# SCR latching trouble

#### Elbowkid

Feb 22, 2017
6
My first post here. I'm trying to create a latching IR circuit which turns on a buzzer when the beam is broken. Screen shot below.

The circuit seems to work on circuit wizard, however when I've breadboarded it, the buzzer sounds when the beam is broken, but it won't latch on.

Any help appreciated

#### Elbowkid

Feb 22, 2017
6
Should probably mention thyristor is 2n5060

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,337
What's the current through the buzzer? The 2N5060 requires a holding current of up to 5 mA. If the current of the buzzer is too low, the SCR will turn off. In that case try a resistor <= 1800 Ω in parallel to the buzzer.

#### BobK

Jan 5, 2010
7,682
If the buzzer is of the electromechanical type (essentially a relay that disconnects itself when activated) then it will break the circuit for each click, turning off the SCR. Haralds suggestion should fix it.

Bob

#### Arouse1973

Dec 18, 2013
5,178
Are you operating this in daylight? Do you think you need to consider that?
Thanks

#### AnalogEL

Feb 21, 2017
11
I agree that the buzzer might look like a "make and break" load, turning off the SCR when the buzzer is in its high impedance state. You might try putting a capacitor in parallel with it, giving the SCR current somewhere to flow during these brief periods. I'd try ~0.1uF first (which probably won't work), and then try a few larger value electrolytics like 4.7uF up through maybe 100uF maximum. The exact values are unimportant, but If the larger ones don't work you'll need a solution based on a different concept.

#### Elbowkid

Feb 22, 2017
6
What's the current through the buzzer? The 2N5060 requires a holding current of up to 5 mA. If the current of the buzzer is too low, the SCR will turn off. In that case try a resistor <= 1800 Ω in parallel to the buzzer.
I've tried a 2k2 resistor in parallel which brought the current up to about 5.7mA but still no latch.

Worth noting I tested the circuit first with a simple +ve pulse input (ptm and pull down resistor) and the thyristor latched no problem.

Arouse, the buzzer is turning on when I place a blockage between the LED and the phototransistor, so the daylight isn't the problem, just the thyristor won't latch!

Any more suggestions? I'll try the electrolytic but that seems to still be operating on the premise that my buzzer is causing the problem.

#### Elbowkid

Feb 22, 2017
6
I've also tried a 3k9 which gave a current of 4.8V. Still no joy.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,855
Try replacing the buzzer with an LED and a current-limiting resistor to make sure the thyristor is (still) working. Shorting the SCR with a NO push-button switch is not as good, IMO, as inserting a NC push-button switch in series with the load.

#### Elbowkid

Feb 22, 2017
6
Thanks hevans. LED and resistor all working fine when I put things back to the old configuration and it's all latching fine. I don't understand why it triggers the SCR but doesn't latch it!

#### Elbowkid

Feb 22, 2017
6
I've tried the following combinations:
1. ptm with LED - works
2. ptm with buzzer - works
3. IR setup with led - won't latch but turns on LED while beam is blocked
4. IR setup with buzzer - won't latch but turns on buzzer while beam is blocked
Can anyone suggest why this might be the case, or an alternative setup to help?! Trying to build a very simplistic alarm system for a year 8 class (12-13 year olds). Don't want the budget to be too high or the circuit to end up too complicated.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,855
Three things will prevent an SCR, once triggered, from latching on: insufficient holding current, insufficient voltage across the SCR, and a defective SCR. Since you have already tried increasing the holding current with a 2.2 kΩ resistor without success, I would decrease this resistance further to 1 kΩ to perhaps as low as 100 Ω to see if you can get the SCR to latch. But first I would substitute a brand new SCR and test for latching with just an LED and a current-limiting resistor in the anode circuit... your combination #3 in your post #11.

The first two scenarios, combinations #1 and #2, don't test the SCR. They simply verify the ptm (push-to-make?) switch in parallel with the SCR will light the LED or sound the buzzer.

In the other two scenarios, combinations #3 and #4, the SCR appears to be behaving as a MOSFET or BJT would. I am not aware of a failure mechanism that would cause an SCR to behave this way, but who knows? Try a new SCR. You are also over-driving the gate terminal when the beam is blocked, essentially connecting the gate to 9 V DC through a 10 kΩ resistor, applying roughly 900 μA to a gate rated to turn on at 200 μA with a forward voltage drop of 0.8 V I doubt this would be responsible for damaging the SCR, but again, who knows? Try a new SCR. Maybe drive the gate from a voltage divider to decrease the amount of forward voltage applied to turn the gate on. I am shooting in the dark here. Try a new SCR.

#### Arouse1973

Dec 18, 2013
5,178
I've tried a 2k2 resistor in parallel which brought the current up to about 5.7mA but still no latch.

Worth noting I tested the circuit first with a simple +ve pulse input (ptm and pull down resistor) and the thyristor latched no problem.

Arouse, the buzzer is turning on when I place a blockage between the LED and the phototransistor, so the daylight isn't the problem, just the thyristor won't latch!

Any more suggestions? I'll try the electrolytic but that seems to still be operating on the premise that my buzzer is causing the problem.

Ok just a thought, thanks.

#### Arouse1973

Dec 18, 2013
5,178
Is it possible the IR transistor is actually turning the SCR off by starving current to the bottom PN junction of the device when IR transistor turns back on. This would explain why the push switches work. Connect a resistor between the gate of the SCR and collector of the transistor, try a 2K2 or something like that.
Thanks

#### tedstruk

Jan 7, 2012
476
Ahem...
suppose you build another circuit called a "latching circuit" and connect it so the beam interuption fires the latch trigger?
I think thats how to do it...

Moderator note: it is best to ignore tedstruck. He seems to live in an alternate reality. If you don't understand something he posts, you're probably not the Lone Ranger.

Last edited by a moderator:

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
I like the possibility that Adam posted regarding adding a series gate resistor but since we're throwing out possibilities I have a one too. First off the assumption is that this is an electromechanical buzzer. The problem I have with this theory is EM buzzers typically draw quite a bit more current than 5mA. If, in fact, it is it's also possible that back EMF arcing across the contacts when they open will also shut the SCR off. Possibly even damage it.

Perhaps modifying the circuit like this would cure his lack of holding current. I didn't spend any time calculating R3 but I think it's good enough for ballpark work .

Chris

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