AC and meter movements

"Mike Cook"

** The simplest possible AC converter circuit use two germanium diodes, one wired across and one in series with the meter. This is what most anlogue multimeters use for the AC volts ranges.

However, the sky is the limit if you go for active circuits using FETs, op-amps and even special AC to DC converter ICs that compute the "true rms" value of all manner of AC waveforms.

.... Phil

If you have power to power the circuit something called a "precision rectifier" works well. It uses an op amp to effectively cancel the forward voltage drop of the diodes.

If you haven't got power . . . and need sensitivity, the customary way is with a "copper oxide rectifier." These are old technology but still manufactured for this purpose, (but not in your local radio shack unless it is inside a meter you purchase there.)

If you are just interested in making a meter for line voltages and starting out with a 50 micro amp movement, the drop of some forward biased germanium diodes is relatively insignificant. If you want to measure a few volts, germanium isn't going to work.

Can one make a copper oxide rectifier?

how to make a Cu rectifier Popular Mechanics - Oct 1935 - Google Books Result Vol. 64, No. 4 - 256 pages - Magazine Dry-contact rectifiers are of various types; however, all of them include a common junction between two dissimilar substances.

Some info on Cu rectifiers along with typical Fwd V drop:

"default"

** Huh ?????????

Ge diodes begin conducting at less than 100mV.

Only a matter of calibrating the scale on the lowest AC range to accommodate this.

Millions of analogue multimeters made since the 1930s cannot be all wrong - eg:

.... Phil

Curves of semiconductor diodes:

Probably loads cheaper to make meters with semiconductor diodes, but they still make copper oxide rectifiers for that purpose.

Please put me back on your plunk list - you are an insufferable egomaniacal idiot Phyllis dear.

Thanks Phil for the simple answer.

This is for measuring output of a simple variac, so yes, the germaniums seem to be the answer.

Regards, M

Small schottkies will behave about the same. If you use a big series resistor, as you will in this case, any diode will work.

John

"Mike Cook" <

** To prevent over any voltage condition on the Ge diodes, the parallel one must be connected across the meter and series diode combination.

..... Phil

"Phil Allison"

** Additional info:

** Just tested an AAZ15 Ge diode - at 1uA the forward voltage is only 13mV.

A BAT46 Schottky showed 60 mV at the same current.

As the OP had a 45 uA meter movement, this makes the Ge diode a useful rectifier to around 10mV AC.

.... Phil

Thanks Phil. It's newbie tips like this that I need.

Cheers.

- meter movement: 968 ohms; 45.8µA DC full-scale

- for 140V AC full scale, V = 0.443344, in V mode: series R = 3046120.5 ohms

- for 15A AC full scale, V = 0.443344, in A mode: shunt R =29.5 milliohms

I didn't take into account the diode drops. Should I? Where?

This is a variac in "boost" mode (hence the max value of 140V).

Thanks.

"Mike Cook"

** Very typical numbers.

** Should be 0.0443... volts DC.

** Only true for DC voltages.

For AC, you need to consider that the rectifier is half wave and the voltage is 198 volts peak and YOU want to see the RMS value on the meter. Meters like yours, however, only respond to the *average value* of the imposed current wave.

For half wave rectified sine waves, the *average rectified* current value is

1/pi the peak current - so the dropping resistor needs to pass pi times the peak current of the meter when the voltage is 140 V rms.

Makes the current: pi x 45.8 = 143.9 uA

- so the needed resistor is 1.38 Mohms.

** Should be 0.0443... volts DC.

in A mode: shunt R =29.5 milliohms

** Only works for DC current and should be 2.95 milliohms.

** Oh dear.

The AC voltmeter is easy to get right but a 15 amp AC current meter is not so easy.

The scale will be very non-linear for amps cos the applied AC voltage from the shunt will be so small.

Better invest in a Moving Iron meter for measuring AC amps - these need no rectifier at all.

.... Phil

Hmm... how non-linear would it be? It's for a variac so approximate current would be fine (best to be more accurate at max 15A).

Meter was included with the cabinet I'm installing the variac, and since it was free, at first I wasn't considering adding any metering at all.

Thanks for your help.

You can find moving iron AC ammeters on eBay for reasonable prices.

Beware, many are actually 5 amp FS (full scale) meters, designed to be used with a current transformer. They may have a printed scale that indicates a range other than 5 amps, but they are really 5 amp meters. For example, this one:

You can see in the picture in the lower right it says "FS = 5A". You can't pass

100 amps through that meter; you have to use a current transformer with it. However, if you wanted a 5 amp meter, you could open it up and change the lettering on the scale, or just remember that it's really 5 amps, and mentally adjust the reading.

But, you can sometimes find meters that are really what their scale says. For example:

I think this is a meter that can measure up to 20 amps without a current transformer.

Here's one where it's not clear whether it needs a current transformer or not:

You'd be taking a chance if you bought it expecting it to measure 30 amps without using a current transformer.

----------------------------------------------------

You can also get moving iron voltmeters which will read true RMS. For example:

This one would be perfect for you. It even has a lower range for better resolution at low voltages.

Or this one, larger and a different style:

If you're not in a hurry, watch ebay for "panel meter" for a few months and you'll find more of these sorts of meters.

"The Phantom"

** Moving iron meters always read "true RMS" - within the limits of their frequency responses.

The force on the pointer is proportional to the average of the *square* of the applied current ( hence it automatically rectifies AC) - and the scale on the face is somewhat non linear as it indicates actual current.

.... Phil

Why are you repeating what I already told the OP? I already explained to him that moving iron voltmeters read true RMS.

** Fraid you told the world that only * SOME* of them did.

Read your own words - d*****ad.

.... Phil

No, you said "You can also get moving iron voltmeters which will read true RMS."

"also"

Phil is correct.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

I have used and calibrated iron vane meters, and I don't remember them as true RMS for all waveforms. Particularly, they have a slightly different response to a DC component, which is part of the true-RMS value (although some meters have an option of RMS and AC+DC RMS). Here is some information about older RMS metering devices:

The other types mentioned, electrodynamic and thermocouple, AFAIK are true RMS with any waveform and DC component. The thermocouple type is especially good for high frequency into the RF range.

Here is more information that may be helpful for the OP:

It also mentions the electrostatic voltmeter, which also responds to AC as well as DC. But they are very specialized, expensive, and usually for very high voltages.

I found other references that state true-RMS performance of iron-vane instruments, and perhaps they do respond accurately, although they are definitely limited in frequency response to power-line and low audio signals. Without actually performing tests I will accept that iron vane types are at least better than rectifier types for RMS readings. Here is a discussion with much detail that seems to prove your assertion:

For the OP's purposes of voltage measurement, a silicon or germanium diode bridge will work well, and can be used with most commonly available meters. AC current is best measured with a CT, and there are some small inexpensive ones that will handle currents of 5 to 30 amps with a ratio of about 500:1 such as this:

The output can be rectified and provides a current of 10 mA for 5 A with enough voltage to produce a linear output through a silicon diode bridge.

Paul

So I did, and it is quite true. In addition to moving iron ammeters, you can ALSO get moving iron voltmeters, and they do indeed read true RMS. I never said anything to the effect that only some moving iron voltmeters read true RMS.

I first described moving iron ammeters which read true RMS, explained about scale problems to watch out for, and then I told him that (as well as ammeters), you can ALSO get moving iron voltmeters (the) which read true RMS.

If you and Phil think that I was saying that only some moving iron voltmeters read true RMS, that is your mistake, not mine.