by EWBrown » Tue Dec 09, 2003 7:11 am
For quick transformer impedance checks, I use a low AC voltage, 16 VAC
in my case, and feed the primary, and measure the secondary voltage(s).
As impedance (Z) is a square function of the number of turns, the impedance will be (Vout/Vin)^2 times the secondary impedance
For example, if you feed 16 VAC into the "unknown" primary, and get 0.506 VAC on the secondary, the impedance will be 31.62^2 or 1000 times the secondary. assuming an 8 ohm secondary, the primary will be 8000 Ohms.
The potted units with the two leads definitely sound like chokes, if you do a DC resistance reading, be careful not to hold the meter leads to the choke leads with your fingers, this will generate a strong high voltage (low current) flyback and you can get a nasty but harmless jolt.
An easy way to measure the unknown choke's impedance is to once again, use a low AC voltage - I should have mentioned earlier - isolated from the line, using a transformer - and put a known resistance in series with the choke, 1K is a good starting value, measure the voltage across the resistor and across the choke. The reactance of the choke is 2*PI*F*L
in Hz and Henries, for US and Canada, this is 60 Hz, so 2*PI*F works out to 377, So, the reactance of the choke, at 60 Hz, will be 377 X the inductance in Henries. In the case of a 5 henry choke, this works out to 1885 ohms, for the common 1.5 henry Dynaco 70 C-354 choke, this will be 565.5 ohms equivalent.
For those in Europe, Asia and other lands where 50 Hz AC power s the norm, replace 377 with 314 and proceed as above.
So, by considering the choke and resistor as a simple series resistive circuit, the total equivalent resistance at 60 Hz will be 2885 Ohms, the voltage across the inductor will be 16VAC X (1885/2885) or 10.45 volts, and the voltage across the 1K resistor will be 16 VAC X (1000/2885) or 5.55 VAC.
I hope I haven't put everybody to sleep.... :sleeping: :clock: :infinity:
D'OH!!!!!!!!!!!!!!!!!!!!!!
Note: I may not have (hopefully) put everybody else to sleep, but I am asleep... In inductors, E leads I by 90 degrees (and in caps, I leads E), the simple AC voltage measurement of chokes method I just described may be misleading. I'll give it a try in real time and see what I really get for results. YMMV, etc...
Just remember: ELI the ICE man...
/ed B in NH :drinking: :morning: :silly:
Real Radios Glow in the Dark