Thread: Meters ..Ray needed

they look good what will you use them for?
aaron

Hi Mike, I have been looking for a voltage meter and power factor meter for a monitoring board I am making for my 75kVa 3 phase generator at my shed. If you dont need one of the voltage and power factor meters would you be interested in selling or swapping for something else. If interested you could Email me on [email protected]
Thanks Mark

Mark

Send me a PM via this forum , or , a email [email protected]

I have problems sending emails for some reason , old computer !

Mike

Hi Mike,
Have sent you an Email. You can send me a PM in reply if you like. thanks again Mark

Hi Mike,

400 Hz power is aircraft stuff, common enough in a lot of avionics, you can have lighter smaller power transformers at the higher frequency.

I can't see why the volt meters wouldn't work just fine on 50Hz.

Not so sure about the power factor, but worth hooking one up and see how it goes.

For $2 each, you got a real bargain..

Regards
Ray

That's a pretty good score, they would have cost a lot to buy, just make sure they didn't come from a Ukrainian Ebay scrap dealer heh heh. You might be able to fit a shunt to the ammeter to make them 50 or 100 amp and print up a new scale for them, there's software for that on the net.

I did some reading on the net .

Apparently the really high quality AC meters use a thermo couple and have a resistor across the AC input . This gives a true RMS value .

The cheaper types uses a DC movement with a diode bridge , the meter is calibrated to give a RMS approx. reading .

It seems that AC frequency isn't a important factor as Ray said .

Mike

Originally Posted by morrisman

I did some reading on the net .

Apparently the really high quality AC meters use a thermo couple and have a resistor across the AC input . This gives a true RMS value .

The cheaper types uses a DC movement with a diode bridge , the meter is calibrated to give a RMS approx. reading .

It seems that AC frequency isn't a important factor as Ray said .

Mike

It depends on what you are measuring really, test equipment tends to have "True RMS" reading like you describe but it's only an issue if you are measuring waveforms other than sine waves (mains waveforms are sine). Your meters are a moving coil type and respond to DC only.

Even if the voltmeters are the diode bridge type, they can be set up to read sine wave AC volts accurately. If you then fed square waves or some other non sine wave into the voltmeter it would be inaccurate for that.

Originally Posted by Graziano

You might be able to fit a shunt to the ammeter to make them 50 or 100 amp and print up a new scale for them, there's software for that on the net.

Just to go a little OT, I thought when you used a shunt to measure current you were measuring the voltage drop over the shunt(thought the scale is labeled amps, its really a voltmeter), or do I have it wrong again??
Seems to me that would be more accurate than paralleled amp gauges(but I only have a basic understanding of how these things work when it really gets down to it)

Stuart

Originally Posted by Stustoys

Just to go a little OT, I thought when you used a shunt to measure current you were measuring the voltage drop over the shunt(thought the scale is labeled amps, its really a voltmeter), or do I have it wrong again??
Seems to me that would be more accurate than paralleled amp gauges(but I only have a basic understanding of how these things work when it really gets down to it)

Stuart

You pretty much have it right, ammeters that have at their heart a moving coil meter, have somewhere on the meter scale a value of current labelled F.S.D. or full scale deflection this is the value of current needed for a 100% reading it varies but is usually a very small amount like 100 microamps. If you also know the meter coil resistance you can use ohms law to work out the voltage to get a full scale reading e.g. a 100uA meter with a 2K coil will need 0.2V across the coil: V=0.0001 x 2000=0.2V.

So....... if you want a shunt to measure 200 Amps you will need a resistance that will develop 0.2V across it at 200Amps so we need a resistance of (ignoring the 2000 Ohm coil in parallel) R=V / I = 0.2/200 = 0.001 Ohms. This is bugger all resistance and is usually made by sizing a bronze or copper strip to be slightly under resistance and then trimming it by experiment to get a FSD reading.

Hi Mike et al,

good find, what else looked a bargain?

If you make medium to high current shunts, you run into practical troubles with temperature changes, shunt corrosion, and the mere fact that some shunt physical dimensions are either too large or too small.

I use 3.15 mm (1/8inch) #316 stainless steel rod, as it is a good all round material especially for temperature (thermal coefficient of resistance) which affects the meter accuracy. It is also practical for higher current shunts, as it can also carry the current without itself heating too much.
For instance, to make a 20 amp shunt for a 0-200 millivolt DC digital display (a la Jaycar inter alia), you would use 0.01 ohm, or 110mm effective electrical length of rod (physical length + allowance for terminal fitting).
Same principle for analogue meters.

73s mike ...._ __. ..._

Originally Posted by mike48

Hi Mike et al,

good find, what else looked a bargain?


73s mike ...._ __. ..._

The same seller where I found the meters , had a table of stuff ..giveaway prices . Two nice 100mhz CRO's ,with manuals and probes , HP brand I think for $20, thats $10 each

I also bought from him a 30 amp 0 to 40 volt variable DC power supply , very cheap , top quality with digital voltage and current readouts - very heavy item , 2 feet long X 18" wide , and two of us just managed to carry it to the car .

He said he bought a lump load of stuff at a Govt. surplus auction , he only wanted one item in the lot , but had to buy the whole lot !

I always look out for screws and bolts , but no luck .

Hi again

groaaaannn....

I always seem to miss these bargains....
But, did pick up a Philips PM6674 550Mhz Counter for $70 a while ago; cleaned up well, and was perfect electrically.

Oh well...
I remain ever hopeful.

_._. .... . . ._. .. ___ , __ .. _._ .

Hi Graziano,
Thanks for that.

lol Funny you should say that, you've jogged my less than wonderfull memory. A long time ago I used to cut 200amp shunts from phosphor(?) bronze, the width adjusted according to thicknees to give the speced a cross-sectional area. That was the course adjustment the fine adjustment was where the leads from the meter were soldered to the shunt(I think). Infact now I think about it more I think the meter was an amp meter so would have been just as you said.

Stuart