Analogue Meter Limitations

[Yorkie]

I don’t want to start a pointless debate about analogue v digital, and I’m not knocking Avos, which have a loyal following, and there are some applications where it’s useful to have a rising and falling needle rather than digits bobbing about in a meaningless fashion - when peaking a tuned circuit for example, using an RF probe. What I’m saying is that we need to be aware of the limitations of using analogue meters such as AVO 8s with an impedance of just 20,000 Ohms per Volt when working on modern solid state equipment.

I know that many forum members own Avos which they enjoy collecting and using to restore vintage radios, and since the original voltages will have been taken using an Avo 8 as a rule, which is 20,000 Ohms per Volt impedance - the norm for its day - the voltage readings in a working set should be similar to the datasheet. And I know that many like the idea of using vintage test gear to restore vintage radios. On valve radios, in which the voltages are quite high, the loading effect of putting a 20,000 Ohms per Volt meter across a resistor at a point at which say 100 Volts is expected would be negligible. It will have the effect of putting a 2 Megohm resistor across the circuit. (100 Volts x 20,000 Ohms).

However, on low voltage solid state gear, a multimeter with an impedance of only 20,000 Ohms per Volt can become problematical. If for example, we expect 0.5V at the base of a transistor, and we plonk an Avo 8 from the base to chassis, we are effectively putting a 10k resistor into the circuit (0.5 x 20,000 OPV). Apart from altering the voltage and giving a lower than expected reading, it may even stop the circuit from working - an oscillator for example.

To give a practical example of what happens, consider a potential divider consisting of two 100k resistors connected in series across a 12 Volt supply: We know from Ohms law that at the junction of the two resistors the true Voltage will be 6 Volts. Now consider what happens when we put a 20,000 OPV meter such as an Avo 8 across it on the 10 Volt range. Effectively, we are placing a 200,000 Ohm resistor in parallel with the 100k one in the potential divider, so the resistance drops to 66,666 Ohms, and as a result, the indicated voltage on an Avo 8 will be 4.8Volts. That will be an accurate measurement of the actual voltage of course, but not the expected or desired voltage due to the loading of the Avo disturbing the circuit. On an Avo 7, which is only 1,000 OPV, the result would be even worse – on the 10V range it would impose the equivalent of 10,000 Ohms across the resistor, so in parallel, the resistance would drop from 100,000 to just 909 Ohms, and hence, the voltage would drop to just 1 Volt. Conversely, a digital multimeter (or for that matter, a Valve voltmeter) with an input impedance typically of 20M OHM, would show the expected 6 Volts.

The formula for calculating the voltage in a potential divider is V2 = V1 x R2/R1+R2

So in the example of an Avo 8 above, the sums are: 12 V x 66,666/100,000+66,666 = 12 V x 0.399 = 4.799V due to the loading of the AVO 8. That is the actual voltage and the Avo is measuring it accurately, but it's 4.799V due to the loading effect of the AVO on the circuit. I’ve attached a sketch which perhaps makes this a little clearer.

There’s a useful parallel resistor calculator at this link:

http://www.sengpielaudio.com/calculator-paralresist.htm


Excellent digital multimeters can be had these days for pocket money. One such meter, which for me, was an impulse buy at just £10.00 at an auto-jumble last summer, is the Toolzone Full Size Large LCD Display Multimeter, which has an incredible spec at any price. I’ve attached a pic of mine. It incorporates a capacitance measurement ranges from 200pF to 20uF and an NPN/PNP. Unusually, it has a 200M resistance range – many such meters are limited to 20M. The large digits are very clear to read, and the auto shut-off is a handy feature to conserve battery power. Although it has AC/DC current ranges up to 20A, I’m not sure I’d want to leave the fairly flimsy test leads connected for very long at the top end of the current range.
Here’s the full spec.

• Auto Power Off (15 minutes)
• Big Liquid Crystal Display 3.5 x 6 cm
• Low Battery Indicator
• Continuity bleep
• Overlaod Protection
• High Impact Rubber Case with stand
• Probes and Leads
• Standard 9v Battery Supplied
Specifications:
• DCV: 200mV-20-200-1000V ........................+/- 0.5%
• ACV: 2v-20v-200 to 700V ..........................+/- 0.8%
• DCA:2m-20-200-20A..................................+/-0.8%
• ACA: 20m-200m-20A..................................+/-1.2%
• Ohms:200-2k-20k-200k-2M-20M-200M..........+/-0.8%
• CAP: 200Pf-20n-200n-20Mf.........................+/-0.2.5%
• Transistor Test: 1b 10MA Vce 3V
• Diode:3V/0.8Ma
• Dimensions: 200mm (8 inches) L, 95 mm (3 3/4 inches) W, 52mm (2 inches) H approx

There a many suppliers. Here’s one such supplier who has the meter at £16.00 with free delivery. (‘Toolzone Digital Multimeter Large’)

http://www.toolspot.co.uk/product/digita...eter-large

(No connection with this or any other commercial outfit).

