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ac/dc operation

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Six-Sixteen

New Member
hello,

originally, the output of my pic16f84a is used to trigger the gate of an scr to light LED's connected to a 12Vac power source. When the trigger voltage from the pic is "0" the scr is off also.

PLease help me figure out a circuit (if possible using scr also) that can do the same function as above but this time the supply is not anymore ac but 12Vdc.

As I understand the scr will remain on even after the trigger voltage is released if using 12Vdc at the anode side.

How do I connect the circuit then? or maybe another approach?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Six-Sixteen said:
hello,

originally, the output of my pic16f84a is used to trigger the gate of an scr to light LED's connected to a 12Vac power source. When the trigger voltage from the pic is "0" the scr is off also.

PLease help me figure out a circuit (if possible using scr also) that can do the same function as above but this time the supply is not anymore ac but 12Vdc.

As I understand the scr will remain on even after the trigger voltage is released if using 12Vdc at the anode side.

How do I connect the circuit then? or maybe another approach?

You don't give any clue as to the current requirements, it may be far simpler to replace the SCR with a transistor or FET. To use an SCR in this way requires a second SCR, which discharges a reverse charged capacitor across the first one. The second one than latches on instead, but when the first one switches on it turns the second one off again with the capacitor. You would probably require a second I/O pin to do this, and the programming altering.

The circuit looks somewhat like an H-Bridge, where the two SCR's are the bottom legs, the load (and a dummy load) are the top two legs, and the capacitor is where the motor would be.

If this isn't clear, I'll draw you a diagram.
 

Six-Sixteen

New Member
good day Nigel,

Thanks for your reply. I would appreciate very much if you could spend a little time drawing the circuit for me.

Also please tell me what software you use for drawing diagrams and how to make that drawing appear in this post, because I would like to show some of my circuit (including problem circuits) to others in the future.
 

panic mode

Well-Known Member
Why not use simple transistor? All you control is few LEDs and
SRCs are rather troublesome to use in DC circuits (or to turn off).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Six-Sixteen said:
good day Nigel,

Thanks for your reply. I would appreciate very much if you could spend a little time drawing the circuit for me.

Also please tell me what software you use for drawing diagrams and how to make that drawing appear in this post, because I would like to show some of my circuit (including problem circuits) to others in the future.

This is the sort of way it's done, one of the two resistors is the load, the second is a dummy load for the other thyristor.

But it's not a good way to design a circuit, unless you need an exceptionally high current capacity. It's usually used in fork lift trucks, dodgem cars - things like that. If your current requirements are modest, a simple transistor would be a lot better and easier.
 

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Six-Sixteen

New Member
Thanks Nigel.

I'll experiment with your circuit.

to Panic Mode:

I decided to use scr because at times I'll be switching to ac supply from dc supply. During ac operation, if I use scr I only have current flowing during the first half cycle, thus reducing power consumption.

By the way the load is a series-parallel combination of LED's (141 LED's total).
 

panic mode

Well-Known Member
Still, thats only 4-5Watt load. Easily switched by transistor...
And if you add rectifier with some filtering capacitor, you should not
worry about AC/DC or polarity.
 
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