[Johnson] Johnson 500 Power Supply RY302 Relay Needed

[Kevin J. Ward]

Most hams think that AC relays can be used in place of DC relays without 
a problem, and while the relay will operate, there are several things to 
consider.

An AC relay usually has a lower coil resistance.  When used on DC it 
will draw a lot more current and may get quite hot.  A series resistor 
will help. 

The core of an AC relay is slotted and fitted with a "D-ring" which 
spans a portion of the coil winding.  The purpose of this ring is to 
keep the relay from chattering in time with the AC power.  When DC is 
applied to the coil, the D-ring and core will be magnetized to some 
extent, depending on how long the coil is energized.  This can cause 
armature release time to vary all over the place.  It can even cause the 
armature to not release at all, depending upon spring tension, relay 
position, and other factors. 

A DC relay can be made to open somewhat more slowly by simply connecting 
a diode across the coil, such that the diode is reverse biased by the 
coil voltage.  When coil voltage is released, a back-EMF is generated.  
The diode conducts and prolongs the coil current until the magnetic 
field collapses.  Then the armature drops out.  Pull-in time is not 
affected.  We're talking milliseconds here, but probably enough time to 
allow transmitter output to drop to zero before the receiver comes back 
to life. 

Kevin  N2IE