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Question on Triggering a Switch and holding in on

R

Thread Starter

RiG615

Joined Nov 13, 2008
50
So heres my problem


I have voltage on a wire @ 40khz. [ DC, just pulsed @40khz ]

Once the above happens I want an LED to go on and stay on even after the pulses stop.

SO, heres what I want to happen

[nothing]...[nothing]...[wire pulsed]...[LED on]...[pulse ends]...[LED on]...[LED on]...[I kill power and reset circuit]



--I can't seem to conjure up anything in my head w/o using an IC, which I perferably dont want to use to keep cost low--
 
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DC_Kid

Joined Feb 25, 2008
1,072
RiG615 said:
So heres my problem


I have voltage on a wire @ 40khz. [ DC, just pulsed @40khz ]

Once the above happens I want an LED to go on and stay on even after the pulses stop.

SO, heres what I want to happen

[nothing]...[nothing]...[wire pulsed]...[LED on]...[pulse ends]...[LED on]...[LED on]...[I kill power and reset circuit]



--I can't seem to conjure up anything in my head w/o using an IC, which I perferably dont want to use to keep cost low--
Click to expand...
pulse the gate of a scr.
 
R

Thread Starter

RiG615

Joined Nov 13, 2008
50
So if I put a transistor in place of the LED(from above) and there was varying current through the transistor would the SCR shut off?

what about slightly varying current? whats the tolerance?
 
CDRIVE

CDRIVE

Joined Jul 1, 2008
2,219
RiG615 said:
So if I put a transistor in place of the LED(from above) and there was varying current through the transistor would the SCR shut off?

what about slightly varying current? whats the tolerance?
Click to expand...
If your thinking that you can use the pulse train to supply power to the Led then you misunderstand how you would use the SCR. Here's a SCR equivalent circuit. In lieu of an actual SCR you could make one like this from two small NPN & PNP general purpose switching transistors.
 

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R

Thread Starter

RiG615

Joined Nov 13, 2008
50
CDRIVE said:
If your thinking that you can use the pulse train to supply power to the Led then you misunderstand how you would use the SCR. Here's a SCR equivalent circuit. In lieu of an actual SCR you could make one like this from two small NPN & PNP general purpose switching transistors.
Click to expand...

No I wasn't planning on doing anything with the pulse train besides to trigger the SCR

my question was about the in general usage of the SRC

1. Once the voltage to the gate is cut off, their is still flow of current through the anode and cathode, right?


2. What causes the SCR to unlatch or turn off?


3. Can there be varying current/voltage through the anode-cathode without the SCR going off
 
CDRIVE

CDRIVE

Joined Jul 1, 2008
2,219
SCR,s have a holding current parameter which vary depending on the type of SCR. A small low current SCR will hold down into the microamp region. If the current through the SCR fluctuates it is not an issue as long as polarity of the current is not reversed. A SCR can be also shut off by shunting the Anode & Cathode. Tomorrow I'll spice the drop out current of the circuit that I posted. It's late now.;)
 
Last edited:
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DC_Kid

Joined Feb 25, 2008
1,072
if you wanted AC to hold you could trip a SCR with your pulse, then that SCR could drive the gate of a TRIAC (aka, solid state AC relay). then when you want it off you short the SCR out.

note: shorting scr cath to anode means you need to do it after the load. otherwise you run the risk of shorting out your V supply, etc.
 
CDRIVE

CDRIVE

Joined Jul 1, 2008
2,219
I want to make a correction here. In an earlier post I stated that some SCR's could have a holding current in the microamp region. Well I did a search of SCR types and didn't find one the held below 2.5mA. Now this doesn't mean that they don't exist. It just means that I haven't found one. To cover my butt, one could say that 2.5mA = 2,500UA! :rolleyes:
 
D

DC_Kid

Joined Feb 25, 2008
1,072
CDRIVE said:
I want to make a correction here. In an earlier post I stated that some SCR's could have a holding current in the microamp region. Well I did a search of SCR types and didn't find one the held below 2.5mA. Now this doesn't mean that they don't exist. It just means that I haven't found one. To cover my butt, one could say that 2.5mA = 2,500UA! :rolleyes:
Click to expand...
but to us physics nuts that would be several magnitudes of uA :)

so you could say "holding current near several magnitudes of uA" :)

lol
 
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