Hope that’s of interest.

David

[BG405]

That's a pretty well specified meter, especially for the price. The capacitance range is a bit limited but I already have a dedicated (digital) cap meter so that's not really an issue, will be fine for most purposes. The resistance range is good, in fact even an AVO 8 doesn't give adequate resolution at those ranges! I'd be tempted to get one of those. Hopefully better quality leads will fit as that is often a problem with cheap meters.

I do prefer an AVO for "proper" work though.


Cheers,
Brian

[ppppenguin]

Please beware of using cheap meters (digital or analogue) on high voltage and/or current circuits. Leaving aside the probes which are likely to be unsafe the meter itself may not be happy for too long. Cheap meters have a lot of uses and are great for leaving around in the car or odd toolboxes but if you're dealing with real power then use something a bit safer.

[Jamie-Skinner]

Thanks! My meter was £6 and I can tell it's cheap.. I am popping to Maplin to get some supplies anyway so will get a new meter too

[Yorkie]

Please beware of using cheap meters (digital or analogue) on high voltage and/or current circuits. Leaving aside the probes which are likely to be unsafe the meter itself may not be happy for too long. Cheap meters have a lot of uses and are great for leaving around in the car or odd toolboxes but if you're dealing with real power then use something a bit safer.

It's a fair point - there's a lot of chap tat around, and generally, we get what we pay for, but this one seems to be well constructed, and I've yanked it on the floor several times, but it just bounced in its protective yellow case and carried on working. It's served me well so far, for the best part of a year or so. The test leads aren't bad, certainly as good as most of what the likes of Maplin sell, and the plugs are fully shrouded, but as I said earlier, I wouldn't want to squirt 20 Amps in them for too long! Many firms sell this particular meter for upwards of £40. I know that people often wax lyrical about Fluke meters but they too have their shortcomings, notably the fading LCD display, for which I think they did a repair kit.

David

[Skywave]

For most day-to-day workshop use, I tend to use either my Fluke 73 or 75 - except when it comes to measuring d.c. current - for which I invariably use an AVO 8. Now I don't exactly know the reason for what follows (I can hazard an intuitive guess, & I'm sure someone here will tell me why), but the Flukes often don't 'see' low d.c. currents, whereas the AVO invariably does. And I would not use a Fluke on its 10A range - I've seen too many Fluke multimeters that have been used for this, and upon opening them to investigate why there is now no displayed reading - it's not a pretty sight! :@

Al.

[AlanBeckett]

I'm sure that this block has been circled many times, although I don't think that it's any disadvantage to do it again. My view is, and has been for a long time, that you really need to know what you're doing. That's easy to say, but more difficult to implement. Particularly with vintage gear it's also very easy to get entirely the wrong impression by using modern meters. Most '50s and '60s measurements will be quoted using an AVO 8, while earlier ones may well be quoted using an AVO 7. Thus, using a modern very high impedance meter, may well give results which confuse those with little experience.
An example, although not with meters, from my past lifetime. I was summoned to Production because apparently the current batch of a particular module (mine) were all failing a very simple test. Sure enough, at first glance, that was true. However, further investigation showed that they were using a 1M Probe not a 10M one as specified.
I've actually seen Posts on other sites saying, effectively, all the voltages are high. Well, with the wrong meter they might well be true.
Alan

[Shaman]

All i can add, and it has been said, i use a vintage Avo to test voltages, excluding rf ( thats another story ) and am quite at home with these, i do have a digital Avo, its nice goes well, but still prefer my old Avo, after all vintage radios were brought up with Avo,s that is analogue types, i do not rate the newer Avo with printed circuit, nice movement thats all i can say, i also use vintage valve scopes, which were the vogue when valve radios were the norm, i do have modern transistor scopes Hameg etc, but prefer the valved scopes.....regards....Alan.

[beam me up schottky]

Good points and advice here. I can add a recommendation for good quality digital meters at realistic but not super cheap prices, the quality exceeds the price though, not a combination often seen these days ! Kewtech. They have many other useful products in their range, including mains socket test adaptors etc. There stuff can be had on ebay for less than their list price.

[tiv48]

i use both an avo 8mk7 and a fluke 25. for solid state i always use the fluke or another unbranded digi. as has been said depends on the job, i speak as some one who tried to measure the base volts of an fet chopper trasistor with an avo 8(dont do it)
rob t