low power SCR?

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tattee

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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?
 

No, once turned on and SCR will not stop conduction even when the gate signal is removed. Current through the SCR has to drop to a very low value (<20ma or less) before the SCR will turn off.
 

hi,
A link for some simple SCR circuits.
**broken link removed**

You can also make your own SCR with two transistors.
 
Hi,

you might want to try BRY42/43 or 44. They are small thyristors. (Metal can similar to TO92)

The datasheet available doesn't give much info though.

Boncuk
 
As Eric noted, you can make an SCR equivalent with two transistors (an NPN and a PNP). That circuit can be turned off by applying 0V to (grounding) the control input.
 
crutschow said:
As Eric noted, you can make an SCR equivalent with two transistors (an NPN and a PNP)That circuit can be turned off by applying 0V to (grounding) the control input.
Click to expand...

No you cant. Once it's on; it's latched on until you disable power.

The MBT3946 are good for this they are only a couple of cents and they're small.You can put a P-FET on the high-side and latch it OPEN ,with the dual MBT3946. Just put a small 10-100n cap on the base,or it might latch as soon as you apply power to it.

Just drive a FET directly off your PIC.

For future reference for the OP a cheap low power SCR is the 2N506x series.

Code: 
http://www.onsemi.com/pub_link/Collateral/2N5060-D.PDF[\code]
 
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Oblong said:
No you cant. Once it's on; it's latched on until you disable power.
Click to expand...
I beg to differ. The transistor circuit allows access to the base drive to the lower transistor. If you short that to ground, it turns off the lower transistor and shuts off the latch. It acts rather like a GTO (gate turn off) thyristor. (See the attached schematic)
 
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I misunderstood I thought using two transistors as an "SCR" I.E this.

**broken link removed**

A pulse to the R1 & R3 node turns it on and it stays on until power is removed " SCR".


If he wants to switch in or out a part of his cct using a control signal from his PIC that is already there then there is no need for an SCR anyways just use a mosfet right off his PIC.All he needs is the one P-FET and invert his control signal.He's asked the same ? elsewhere and he's been getting the same response.
 
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Take a look at an SSR (solid state relay) like the Fairchild HSR312. These are small 6 pin DIP or SOIC packages like an opto-isolator. You need a few milliamps to turn on the LED and the outputs pins can be wired to switch AC or DC loads up to a couple hundred volts at 100 to 200 or so milliamps.

Happy Holidays. Mike
 
Thanks for your help guys.. I will try to look at your suggestions and update you on what I've come up with.
 
Why would you need a SCR at all? Why not have the PIC turn a transistor on and do the latching in software? If you are worried about power consumption, use a logic level FET and put the PIC to sleep and use wake up on pin change to rouse it from it's slumber.
 
Thanks for your suggestion kchriste, it looks very interesting.. But I dont know how to start.

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
 

Attachments

  • scr circuit.JPG
    8.2 KB · Views: 413
From your textual description, you could do this:

When S1 is pressed, it turns on Q2 which supplies power to the PIC and the load. The PIC, after reset/power up, outputs a high on the I/O pin which turns on Q1. Q1 turns on and keeps Q2 turned on also. Then the PIC does whatever, timing, etc that it needs to do. When done, the PIC outputs a low on the IO port which turns off power to itself and the load.
 

Attachments

  • PIC SCR Latch Sch.GIF
    4.4 KB · Views: 2,318
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Guys,

This is what I did with my design. It works perfectly base on what I want it 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 stops conducting

But I haven't tested it with the PIC yet. What I did was connect the wire intended for the PIC to ground to stop the circuit from conducting. Its really weird though if you try to remove the 1uF capacitor. What happens is that, once you remove the battery and then connect it back, the circuit doesnt reset. So if the last state was ON when removing the battery, the circuit will be ON when you connect it back.
 

Attachments

  • 1.JPG
    15.4 KB · Views: 663
The PIC will not be able to turn off the circuit as shown, only turn it on. You need to pull point G to ground to shut the circuit off. To do that you also need to add a resistor in series with the collector of Q1.

If you place the PIC output in a high impedance state, then you don't need the diode to the PIC. To turn it off, just put the PIC output in a logic 0 state. Or you could add another transistor from point G to ground contolled by the PIC.
 
I have some questions regarding the behaviour of my ciruict shown above. what happens is that when I activate the SCR and remove the batteries, the system doesn,t reset as soon as I connect the batteries back. It shows some form/behaviour of a memory which stores the last configuration and runs that configuration when batteries are reconnected. Is this how SCR really behaves?
 
By state, you mean whether it was on or off? Or something more complicated?
 
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yes by state, I mean ON or OFF..

the circuit automatically turns ON when the last state before removing the batteries is ON.
 
tattee said:
yes by state, I mean ON or OFF..

the circuit automatically turns ON when the last state before removing the batteries is ON.
Click to expand...

But does your circuit actually turn off? Crustchow says it won't. He's seems more knowledgable about this than I am. All I can say is try his recommendations.

 
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