Ammeter Configurations Discussion
from the Tesla List
edited: 5-10-11
Date : Mon, 04 Oct 2004 19:09:40 -0600. - Subject : Re: Ammeter ConfigurationsAbout your amp meter, first I will assume you also own a digital multi-meter as they are not expensive and most technical people have one. Rather than do this with DC meters and diode bridges, plus opening the neutral (a safety no-no) I would offer this. Use a shunt. We want to keep the actual and heavy load off the multi- meter so let a shunt do the work. It is very easy to do.
Take advantage of the fact that your power feeds have resistance. If your
power system there in "Down Under" keeps the neutral near ground like it does
here in the States, all the better. If your target current is to be 35
amps, then simply measuring along your feed wire between two points on it will
give you all the current reading you need. If you have a 35 amp load, read
along the wire until you read 35 milli-volts, at which point you will also be
reading a .001 ohm section of the wire. Want to calibrate it easy? Place a known
5 amp load (light bulb, anything steady) and move the meter along the bared wire
until you read 5 milli-volts. Presto, you are at the .001 ohm point
between the two meter leads. You can always loan a calibrated meter to set
this up then you know your .001 ohm sample points. Sure, you could add a 1
ohm resistor in series but at 35 amps it would have to be a 1.225 kilowatt
resistor, though it would yield a 35 volt reading at 35 amps. With the tap
points on the non-interrupted wire, you can read one milli-volt per amp, direct
conversion. You then have the option of direct reading on your multi-meter
or you can take the signal into an op-amp and amplify it, feed comparators to
switch high or low limits, all sorts of things. Because you are reading the
signal across such a low resistance, to the meter it is nearly a short and noise
getting in across that short is not likely. At this .001 shunt distance on
the wire you get one milli-volt per amp. That is also typically the lowest your
meter will read. If you wish to resolve to 1/10 amp steps or between, move the
sample points further apart. If you wish 10 milli-volt reading at 1 amp, that
will be at the .01 ohm place on the wire.
I do this all the time in the system we have here. 4 digital meters with computer interface, each function has its own old computer so we have 4 readings on 4 screens and logging. We can read them across the room with ease, use direct number display or a scope history pattern. Shunts are really nice, we have several calibrated ones but you can make your own if you know the current. Amps or hundreds of amps, the only limit is the wire size. When you find the proper points, simply solder small sample wires that will go to your meter. - Regards, Mike.
Date : Tue, 05 Oct 2004 07:37:25 -0600.
Subject :
Re: Ammeter Configurations - Original poster: "Jim Lux"
One useful trick on metering AC line current is to put a standard bridge rectifier (with sufficient current capability) in series with the load. The wiring is like
AC -> line
AC -> load
+ -> to DC meter +
- -> to DC meter-
This solves the forward voltage drop problem.
Then, all you've got is the shunt/measurement problem. If you want to use cheap DPMs, they usually have a full scale of 200mV. You want that to correspond to 20A (or 2A.), so you need .2 V/20A or .001 ohms. That's about a foot of AWG 10 wire (call it 2mm diameter). A bit of tweaking of the DPM or the shunt length will get the calibration right. Don't forget that you probably want to measure RMS amps, but the meter will read some weird hybrid of peak and average (depending on filter time constants and when it samples).
To filter out the AC fluctuations, you'll want to put a capacitor across the DPM, too. If you put a series resistor in front of the cap, it will change the time constant. Most DPMs have an input impedance of several Megohms, so something like a 10k resistor and a 10 uF cap will give you a time constant of 0.1 second. (the source is effectively zero ohm impedance driving it).
----- Original Message -----
Sent: Monday, October 04, 2004 7:16 PM
Subject: Re: Ammeter Configurations
> Original poster: "Ozone Junkie"
>
> G'Day Again,
>
> The problem is in getting a 0-250 full scale AC voltmeter. I have scoured all my usual parts sources here in Aus, and can't find any AC meters for affordable prices. The coil is only a 12/30 (360VA) but i want to be able to read upto about 6A (1.44kVA). From what I thought, there is no point in trying to build a small current transformer for this small amount.
