amps adaptor for DMM

My DMM gives erratic readings on the 10-amp range. I'll bet that happens to a lot of meters. If you try to check voltage without noticing your lead is plugged into the 10-amp jack, you cook the circuit in an instant.

How about measuring current with an external load of 0.1 or 0.01 ohm? A measured length of wire wrapped around a stick could provide the resistance and dissipate a couple of watts. Before wrapping, I could fold the wire at the middle (double it) to avoid inductive pickup.

Maybe there's a fuse with a suitable resistance. What's a good way to make a resistive device for measuring 10 amps or so?

Reply to
Sawney Beane
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The rather obvious answer is that you have a low value precision resistor rated for the current already - in the meter and connected to the 10A terminal.

The other answer is to use a clamp type multimeter. Mine certainly measures dc and ac amperes well enough for me. It doesn't get blown up and has several other advantages too.

However, if your real problem is because you can't get into a good routine with your meter:

What you could do is to use a set of fused leads, at all times. The fuse will drop a voltage that varies with current as it heats - but that typically won't matter as you won't be drawing much current when measuring volts on a volts range. On a current range, however, and the fuse will go bye-byes, hopefully before the meter and/or the circuit being measured does.

Using a fuse as a precision resistor is not going to be a recipe for success - particularly at any currents even remotely near the fuse rated current.

Reply to
Palindr☻me

You can buy or make a current shunt that yields a known voltage across it when a given current is drawn through it. The disadvantage of buying is cost; the disadvantage of making one is accuracy. If you have access to the needed equipment, and are careful, you can make a shunt that will be accurate enough for most purposes.

The shunt works just like you said - it is a known low value, high dissipation resistance - except it is placed in series with the load, it is not the load itself.

Ed

Reply to
ehsjr

I was going to call my external adaptor a shunt, but isn't a shunt a bypass? I couldn't think of a term for a small resistance placed in series with the load.

Reply to
Sawney Beane

I wouldn't call it a precision resistor. It's a couple of inches of wire, maybe 14 gage. A lead going to the milliamp input of the DMM is connected to that wire at a certain point.

The resistance of the milliamp input is 10 ohms, so they are looking for 0.01 from the heavy wire. The lead is attached about halfway along the wire. Instead of installing a precision resistor, they install a wire of about 0.02 ohm. At the factory they probably run a known current through it and use a volt meter to find the point to tap.

Whatever the wire is made of, it has gotten hot. I assume hairline cracking causes the erratic readings.

I'd be out of business until I could dig up a suitable fuse. If it happened only every three years, it might take me a long time to find the spare fuses.

A meter set up for current looks pretty much like the same meter set up for voltage, so every once in a while I'll make a mistake. If I had an adaptor for current, it would not look like a device for measuring voltage, so I would be very unlikely to shunt it across the load.

If I had a 40-amp fuse, how many amps could I pull without significantly increasing resistance? How about a breaker?

If I found a fuse with .0091 ohm, for example, I could add 10% to my millivolt readings to get amps, or I could add .0009 ohm of wire to bring it up to .01 ohm. I could determine the exact resistance by running it in series with an accurate ammeter.

Reply to
Sawney Beane

If it is a quality instrument with a calibration certificate then, IMHO, the resistor value is certainly precise. I would agree that they get that precise value by individual "tuning", rather than actually buying precision resistors.

I have never had erratic current readings, other than when the current was erratic. But then, I use a clamp ammeter.

Presumably you would also be out of business if the meter went u/s, or you dropped it, or whatever? I would maybe tape a spare fuse to the test prod, if I only had one meter and one set of leads. The fused test leads I have take normal 1 1/4" fuses - I think. It has been a long time since I used them.

Very true. But a clamp ammeter means no breaking the circuit, no bare wires, no worries of a loose connection, no worries of overloading the meter, is much quicker to use, etc.

No idea. Some fuses are incredibly ingenious internally. Breakers even more so. A simple bit of copper wire would be more predictable. For higher current, a bit of copper sheet, with sawcuts used to get the right resistance, works well.

Finding a fuse of exactly that value would possibly take longer than hacksawing slots in a small copper sheet.

Reply to
Palindr☻me

Both are correct descriptions. Sometimes it is a bypass, and sometimes it is a small resistance in series with the load.

You can make them from #12 solid wire, cut a little long by formula, and connected with a slider (which is used in setting the exact point on the wire, then soldered). You might do better (I don't know if you will - it's just a thought) by taking one out of a cheap meter from Harbor Freight. They go for $2.99 when they are on sale, and include a 10 amp scale.

In fact, those cheap meters are surprisingly effective. They are likely suitable for most measurements you will make, and unless the circuit is critical, any lack of accuracy is irrelevant. I wouldn't use one if I needed to know the exact value, but I have used 4 of them plus two good DMMs when testing output regulation on a DC-DC converter I made that has dual regulated outputs. First, I set the ouput V for each output with the Fluke. Then I set up the 4 meters to monitor output current & voltage, with the good meters monitoring V and I on the input. That way, I got precise input readings as I varied Vin, and only needed to look for a change on the output. The same setup was used with fixed Vin and varying loads on the outputs to record Iin variation. In use, I did not notice any glaring innacuracies.

Ed

Reply to
ehsjr

I guess the adaptor would be a bypass for the meter.

At 10 amps, 0.01 ohm would produce 1 watt.

with 12 gage I would need 185 cm. 14 gage 117 cm. 18 gage 46 cm. 20 gage 29 cm. 22 gage 19 cm. 24 gage 11.4 cm.

To me, they all sound long enough to dissipate 1 watt without getting hot. (At 10 amps, the wire in my meter dissipated about 2 watts in about 5 cm.)

The 12 gage would dissipate heat from higher currents, and fabrication errors would affect accuracy less. I wonder how much variance is normal from the nominal gage and the nominal resistivity of copper. If manufactured wire is consistent, I could simply tap two points 46 cm apart on 18 gage wire, for example, without trial and error.

Thanks, I'll look. Some cheap meters function reliably for a long time.

Reply to
Sawney Beane

Ahh... dropping meters! It brings back memories of the Simpson

260. Bulletproof! It became the standard in the American armed forces in WWII, and it's still in demand. In 1946, Ray Simpson moved his production to Indian reservations because he found their workmanship superior. The Lac du Flambeau Band of Lake Superior Chippewa Indians purchased Simpson twenty years ago.

You got me interested. Clamp ammeters for DC look expensive.

I meant if I happened to find a .0091-ohm fuse, that's what I could do. A suitable length of wire could be added to any fuse with less than .01 ohm. I'd be less likely to short a source with an adaptor than with a DMM, but if I ever did, the fuse could prevent damage to the wiring or the source.

Reply to
Sawney Beane

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