Thread: Thermal Ammeter

I want to purchase a thermal ammeter. However, I have been unable to locate a manufacturer or distributor of this type of ammeter. I might be using the incorrect name for this instrument. A thermal ammeter is an ammeter that uses the heating effect of a current to determine the current that is running through a circuit; this method is instead of the more common method of using an ammeter that measures the deflection of an electromagnet in which the current runs through.

David

Why do you want this unusual type of ammeter? What will it do that an ordinary one can't?

I guess you could make such a measuring device using a thermistor - current flowing through it will cause it to heat and reduce its resistance.

If you want a true RMS current measurement, then heat is the way to do this.

You need to isolate the resistance measurement from any AC flowing through - e.g. a series inductor with a capacitor across the ohmmeter will do this

An alternative, although clumsy, may be to get hold of an old HP RF Power meter. These use a "head" based on thermal power measurement (RMS). The input impedance will be 50 Ohms so you need to convert power in watts or dBm to . However these run off mains and are not isolated if you need high side current measurements.

Depending on the voltage differential (i.e. not Tesla values!) you might be able to make a simple toroid transformer to provide isolation to a few 1000 volts.

I hadn't heard of these before but this is what I found out with a little bit of googling.

The thermal ammeter was more commonly called a hot-wire ammeter. They worked by thermal expansion of a heated wire. These are basically antiques. They used to be used to measure alternating current because the analog meter movement is d-c. Nowadays it is a simple matter to rectify the a-c and use the d-c meter movement for either a-c or d-c.

A variation of the thermal ammeter was the thermocouple ammeter. This used a thermocouple to generate a d-c voltage from the heated junction which allows the use of the d-c meter movement. They were used up through WW2. Maybe some antique radio buffs could help you.

http://www.vias.org/basicradio/basic_radio_06_05.html

http://www.antiqueradio.com/Jul04_Bi...oastguard.html

Interesting research Harold14370. The thermal approach is best for RMS detection and NTC or PTC thermistors could be used representing a more modern approach. AC current flowing through either causes heat and therefore a measurable change in resistance. They are cheap components available from Farnell, RS etc for less than $1.[b]

Diodes, biased or unbiased are also OK but these tend to measure peak voltage, not RMS voltage (or current with a resistor). If the AC current is sinusoidal then the conversion is simple () but if the AC signal has harmonics, this relationship will be invalid. If real power measurement is required, an RMS measurement is needed, rather than a peak or peak measurement.

Thanks for your input.