>
> That is why I am trying to use the smaller, cheaper, easier to get DC versions. I was originally using 0.1r across a digital panel meter, but it has died a rapid death. I am trying to avoid using external DMM's for a 'neatness' factor, but I am thinking that that might be the only way.
>
> Tristan
>
> On Mon, 04 Oct 2004 19:06:38 -0600,
> > Original poster: "Dr. Resonance"
> >
> > Tristin:
> >
> > This is how we do it. Since the coil is powered by an AC transformer, just use a 0-250 full scale AC voltmeter that the dial faceplate is re-calibrated in 0-16,363 Volts. We usually round this off to 16,500 VAC full scale. In normal operation the meter will read 14,400 Volts at 220 Volts input. This is the Erms potential and the cap will fire at 1.414 * Erms actually this depends on your sparkgap setting for exact value).
> >
> > A cheap easy way to accomplish the task and no messy HV dividers necessary. Also, very accurate for TC work requirements. We also do this with NSTs, just using a different value. The math is a simple ratio proportion:
> > X = (250 V. * 14.4 kV) / 220 VAC X = 16.353 kV fullscale reading for a pole xmfr
> >
> > or X = (150 VAC * 12 kV) / 120 VAC X = 15 kV fullscale for a 12 kV NST
> >
> > Dr. Resonance
> >
> > Resonance Research Corporation
> > E11870 Shadylane Rd.
> > Baraboo WI 53913 > >
> >
> <SNIP > >
Date : Tue, 05 Oct 2004 07:40:03 -0600. Subject : RE: Ammeter Configurations - Original poster: "Lau, Gary"
I would recommend the use of an analog meter over a digital multimeter for a couple of reasons.
I have not tried it to measure current, but I have seen my multimeter wink out to never-never land when I monitored my mains voltage, due to the transient spikes present. Happily it came back after power cycling the meter (I GOTTA get some MOV's!). I can't say whether the same will happen when measuring current, but I personally wouldn't risk my DMM. DMM's are good for measuring stable readings, but the load current to a Tesla coil will vary widely, rapidly, and randomly, and the display will be an utterly useless jumble of numbers. An analog meter will time-average the fluctuations and provide a far more useful reading, as well as being more tolerant of transient spikes.
Regards, Gary Lau MA, USA
Date : Tue, 05 Oct 2004 23:25:20 -0600. Subject : Re: Ammeter Configurations - Original poster: "Mike"
Hi Gary and all:
Returning the message from work here. Reading in the current mode the meter is
so heavily shunted spikes are not a problem. This is done on large plasma arc
discharges and I would say the trash on the line is about as bad as a coil. Same
variables. Using larger value "resistors" I can get enough rise voltage
(especially at
the current I pull, testing the rating of 3 pole pigs on our 3 phase drop) to
drop a dc signal into small averaging cap. A one or two second time constant
gives nice readings, the meter never sees the trash. Analog meters are nice, I
like them too but in our automated reading system the computers want a digital
interface. I meter the HV earth grounded DC output this way also and those are
~860 Amp large cap dumps to plasma with
2600 volts. Well, I guess he will do what he will do. Regards, Mike.One useful trick on metering AC line current is to put a standard bridge rectifier (with sufficient current capability) in series with the load. The wiring is like
AC -> line
AC -> load
+ -> to DC meter +
- -> to DC meter-
This solves the forward voltage drop problem.
Then, all you've got is the shunt/measurement problem. If you want to use cheap DPMs, they usually have a full scale of 200mV. You want that to correspond to 20A (or 2A.), so you need .2 V/20A or .001 ohms. That's about a foot of AWG 10 wire (call it 2mm diameter). A bit of tweaking of the DPM or the shunt length will get the calibration right. Don't forget that you probably want to measure RMS amps, but the meter will read some weird hybrid of peak and average (depending on filter time constants and when it samples).
