I need to build a surge suppressor to go on the input of an alarm
circuit. The circuit operating voltage is 24 volts DC. This is a two
wire input with an end of line resistor. I pulled a toroid input
filter off an old RCA TV chassis and plan to connect my circuit
throught this first. At the output of the toroid I plan to put two 33V
MOV's in series with the middle two leads connected to chassis,
(building) ground. This output will connect to the alarm circuit
input. There will also be 1.0 uf disc capacitor across the alarm input
as well. My theory is that a transient will first see the buck effect
of the toroid, then the transient magnitude will be limited by the
series MOV's and shunted to ground. Anything left will be absorbed by
the cap. I know that there are commercial surge suppressors available
but I would appreciate any opinions on my plan. Thanks, Lenny

wrote:
I need to build a surge suppressor to go on the input of an alarm
circuit. The circuit operating voltage is 24 volts DC. This is a two
wire input with an end of line resistor. I pulled a toroid input
filter off an old RCA TV chassis and plan to connect my circuit
throught this first. At the output of the toroid I plan to put two 33V
MOV's in series with the middle two leads connected to chassis,
(building) ground. This output will connect to the alarm circuit
input. There will also be 1.0 uf disc capacitor across the alarm input
as well. My theory is that a transient will first see the buck effect
of the toroid, then the transient magnitude will be limited by the
series MOV's and shunted to ground. Anything left will be absorbed by
the cap. I know that there are commercial surge suppressors available
but I would appreciate any opinions on my plan. Thanks, Lenny
If you need to have your protection closer to 24, you can either use
a zener diode with a thyristor, or a zener diode with a hefty power
transistor darlington.
The solutions make sure the input fuse blows, but the first solution
only recovers after power removal, the second one recovers when the
24 volt supply returns to normal.

wrote:
I need to build a surge suppressor to go on the input of an alarm
circuit. ......

Don't forget a fast blow fuse. That way when the MOV's fail, they
will just blow the fuse and not explode.

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM