Measuring Current

Well, I do approximately what you want. I have around the shop 3-4 miscellaneous meters in plain old sloping front panels. The meters are almost free at the swap meet, the cabinets are the expense, but you could fashion something yourself. Let's see, I have a 50 microamp, 1 milliamp, 500 milliamp and 5 amp units.

Leads are brought out through the top through a dual banana jack. For some frequently used current ranges, I put some shunt resistors across a stackable dual banana plug. By plugging the right shunt plug into the meter, I get some ranges I commonly use; 10 mA, 100 mA, 1A, 15A.

You can laugh, but it sure beats wiring up range switches. Too, using the dual banana jack forces you to unhook the meter from the source of power before changing the shunt, which protects the meter from an open shunt (burn-out) situation.

Hi, Jim -

For my current measurements, I bought an HP 428B (toob-type) off Ebay and repaired it. The reason I wanted it was so I would not have to break connections to measure current. Of course, one must have enough slack in the wire to allow inserting the clamp-on probe.

Just an idea. Good luck.

John (kes is not required)

I currently have one nice digital VOM. So I measure voltage, then current, back and forth having to move the cable each time, which sucks. I want a stand-alone ammeter that I can quickly place within a circuit, I think. I would rather not purchase another meter just to measure current and am thinking that this might be something easy to build but didn't find alot searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a hobbyist that likes playing around with things. One goal is to eventually build a SW transmitter. Should I try to purchase, build? I would prefer building. I am thinking I would like to be able to measure down to the microampere range.

Thanks for any input!

I read in sci.electronics.design that Jim Douglas wrote (in ) about 'Measuring Current', on Sat, 29 Jan 2005:

An analogue ammeter with a decent milliammeter movement is costly, and you have to hand-craft the shunts. You could buy a bare digital panel meter module, which will make an ammeter with 200 mV drop at full scale on all ranges. This could cost less and be easier to make.

If it were me, I'd buy another inexpensive digital multimeter.

Yup!!! I bought three of those HP 428B meters on ebay in one auction. One of them had a problem, but was easily repaired. Old technology, but they sure save the day when you need to jump around in a circuit and measure currents quickly. They are capable of current measurement from 1ma full scale to 10A full scale. Great equipment survives the ages.

I agree with the other posters that it would probably be cheaper to buy another multimeter. However, I would add that your desire to measure in the microampere range is probably wishful thinking. The equipment and techniques involved in _accurately_ dealing in those ranges are, IMHO, beyond what those of what you're likely to encounter in a typical hobbyist setup.

Or learn to avoid current measurements as much as possible. It takes time to open a connection and insert the amperemeter, and then you have to restore the connection after measuring.

In most cases you can measure voltages and calculate the currents.

Make a few homemade shunts, like 1 Ohm and 0.1 Ohm, so you can insert the shunt and measure the voltage over it, when there is no other way to find out the current. The shunt can be a resistor or a suitable piece of wire.

Thanks for all the good input. I will be picking up another VOM for my current measurements.

SW

am

In article , Jim Douglas wrote: [...]

Harbor Freight has DMMs on for $3.99. Buy one for each thing you want to measure.

Like these ones I put up (these particular ones are sold)?

The clamp opens up and you can put it round the wire. They are *not* for small currents-- scores of mA up to hundreds of A, depending.

If you put multiple wires through the core (like an AC line cord) they will read about zero, unless there is a fault to ground, as they read the difference between the two currents.

They are great for working on power stuff- you can poke around in a panel and check motor currents etc., but for electronics you really want some DVMs.

Best regards, Spehro Pefhany

I've found common multimeters fairly accurate for measuring very small currents. The typical 1Mohm input resistance at 200mV full-scale drops 200nA. (when I checked it, it seemed to be accurate). Even my cheap $4(us) meter has a 200uA range.

Err...most DVMs have a 10Meg input resistance, not 1Meg. However, one can use a resistive shunt (previously suggested) and measure the voltage across it (to 200mV full scale).

Is it possible to make a hand-held probe to do this?

Best regards, Spehro Pefhany

Though there exist probes like this that using some assumptions about the geometry of the wire, and several sensors closely spaced arouind the cord, can actually measure the current flowing through a balanced conductor.

AIUI, the probe that I referred to was 'just' another clamp-on probe, for attachment for the meter of your choice. Sorry, I don't recall when. It was probably on sci.electronics, on a thread I was involved in. "ian stirling" multiple current wire hall may or may not turn up something on google.

No, the listings you referred to are not the HP428 DC current meters that were mentioned previously. The HP428 meter uses a small clamp-on probe that saves the user from having to open the circuit to measure current. The HP428 meter has full-scale ranges from 1ma to 10A. Yes, these are vacuum tube models, but easy to maintain and use. Well worth what you'll pay on Ebay for these meters if you need to measure small currents without breaking the circuit to insert a current meter. They are not the hallmark of accuracy when compared to the digital instruments of today's technology; +/- 3% full scale on every range, but will absolutely get the job done.

The probes are not hall-effect sensors, but are esentially small transformers that are excited by a 40 Khz signal. The output of the transformer is modulated by the change in core saturation caused by the DC current being measured. You can see great pictures of the meter and probe at

I've seen the ad, but I don't think it was particularly "hand held." We decided that the instrument had to make certain assumptions about the geometry of the cable you put in there. Or maybe you had to tell it about the geometry.

In any case, I don't think they made any claims of being able to measure balanced currents in coax.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I saw some of those on Ebay today, It seems to have a scope type thing that simply wraps around a wire?? Are they pretty accurate, is the HP a tube based product?

current,