# true rms multimeter

#### denison

Joined Oct 13, 2018
263
Hi, Just bought a true rms digitech multimeter. It is reading the same as my ordinary meter.
I am programming a nano board using Arduino. I am using phase control of a 240 volt signal at 50hz. I am setting different delay times for switching on the mains. For instance I set turn on at 9ms. A half wave is 10ms.
Both meters are reading 137 volts across a filament light globe. The maths for a sine wave is Vt= 336 sin(2p.009/.02) in radians. p is the pie symbol which I don't have on my keyboard. .02 is the time of one full cycle of the mains in seconds. Vt is the voltage at 9ms. The instant voltage at time 9ms is then 103 volts. Then the true rms should be the average of 103v down to zero volts over 1ms. The correct voltage should be the DC voltage that gives the same brightness as the sine wave.
Can anybody suggest a multimeter which will give me the true voltage? The digitech model from Jaycar is certainly not it.

#### DickCappels

Joined Aug 21, 2008
8,159
1. Check the specification for your meter to see the frequencies and crest factors that it can handle with acceptable error, then decide what specs you really want, then how much you are willing to pay. Your meter is not far off given the signal you are feeding it.

#### SamR

Joined Mar 19, 2019
3,835
π = hold alt key and press 2, 2, 7 and release alt key

#### Ian0

Joined Aug 7, 2020
4,119
π = hold alt key and press 2, 2, 7 and release alt key
Or click the symbol at the top of the pane that looks like a Greek temple and says "Insert symbol" when the mouse is hovered over it.

#### Ian0

Joined Aug 7, 2020
4,119
Can anybody suggest a multimeter which will give me the true voltage?
Most modern digital oscilloscopes can.