help with voltage control circuit

Thread Starter


Joined Aug 16, 2009
I need to design a circuit that take 115VAC input and delivers about 100VDC output then after a set time period say 5 seconds the voltage drops to 55VDC.

I was think that I could use a bridge rectifier circuit and that would get me my 100VDC but i'm sure on the best was to drop it down to 55VDC. I was thinking that I could open up the circuit on one of the diodes but i'm sure sure of the best was to do this.

My other problem is that the packaging needs to be pretty small, about 2"x2"x1" give or take alittle.

does anyone have any ideas?


Joined Jan 8, 2009
Two options - SMPS or a stepdown transformer. Everything depends on your current requirement.


Joined Jan 19, 2009
> i need 10A
> 2"x2"x1"

Since you need 10A at 55V, this sounds too small for the step-down
approach -- a transformer rated for 500VA is going to be larger than
that. To get that size you will need to do without a transformer, which
means that your circuit will not be isolated from the power lines and,
hence, only safe to use if all electrified parts are carefully hidden and
insulated where a user could not bump into them. In particular, there
will be no safe grounding point to which you could, say, connect any
exposed metal parts.

Another way of dropping from 115 to 100 to 55 is via a TRIAC
arrangement as used in dimmers. By using an arrangement
consisting of some fixed resistors and a transistor instead of
a variable resistor, the circuit would output 55V or 110V depending
on the logic level of a control wire. Then you could use an RC
circuit to change the control wire after 5 seconds.
Last edited:

Thread Starter


Joined Aug 16, 2009
What I have now is a full bridge rectifier with an SCR replacing one of the diodes. If I just connect the gate to a positive voltage it trigger just fine. The problem I'm having is triggering it with an RC circuit. I've got a 10K resisitor and 25uf CAP. The timing may not be quite right but I can seem to get the SCR to trigger at all. Any suggestions.