Metering current and voltage - the basics

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Richard Hull
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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, wattmeter, 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, 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. You are done.

...................................................................................................

For a current meter.....I have a FS 100ua meter and it is label 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 not series internal resistor and the meter is a true 100ua meter movement. The gear it was in demanded an external resistor.

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 5% 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 it. 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 tapped length of nichrome wire and slide an alligator clip along the wire until half scale is reached and cut the wire about 1 inch longer than needed. I bolt one end of this wire in a coil to a small blank circuit board leaving one end open through a bolt and then repeat the test until half scale is reached by sliding it through the bolt and then tighten down the last bolt. Your special custom low ohm resistor is now finished.

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

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

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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