To filter out the AC fluctuations, you'll want to put a capacitor across the DPM, too. If you put a series resistor in front of the cap, it will change the time constant. Most DPMs have an input impedance of several Megohms, so something like a 10k resistor and a 10 uF cap will give you a time constant of 0.1 second. (the source is effectively zero ohm impedance driving it).
----- Original Message -----
Sent: Monday, October 04, 2004 7:16 PM
Subject: Re: Ammeter Configurations
> Original poster: "Ozone Junkie"
>
> G'Day Again,
>
> The problem is in getting a 0-250 full scale AC voltmeter. I have scoured all my usual parts sources here in Aus, and can't find any AC meters for affordable prices. The coil is only a 12/30 (360VA) but i want to be able to read upto about 6A (1.44kVA). From what I thought, there is no point in trying to build a small current transformer for this small amount.
>
> That is why I am trying to use the smaller, cheaper, easier to get DC versions. I was originally using 0.1r across a digital panel meter, but it has died a rapid death. I am trying to avoid using external DMM's for a 'neatness' factor, but I am thinking that that might be the only way.
>
> Tristan
>
> On Mon, 04 Oct 2004 19:06:38 -0600,
> > Original poster: "Dr. Resonance"
> >
> > Tristin:
> >
> > This is how we do it. Since the coil is powered by an AC transformer, just use a 0-250 full scale AC voltmeter that the dial faceplate is re-calibrated in 0-16,363 Volts. We usually round this off to 16,500 VAC full scale. In normal operation the meter will read 14,400 Volts at 220 Volts input. This is the Erms potential and the cap will fire at 1.414 * Erms actually this depends on your sparkgap setting for exact value).
> >
> > A cheap easy way to accomplish the task and no messy HV dividers necessary. Also, very accurate for TC work requirements. We also do this with NSTs, just using a different value. The math is a simple ratio proportion:
> > X = (250 V. * 14.4 kV) / 220 VAC X = 16.353 kV fullscale reading for a pole xmfr
> >
> > or X = (150 VAC * 12 kV) / 120 VAC X = 15 kV fullscale for a 12 kV NST
> >
> > Dr. Resonance
> >
> > Resonance Research Corporation
> > E11870 Shadylane Rd.
> > Baraboo WI 53913 > >
> >
> <SNIP > >
Date : Tue, 05 Oct 2004 07:40:03 -0600. Subject : RE: Ammeter Configurations - Original poster: "Lau, Gary"
I would recommend the use of an analog meter over a digital multimeter for a couple of reasons.
I have not tried it to measure current, but I have seen my multimeter wink out to never-never land when I monitored my mains voltage, due to the transient spikes present. Happily it came back after power cycling the meter (I GOTTA get some MOV's!). I can't say whether the same will happen when measuring current, but I personally wouldn't risk my DMM. DMM's are good for measuring stable readings, but the load current to a Tesla coil will vary widely, rapidly, and randomly, and the display will be an utterly useless jumble of numbers. An analog meter will time-average the fluctuations and provide a far more useful reading, as well as being more tolerant of transient spikes.
Regards, Gary Lau MA, USA
Date : Tue, 05 Oct 2004 23:25:20 -0600. Subject : Re: Ammeter Configurations - Original poster: "Mike"
Hi Gary and all:
----- Original Message -----
Sent: Tuesday, October 05, 2004 9:40 AM - Subject: RE: Ammeter Configurations - Original poster: "Lau, Gary"
>
> I would recommend the use of an analog meter over a digital multimeter for a couple of reasons. I have not tried it to measure current, but I have seen my multimeter wink out to never-never land when I monitored my mains voltage, due to the transient spikes present. Happily it came back after power cycling the meter (I GOTTA get some MOV's!). I can't say whether the same will happen when measuring current, but I personally wouldn't risk my DMM. > DMM's are good for measuring stable readings, but the load current to a Tesla coil will vary widely, rapidly, and randomly, and the display will be an utterly useless jumble of numbers. An analog meter will time-average the fluctuations and provide a far more useful reading, as well as being more tolerant of transient spikes. - Regards, Gary Lau - MA, USA
- 5-22-11
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