Analog Meter Protection

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Ron Reeland
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Analog Meter Protection

Post by Ron Reeland » Mon Aug 17, 2009 5:25 pm

Hi: I restore and tinker with old test equipment, particularly vacuum tube testers. The analog meters on these old instruments are easily damaged by pegging the pointer. In severe overloading , burning out the meter coil is the unhappy result.

Conventional wisdom is to attach a silicon diode across the meter terminals to shunt the excessive current around the meter. However, a typical diode such as 1N4007 (1 amp,1000PIV) has a forward voltage drop of .7 volts and that increases with higher current flow. Since the diode is wired across the meter terminals, the .7 volts is applied to the meter coil.

Now this may not sound like much voltage, but consider the meter on an Eico model 667 tube tester. It is rated at .0002 amps(200uamp), 1000 ohms, and .2 volts. Thus, the .7 volts dropped by the diode applies 3 1/2 time more than the rated voltage. And 3 1/2 times more current thus severly pegging the pointer and often destroying the meter. Meters on other instruments are even more critical as they may be rated at 50 uamps full scale.

I am hoping someone can furnish a schematic for some sort of protection crowbar circuit that can operate with such low voltages. The circuit cannot have any shunt resistance across the meter as that would make the meter readings inaccurrate. It must be totally passive unless and overvoltage/overcurrent situation arises. Might there exist some sort of circuit using an LED and light sensitive device to cut off excessive voltage/current? There are no fuses for such tiny currents, by the way. Thanks, Ron Reeland

Dean Huster
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Re: Analog Meter Protection

Post by Dean Huster » Mon Aug 17, 2009 7:36 pm

Check out this current discussion on the topic at the Antique Radios web site:

http://antiqueradios.com/forums/viewtopic.php?t=115731

Oh, Ron! You rascal! You started that discussion over there, too, now that I look at it more closely!

BTW, 31.25ma is the smallest fuse I have in stock.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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reloadron
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Re: Analog Meter Protection

Post by reloadron » Tue Aug 18, 2009 6:05 am

Hi Ya Ron

Would something like the overcurrent protection circuit used in the later Simpson 260 Series 6 meters work? Something LIKE THIS maybe? I know those multimeters eventually got to that protection scheme to protect the 50 uA meter movements.

Ron

Ron Reeland
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Re: Analog Meter Protection

Post by Ron Reeland » Tue Aug 18, 2009 4:04 pm

What type of solid state device is connected across the Simpson 260 meter? It is labelled V1. Is it a standard industry type? Seem as though if it can protect a 50 uamp meter that it would be suitable for use on less sensitive meter. I notice that the Simpson needs 80 millivolts for full scale reading although there is some sort of calibration pot in the circuit and also a rheostat shunt across the meter.

An Eico 667 tube tester uses 200 millivolts, but I guess a series resistor cold be added in case the Simpson safety device is limited to something under that 200 millivolts.

Ron

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reloadron
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Re: Analog Meter Protection

Post by reloadron » Tue Aug 18, 2009 4:28 pm

V1 is a varistor and this is a better LINK. The owners manual with the schematic and parts listing. Maybe that will help. Also as to the shunt resistor across the meter movement. If I recall correctly when calibrating meters like that we would apply 50 uA right at the start and adjust that for a perfect 50 uA full scale deflection. After doing the mechanical zero. Matter of fact we would first adjust the weights on the movement turning the thing every which way and get the movement balanced, Then the mechanical zero and then the 50 uA FS adjustment. Then the rest of the meter.

Ron

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Externet
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Re: Analog Meter Protection

Post by Externet » Thu Aug 20, 2009 11:21 am

Use a Schottky diode, selected by hand for the lowest forward voltage drop, or a germanium one. :smile:
- Abolish the deciBel ! -

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haklesup
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Re: Analog Meter Protection

Post by haklesup » Thu Aug 20, 2009 12:37 pm

A varistor across the meter will obviously help but is not fool proof as evidenced by parr 4.3.1 in the Simpson manual. THey don't give the value of the varistor they used either. The smallest varistor at Digikey is 5VDC. Above that voltage it starts to act like a short, below that its open. You can still fry a varistor with a very large overstress and then the meter will go next. A long duration overstress below the varistor activation voltage but above the movements rating will also do damage.

The main protection in the Simpson diagram is the relay at the bottom. One set of contacts is in series with the meter input and the other contacts are used to latch the relay when set. the circuit next to it is used to sense if an overload is occuring. It basically does that by sensing the voltage across the selected current sensing resistor. I didn't fully analyze how this circuit works but if it's about right, we can help unravel its secrets

If you want something fast acting that otherwise dosen't change the impedance of the measurement loop, you'll have to go a similar route. Put a relay in series with the meter and control the coil with a circuit that compares the voltage across the shunt resistor and a reference.

Are you trying to protect the movement when the equiptment is in normal use or just while you are servicing it. For service, I would just disconnect it and insert a DMM, make some common sense checks and once confirmed in spec, then reconnect the movement. For use, you might just perminantly substitute a digital display. If you're trying to restore this like new, then adding circuitry works against that goal.

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Bob Scott
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Re: Analog Meter Protection

Post by Bob Scott » Thu Aug 20, 2009 3:01 pm

Externet wrote:Use a Schottky diode, selected by hand for the lowest forward voltage drop, or a germanium one. :smile:
My first job out of high school was as a repair tech at hi-fi stereo store in 1969. They supplied me with a Japanese VOM. The brand name was Hioki or Huaku something. At one time it needed a new protection diode, amongst other parts like smoked resistors. I probably fried it by measuring Volts while the rotary switch was in the Ohms position. I tried a germanium diode, but it conducted at too low a voltage for that particular meter movement.

A silicon diode worked. :grin:

Bob
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