# Line frequency meter

#### pbudne

Joined Jan 5, 2014
2
I'm a software guy and I'm trying to understand this line frequency meter circuit (on my way to trying to replicate a passive component only tachometer circuit):

The same circuit (different semiconductors) appears in multiple sites:

http://circuit-diagram.hqew.net/Power$2dline-frequency-meter_17270.html http://www.next.gr/meter-counter/frequency-meter/power-line-frequency-meter-l12726.html http://circuit-diagram.hqew.net/Power$2dfrequency-meter_16885.html
http://circuit-diagram.hqew.net/Power-line-frequency-meter_10894.html
http://www.eleccircuit.com/simple-ac-wattmeter-circuit/

If the input was DC, 110V thru 100KΩ would only be 1.1mA (and 240V would be 2.4mA), so I don't understand how 60Hz AC input would deliver 60mA to the movement!!

And if 1A = 1 Coulomb / second, and 1 Coulomb = 1 Volt * 1 Farad, I expect that capacitor charging current seen by the meter would be:
Volts * Capacitance * Hz.

For 6.8V, 0.22μF and 100Hz, this only comes to 149.6μA

What am I missing???

#### MrChips

Joined Oct 2, 2009
26,469
Two points worth noting.

1. You cannot use the formula Q = C x V for the function of that capacitor in the circuit.

2. Check those links. One of them shows a 100μA meter.

#### wayneh

Joined Sep 9, 2010
17,175
1. You cannot use the formula Q = C x V for the function of that capacitor in the circuit.
?? By my calculation, the capacitor fully charges and discharges on each cycle. So I think the equation is useful.

Agreed on the meter current.

#### MrChips

Joined Oct 2, 2009
26,469
I would think that you would have to look at the duty cycle of the 60Hz square wave as well.

#### THE_RB

Joined Feb 11, 2008
5,438
?? By my calculation, the capacitor fully charges and discharges on each cycle. ...
...
It has to, for the circuit to have any accuracy.

The cap forward charge current goes through the meter, while the cap discharge current bypasses the meter.

So assuming the cap fully charges and discharges each time, the average meter current (and deflection) is based on how many charge cycles occur each second. Deflection linear to frequency.