
Originally Posted by Phil G4SPZ View Post
Yes you're right, I have 30k, 40k and 100k OPV meters in my collection. The 100k meter actually isn't as useful as you might think, as the 10uA movement is quite heavily damped (presumably for its own protection) and takes rather too long to settle at its final reading.
30k, 50k and 100k here- the 100k/V one (Simpson 269) is a tad slow and ponderous off the mark, but has useful ranges, a lovely big scale and, with 1.6 and 4kV scales, is good for 'scope stuff (or less bad than many).

Maybe I shouldn't say things like this, but the 30k and 50k multimeters in question were widely sold and can be fairly easily found secondhand and would at least offer a get-out-of-jail option for a well-known valve tester with a particularly sensitive movement that is prone to failure, the circuit arrangement meaning that the basic meter movements are more sensitive than the 33.3 and 20uA that might be assumed.
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Old 24th Mar 2017, 9:39 pm   #14
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Default Re: High impedance DIY analogue voltmeter design

Going back to PW, some years ago they ran an excellent series of articles called "Are The Voltages Correct?" which was also later released as a reprint. It covered fault-finding using your multimeter, and included numerous examples of how meter loading affects readings in both valve and transistor circuits. PW did tell me that they intended to release this, together with several other reprints, on CD-ROM but never confirmed a release date. If they received a few more enquiries, perhaps they'd get a move on...
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Old 25th Mar 2017, 12:47 am   #15
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Default Re: High impedance DIY analogue voltmeter design

I was given a HeathKit moving coil meter that was out of a Heathkit IM-17. I only had the meter and nothing else, so I rebuilt the thing in a plastic box and it looks quite nice.

The original meter only had limited ranges, 1, 10, 100, and 1000 Volts, so I extended it to 1, 5, 10, 50, 100, 500, and 1000 Volts

I used the same transistors as specified in the original design, which was an FET for the input and a couple of general purpose transistors for the balanced bridge. It also used the same transistors connected as diodes as an overload protection

I was very surprised how accurate it was, at least when compared with several of my DDM's. It was also very linear across the scale.

The original design used carbon composition resistors I believe, but I used some 5% metal foil resistors and had to connect some of them in series to get the right value, which gave me a chance to mix and match to achieve better accuracy.

The original device could measure both AC, DC, and resistance, but I decided only to use AC and DC. The DC input is 11M, and the AC as you might expect was only about 1M.

I checked the frequency response on the AC and found it was very linear between about 20 Hz to about 15 KHz, just using a regular 1N4007 for the rectifier.
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Old 25th Mar 2017, 9:57 am   #16
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Default Re: High impedance DIY analogue voltmeter design

If you replace that 1N4007 for a UF-series diode you should get 100KHz out of it easily.
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Old 25th Mar 2017, 12:11 pm   #17
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Default Re: High impedance DIY analogue voltmeter design

I'm sorry, but if a meter so sensitive that it is easily destroyed is used in a piece of equipment with mains power available to it, then it comes down to incompetent design.

A couple more windings on the power transformers, an ECC82, and a 1mA movement and the AVO VCM would have been far better, and had better leakage scales.

Since the 70s with opamps for pennies, the only reason for high sensitivity movement would be the avoidance of batteries, but if you want an Ohms range, that reason vanishes, and you might as well have an electronic meter... but watch out for self-rectification of RF if you work on/near transmitters.

I Had an AVO CT38 at first.
A TMK was more portable
Then I built a homebrew one based on LM308s and an HP 1mA large format meter.

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Old 25th Mar 2017, 2:57 pm   #18
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Default Re: High impedance DIY analogue voltmeter design

That's a very good point- I'm not familiar with the circuit in question, but surely whatever parameter was driving the infamous 30uA meter could have been used to alter the balance of the classic twin-triode VVM and mA-range meter circuit. I guess it reinforces the point that AVO VCMs aren't quite the holy, sacred and incontestable arbiters of matters thermionic that some circles revere them as, but just another piece of workshop test-gear subject to thrifting and price-pointing.

The op-amp circuits under discussion are, after all, a development of the classic twin-triode VVM circuit that strives to get to equilibrium under external influence.
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