# Transistor Latch Circuit

#### jack33

Joined Dec 31, 2010
42
I want to build a latching switch circuit with transistors (no relay). It needs to remain on after pressing a momentary contact switch and shut off when power is switched off. I would like to use as few components as possible.

I tried the circuit on the link below, but the circuit goes on and stays on when power is applied without pressing the momentary contact switch. The switch has no effect. Can someone tell me why this circuit doesn't work? I used 2N4424 NPN transistors. I checked all my wiring.

http://www.zen22142.zen.co.uk/Circuits/Switching/latch.htm

#### MKCheruvu

Joined Nov 20, 2012
30
1) The DC Forawrd Current Gain of 2N4424 as given in Data Sheet is :
@ 2mA/4.5V - 180(min) - 540(max)
2) As per the Current Vs hFE Characteristics @ 10mA/5V is :
So the Base current required for Q2 @Ic=10mA is about 0.03ma.Hence R2+R3>=300K.
In addition we have to consider max hFE and also the variation of hFE between Q1 and Q2.
Hence you may try reducing the base Drive by increasing R1 and R3 and also craete slight imbalence in the drive conditions of Q1 and Q2 with values like : R1=270K and R3= 360k

#### tkng211

Joined Jan 4, 2008
65
When the circuit is switched on, the whole supply voltage +V applies across the B-E junction of Q1 (assuming the voltage across C1 is zero or very low). Since there is no current limiting resistor for charging up C1, the high pulse current will easily burn the B-E junction of Q1, hence it won't work any further. That's why the circuit stays on once swithed on.
If you want to have a simple transistor latch circuit, better to use a NPN-PNP type or simply use a SCR instead.
http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P2.html#Latch

#### #12

Joined Nov 30, 2010
18,210
Look for "transistor analog of an SCR".

#### jack33

Joined Dec 31, 2010
42
Thanks to all for the advice.

I tried changing R1 and R3 on the original circuit as advised. With those changes, the LED flickered then went out on power-up. The LED went on when the momentary switch was pressed but did not latch on when the switch was released. I also tried adding a 10k resistor between the cap and the base of Q1 but that didn't change anything.

I also tried the transistor latch circuit in Fig. 75B (transistor analog of an SCR) in the link given by tkng211. The LED did not light at all, on power-up or with the switch depressed. I didn't have the specified transistors, so I used a 2N3906 PNP and a 2N4424 NPN. I don't know if that is a problem or not. Earlier I had tried a circuit similar to Fig. 75B which also did not work.

I have seen some circuits with more transistors, but I like the idea of a two transistor circuit. I have some salvaged SCRs, so I think I'll try that next.

#### crutschow

Joined Mar 14, 2008
25,135
An SCR should work as they are designed to do exactly what you want. Just be sure you have a resistor in series with the SCR gate to limit the current to a safe value. A resistor from gate to cathode may also be needed to minimize false triggering.

#### jack33

Joined Dec 31, 2010
42
Thanks. I tried the circuit in Fig. 75A in the link given by tkng211 with a 2N5061 SCR and it worked perfectly.

#### tkng211

Joined Jan 4, 2008
65
jack33, please note the central terminal of 2N4424 is the Collector whereas the central terminal of 2N3906 is the Base. You might have made a wrong connection. the circuit is quite simple and should work properly. Good luck!

#### jack33

Joined Dec 31, 2010
42
Thanks, tkng211. I did have the transistors connected properly. However, I had left out the "off" switch, thinking the circuit would reset when the power was shut off. This time I added the "off" switch. Now, if I turn off the power while the LED is on and then back on, the LED comes back on and stays on. But it goes off when I press the "off" switch. After that the circuit functions normally using the "on" and "off" momentary switches.

If I turn the power off when the LED is off and then turn it back on, the LED stays off. It will go on when I press the momentary "on" switch, and the circuit works normally.

I'm baffled as to why the circuit does not reset to the off condition when the power is off. It seems that both transistors should be off on power-up until the "on" switch is pressed, just like the SCR circuit.

#### Ron H

Joined Apr 14, 2005
7,014
Here's what I think needs to be done to make the original circuit reliable.
The problem with the original scheme is that switching is dependent on the beta of Q1. If it is reasonably high, Q1 will remain saturated even when the trigger switch is on. Moving R1 to the base of Q2 removes this possibility. It also allows you to raise the value of R1.
I added R6 to protect the base of Q1 from the high current transient that otherwise may flow through C1.

