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I'm a hobbyist/newby and I buy some items to build a cheap nichrome wire heater.First I buy a 50feet of 22gauges nichrome wire. For 120V AC source , I need 8feet of this nichrome wire to get desired temperature and this consume near 13A. Second I buy a SCR voltage regulator to controle the heat of my wire. And recently I buy a two others items , one to keep a eye on volts with a analog voltage indicator (150V AC max) and another one for amps with a analog amps indicator (35A AC max).

I wire for testing purpose the SCR regulator to a AC motor and put the volts indicator in parallel. Everythings goes fine from here. But when come time to connect amps indicator , I don't what to do exactly with it. I rectifying the current with a diode before does in the amps indicator in series ... the needle stop to giggles but snaps really fast at the maximum ( 35A ) end. If I put a 0.15omhs resistor in parallel to the amps indicator , the needles move more slowly and stop at ~20A ... but with a Multimeters I know the true value is ~2A for now.

So there a way to connect correctly this kind of indicator ? I going in wrong way to find the good resistor ohms value until I get the good reading value ? No instructions come with the amps indicator. Here is a shematic of what I'm doing with the diode and the resistor :

enter image description here

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  • \$\begingroup\$ I think a True-RMS meter would be in order here... \$\endgroup\$ – ThreePhaseEel Aug 7 '15 at 22:36
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The output of the SCR regulator at less than full voltage will be a short positive voltage pulse and a short voltage negative voltage pulse during each cycle of the input voltage. The meter may not respond well to that waveform. The multimeter may not be accurate either, but it might be better designed for a distorted waveform than the panel meter. Inserting the diode into the circuit will convert the current through the meter to DC. That may interfere with the SCR regulator and cause it go to full output, but the diode would only pass the positive half cycles of the AC from the regulator.

You could test the meter by connecting it to measure the current through an ordinary AC load without the regulator. If it works properly with 10 amps AC without the regulator, it may work properly at higher current from the regulator without the diode and resistor in the circuit.

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  • \$\begingroup\$ without the regulator and resistor, the needle snap to the maximum ( 35A ) really fast. If I remove the diode , the needle go back and forth really fast from 0A to 35A. \$\endgroup\$ – Jean-Francois Gallant Aug 10 '15 at 14:49
  • \$\begingroup\$ It seems like the ammeter is really a 5 amp ammeter designed to work with a 35:5 or 30:5 amp current transformer. You can verify that by connecting to a load that draws 2 amps without the regulator. There are a lot of people selling cheap products from China that they don't describe properly because they don't understand the Chinese description or they don't understand the product and can't describe it in the language they are using to describe it. It could be a DC meter also. You need to learn how to test and analyze the meter. \$\endgroup\$ – Charles Cowie Aug 10 '15 at 15:22
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That current meter does not look to me like it is designed to directly handle 35A, nor does it say whether it is AC or DC on the faceplate. Frankly, it looks like a moving coil DC meter with a low current coil rather than an AC moving iron type.

It may be designed for use with an external shunt of some description and DC current.

You might be better off throwing it away and getting a better panel meter.

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  • \$\begingroup\$ I just want a approximative visual of the current going through the circuit, it's with I buy this cheap one. The description of the item said it's AC ... but like you said I'm not sure of that and it's why I ask the question here if nobody know how to connect this kind of meter. You have a link of a cheap ammeter with AC moving iron type can mesure 0A to 20A ? \$\endgroup\$ – Jean-Francois Gallant Aug 10 '15 at 14:53
  • \$\begingroup\$ You can probably fiddle with the 'shunt' and get it to read similar to your other meter. Maybe that's good enough for your purposes. 15m\$\Omega\$ is about 1.5' of AWG 20 copper wire. It will change with temperature etc. but for a rough 5-10% indication probably fine. \$\endgroup\$ – Spehro Pefhany Aug 10 '15 at 15:50

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