low power SCR?

Thread Starter

tattee

Joined Apr 24, 2008
16
Hi,

I'm planning to use SCR in my circuit but im having a problem of trying to find a low power SCR. What I have is a circuit with 5v supply and current consumption of 20mA maximum. Is there any SCR suited for this operation? If not, what can you suggest for replacement? What ive come up so far are SCR's for high voltage/amps circuits.

By the way, here's what my circuit will do:
  • A pushbutton will be push momentarily, thus enabling the current flow to the circuit.
  • A PIC will disable the SCR by applying '0' to the gate of the SCR
Question: Will a '0' from the output of the PIC disable the SCR?
 

jpanhalt

Joined Jan 18, 2008
11,087
Hi,

[/LIST]Question: Will a '0' from the output of the PIC disable the SCR?
No, at least not for the usual SCR. Read about them in the AAC book.

If you are using a PIC anyway, why not have the switch input to the pic and have the PIC control some other switching device?

John
 

Thread Starter

tattee

Joined Apr 24, 2008
16
Thanks for your help guys.. I will try to look at your suggestions and update you on what I've come up with.
 

Thread Starter

tattee

Joined Apr 24, 2008
16
By the way, here was my initial plan.

Attached is the circuit im planning to do:
  • The pushbutton will be pressed to start the operation
  • The circuit will run for sometime after button is pressed
  • The PIC will send a signal to the gate of the SCR to disable it (if possible)
  • The circuit stop conducting
 

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Wendy

Joined Mar 24, 2008
23,415
Here is the transistor version. The 10MΩ resistor help raise immunity from noise. It drove me crazy when I used it and didn't have the resistor, a 1MΩ. It kept false triggering.



A SCR is a PNPN device, this is a direct equivalent. It really is a true SCR.

The only way to stop a SCR conducting once it is on is to stop the current flow. With AC this happens automatically on the alternate cycle, in my design (which was a DC alarm) I used a simple push button to break the current flow.
 

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Last edited:

KMoffett

Joined Dec 19, 2007
2,918
By the way, here was my initial plan.

Attached is the circuit im planning to do:
  • The pushbutton will be pressed to start the operation
  • The circuit will run for sometime after button is pressed
  • The PIC will send a signal to the gate of the SCR to disable it (if possible)
  • The circuit stop conducting
I saw this circuit (as best as I can remember) on a forum recently. Can't remember which one. It uses R2, sized to maintain the hold current, pulled high to hold the SCR on, once started. To shut down, the μC could pull R2 low, depending on the rest of the working circuit load, or go into a low-current sleep mode to starve the SCR off.

Ken
 

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nanovate

Joined May 7, 2007
666
Here is the transistor version. The 10MΩ resistor help raise immunity from noise. It drove me crazy when I used it and didn't have the resistor, a 1MΩ. It kept false triggering.
Most commercial SCRs have a gate-cathode resistor that ranges from a few tens of ohms to a couple hundred. There are also "sensitive-gate" -- which you created ;) -- that have very high gate-cathode resistance but even they are actually spec'd with a 1k resistor externally. One thing to watch out for is the dv/dt spec. Sometimes applying a very fast pulse to the anode can trigger it on.
 

Wendy

Joined Mar 24, 2008
23,415
Yeah, mine needed to trigger off a pulse. It was a water monitor for the factory, I wanted an alarm mute. It amazes how many companies don't install that simple feature, once people are responding then you don't need loud audio driving you crazy.
 
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