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#### jack33

Joined Dec 31, 2010
42
Thanks, Ron. I tried your modified circuit. When S2 is closed to apply power, the LED lights up very slightly and stays that way until S1 is pressed. When S1 is pressed, the LED lights fully and stays that way until S2 is opened when it goes off. I changed R1 to 470k, and the LED is barely lit at the start, but the LED still fully lights when S1 is pressed. With R1 at 1M, the LED lights fully when S2 is opened. I guess for this circuit to work some current needs to go through the load in the off state.

#### Ron H

Joined Apr 14, 2005
7,014
Thanks, Ron. I tried your modified circuit. When S2 is closed to apply power, the LED lights up very slightly and stays that way until S1 is pressed. When S1 is pressed, the LED lights fully and stays that way until S2 is opened when it goes off. I changed R1 to 470k, and the LED is barely lit at the start, but the LED still fully lights when S1 is pressed. With R1 at 1M, the LED lights fully when S2 is opened. I guess for this circuit to work some current needs to go through the load in the off state.
You did something wrong. S2 interrupts the current, independent of any circuit values.

Yes, the LED has a current path through R1 and the base of Q1, as you observed. I think the LED was in the original circuit to indicate when the circuit is latched. If your goal is to turn the LED fully off when the circuit is "unlatched", it needs some changes.

What are you trying to do?

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#### jack33

Joined Dec 31, 2010
42
I wired S2 as on the circuit diagram and used it to interrupt the current. The circuit works fine with your design. I increased R1 just to see how much I could reduce the current through the LED when the circuit is unlatched.

I want to use the circuit to turn power on to another circuit using a momentary contact switch. If I put the load in parallel with the LED and R5 it would work, but a small current could flow through the load while the circuit was unlatched. Ideally, I would like the power to the load to be fully off when the circuit is in the unlatched state.

#### Ron H

Joined Apr 14, 2005
7,014
I wired S2 as on the circuit diagram and used it to interrupt the current. The circuit works fine with your design. I increased R1 just to see how much I could reduce the current through the LED when the circuit is unlatched.

I want to use the circuit to turn power on to another circuit using a momentary contact switch. If I put the load in parallel with the LED and R5 it would work, but a small current could flow through the load while the circuit was unlatched. Ideally, I would like the power to the load to be fully off when the circuit is in the unlatched state.
How much current does your load draw?
What is the supply voltage?
Do you need an LED indicator?
Do you want to switch the high side, or the low side?

#### jack33

Joined Dec 31, 2010
42
How much current does your load draw?
What is the supply voltage?
Do you need an LED indicator?
Do you want to switch the high side, or the low side?
The latching switch could be used for various projects. Here are typical specs:

Circuit description: microcontroller with various inputs and outputs
Current draw: 200mA
Primary supply voltage: 9V DC from a wall wart or battery
Internal power supply: 5V DC from a 7805 voltage regulator with appropriate capacitors
LED indicator: Yes
Switch the 9 V DC high side

#### Ron H

Joined Apr 14, 2005
7,014
The latching switch could be used for various projects. Here are typical specs:

Circuit description: microcontroller with various inputs and outputs
Current draw: 200mA
Primary supply voltage: 9V DC from a wall wart or battery
Internal power supply: 5V DC from a 7805 voltage regulator with appropriate capacitors
LED indicator: Yes
Switch the 9 V DC high side
Why can't you use the microcontroller as a latch?
If you do need an external latch, why do you want to use discrete transistors? Why not use an IC?

#### jack33

Joined Dec 31, 2010
42
Why can't you use the microcontroller as a latch?
If you do need an external latch, why do you want to use discrete transistors? Why not use an IC?
Would I not need to switch a transistor with the microcontroller, since I could not put all the current through the microcontroller? I agree, using an IC would be simpler and take less space. I'll look into that.

#### Ron H

Joined Apr 14, 2005
7,014
Would I not need to switch a transistor with the microcontroller, since I could not put all the current through the microcontroller? I agree, using an IC would be simpler and take less space. I'll look into that.
Yes, in fact, if you want a high side switch for +9V, you will need 2 transistors: One for level shifting, one for the switch.