# shunt for AC ammeter

#### boatsman

Joined Jan 17, 2008
186
I am using a digital multimeter to monitor AC currents through a circuit that dims fluorescent lamps. I need to have a steady reading of for example 70 mA. I find that with the digital meter the readings keep jumping about. I have an AC ammeter with a needle but the scale is up to 5 amps. What size shunt would I need so that I can easily have readings from about 50mA up to 150mA?

#### hgmjr

Joined Jan 28, 2005
9,029
Do you know what the true sensitivity of the AC meter is? Since it is configured to read 5A it must already have a shunt on it.

hgmjr

#### hobbyist

Joined Aug 10, 2008
887
Instead of a shunt wouldn't you need a current amplifier to give more meter movement?

Just throuwing this out there but send current through a impedance and read the voltage across or something like that?

Joined Apr 27, 2009
97
You would use a shunt to measure currents higher than the ammeter capability, not smaller. Like hobbyist says, you might need an amplifier and some filtering to give you the average value of the measured current.

#### boatsman

Joined Jan 17, 2008
186
Thank you hgmjr, hobbyist and AdrianN for replying to my query. I really need to find some way to regulate the current that I'm monitoring. The current is in AC milliamperes and ranges between 50mA and 150mA (it could be narrowed down to the range 60mA to 90mA) and the voltage is 110VAC -120VAC. Is there any simple way to regulate the current so that once I've set it up using a potentiometer I could make a scale so that any time I needed to alter the current I could just turn the knob of the potentiometer to the required mark without having to bother with a meter reading? Again many thanks, boatsman.

#### DickCappels

Joined Aug 21, 2008
5,755
The problem is you are using a meter that measures peak and assumes a sine wave to calculate RMS. This isn't going to work when you are chopping up the waveform with a phase controlled triac, as you find in light dimmers. If you want to know the RMS current, then you would be best off using a true RMS meter. Evan using a current probe on an oscilloscope will tell you a lot more than a DVM (unless it is a TRUE RMS meter).

True RMS meters find the RMS values of a waveform within that vary from a true sine wave by a parameter refered to as "crest factor". The high the crest factor rating of the meter, the more accurate your measurements will be.