FAQ - Metering current and voltage - the basics

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Richard Hull
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FAQ - Metering current and voltage - the basics

Post by Richard Hull » Tue Aug 22, 2017 4:06 pm

In this day and age of free digital voltmeters, (Harbor Frieght coupons), or list price $5.99 digital meters, (also Harbor freight), the use of old d' Arsonval deflecting needle movements seems a bit rustic. However, this old metering can be mastered rather easily such that most any old meter movement can be turned into most any metering you might require.

All d' Arsonval meters are current meters only as they deflect by magnetic action and this demands a specific current flow to make the meter needle deflect full range. We usually refer to the meter based on is manufactured printed scaling, voltmeter, ammeter, watt-meter, etc. All good meter manufacturers, regardless of printed face scaling, will usually place the meter's full scale, (FS), "current value" down low, out of direct view, just below the glass base level. Look for it by peering down on the lower left or right side of the printed scale through the glass. It might look like..... FS 50ua or FS 1ma, etc.

Once armed with this key info you can make the meter into most anything you wish. A voltmeter is made by adding a series resistor and an ammeter is made by adding a paralleled resistor. The values are determined mathematically using very simple math.

If you are smart, you can use the existing scale provided you figure the resistor to match the printed deflection range. (A 0-100ua meter can easily be made into a 0-100 volt voltmeter or a 0-100ma current meter.)
Of, course you can go through the tedium of making up a nice new scale by opening the meter and printing your own scale, then gluing it over the old scale. (A possible, but usually terrible, idea.)

Smart money latches onto a FS 50ua meter movement if wanting a meter for a fusor, though a FS 100ua meter movement is fine, too.

Let's do the math........

For a volt meter...I have a FS 50ua meter, but the scale is 0-100ua....Let's make this a 0-100 volt voltmeter

We must be cautious here. Does this meter have an internal shunt already to make it a 100ua meter or did it have the shunt externally added in its original gear?? If so, we have two paths. if internal, we have to assume it is now a 100ua meter. If it is a 50ua meter we can proceed directly as below.
The ohms/volt is a key factor in making up the proper resistor for a voltmeter. Take the FS, 50ua and find its reciprocal. 1/.000050 = 20,000 ohms per volt. Since this meter demands 20,000 ohms per volt, we multiply this by the desired voltage. 100 X 20,000 = 2,000,000 or 2 megohms. This is the value of the resistor we need to place in series with the meter to have this meter read full scale 100 volts.
Test the new meter. Place a 9 volt battery across the series connected meter and resistor. If it reads 9 volts (45ua), you have it right. However if it reads 4.5 volts, you have a 100ua meter with an internal shunt. NO PROBLEM! Halve the resistor above to 1 megohm and all will be well as the meter will now read 9 volts, (90ua). You are done.

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For a current meter.....I have a FS 100ua meter and it scale is labeled 0-100 volts. I want a 0-100 ma meter.

Here we must be very careful. Does this have an internal series resistor? Let's see. Take a 1000 ohm resistor and hook it in series with the meter. Very briefly touch a 1.5 volt battery across this series meter arrangement. If the need bangs over to full scale then there is no series internal resistor and the meter is a true 100ua meter movement.

If, however, the meter didn't even wiggle with the 1.5 volt battery, the resistor is internal and you might have to cut it loose internally or wire a short around it. A nasty business.

Let's assume it has no internal resistor and is a true 100ua meter. How do we figure the shunt (parallel) resistance needed across the meter terminals?

We need a thousand to one reduction in current on the meter. (0.1/.0001). We need to know the meter's resistance!

This is not a great way to find the meter's internal resistance but it works. Put a digital VOM on the 2k ohms range and place it momentarily across the naked meter. the needle may pin hard, but rapidly read the resistance and take the leads off. Lets say the resistance is 750 ohms. you will need one thousandth of this as a shunt resistor across the meter or .75 ohms to make this a 0-100ma meter. Such goings on here are rather imprecise and you may need to trim this .75 ohm value to a custom value like .79 or .71 ohms. The way to tell is to place the shunted meter in a circuit where 50 ma will flow. (half scale). You will need a very accurate voltage source let's say 10 volts from a variable DC supply set to 10.0 volts. 50ma will flow when a load of (10/.05) = 200ohms is present. Get a good 1% or better 200 ohm resistor and place it in series with the supply and your, now .75 ohm shunted, meter. The needle should be exactly half scale. If not, then you will need to adjust the shunt resistor value. If more than half scale, you will need to decrease the shunt resistance on your meter a tiny amount. If under half way, you will need to increase it slightly.

I, personally, use a length of 20 gauge nichrome wire wrapped around one meter bolt and tighten it well. Connect an alligator clip to the other meter bolt. With the other clip attached to the nichrome wire, place it in the circuit above and slide the clip along the wire until half scale is reached. Now, cut the wire between the meter and the clip about 2 inches longer than needed. Turn power off and remove the clip lead.

I bolt one end of this wire, wound into a coil, to one of the meter bolts and tighten it down. The other end I place between two washers on the other meter's bolt terminal and bolt it down. Hook this shunted meter into the above circuit and see if it reads to the half way mark on the meter. If it is not half scale, turn the circuit off and loosen the bolt & washers and drag more or less wire through and tighten it down. turn on the circuit. check again. If not half way, turn off the power. keep turning the circuit off and on after moving the wire in or out of the washer terminal while re-tightening until it is exactly on half scale deflection. Now you must tighten the nuts very firmly as you now have a 0-100ma meter with a suitable, fusor-ready, heavy shunt.

Making an accurate ammeter is difficult, but key to safe operation. For a fusor, it is hyper important that you do not buy and use a simple 50 or 100 ma meter movement!!!! You might be in danger of the meter opening up during a high a current surge and floating the fusor shell to thousands of deadly volts as this meter is in the ground circuit of the fusor. You must use a 50ua or 100ua meter movement and shunt it with a 2 watt or larger resistor to make it a 0-50ma or 0-100ma meter to measure fusor current. In this fashion, if the meter goes open, you still have a very low resistance path to the fusor ground in the circuit.

Example of the above is in the drawing below.

Never use a digital voltmeter VOM to measure fusor current unless you are very adept in placing it on the 200 millivolt, (0.2 volt), range and externally shunting it, much as noted above. This entire business demands a small amount of electrical familiarity.

Richard Hull
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