Choke questions

[quomaya2]

What does a choke actually do and how do you install one in an St-35

[dcriner]

A choke is an inductor coil that smooths the B+ in a DC power supply. When used, they are in series with the B+ supply and usually in between two filter caps across the B+ line. That would be a pi-type C-L-C filter.

If you add a choke where there wasn't one before, the B+ voltage will drop due to the resistance of the choke. So, that might not be a good idea.

[mesherm]

When the current increases in a choke the magnetic field increases also and self generates a EMF which impedes the original current flow.
When the current decreases in a choke the magnetic field collapses and the direction of the self generated EMF is reversed and tries to continue the original current flow.
As an aside, the spark in older auto ignition coils was generated in the secondary from the collapsing magnetic field in the coil when the points opened in the primary.
The choke works kind of in reverse from a capacitor in that a choke's AC impedance increases as you try to increase the rate of current change whereas a capacitor's AC impedance decreases.
There is a difference in that a choke has a DC impedance that is equal to the resistance of the wire in the coil and a capacitor has in theory an infinite DC impedance.
The choke therefore acts like a low value resistor to steady flowing current (DC) a high value resistor to fluctuating current (AC).

Mostly the choke is placed in series between two capacitors where it is more effective than a simple resistor. Some people prefer to have the choke first where it sees the most current rate of change. However when it is placed first it also creates a greater voltage drop so it depends on your personal preference.

[EWBrown]

To estimate the impedance or inductive reactance of the choke, the formula is XL = 2*pi*F*L, with F in Hz and L in Henries.

For 120 Hz (the usual ripple frequency freq, right off the rectifier cathode,

6.28*120*L (henries)

Effectively Impedance would equal 754 ohms (xL) per Henry.

So, if you are using a 5 henry choke, with 200 ohms DCR, the
choke, in terms of a resistor, would look like 200 ohms to DC, but
3770 ohms impedance or inductive reactance to the 120Hz ripple.

HTH

/ed B in NH