Author Topic: Permanent Latch Circuit  (Read 1866 times)

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Offline PentoadTopic starter

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Permanent Latch Circuit
« on: March 23, 2020, 08:02:53 pm »
Hello all,

This circuit here switches on a relay when a trigger voltage is applied and keeps the relay on permanently until power is reset.

What I need though is a system where the relay switches on when the circuit is powered up and switches off permanently when a trigger voltage is applied and stays off until power is reset. How would I change the circuit to allow this?

 

Offline jbb

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Re: Permanent Latch Circuit
« Reply #1 on: March 23, 2020, 09:38:58 pm »
Use the normally closed relay contact?
 

Offline PentoadTopic starter

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Re: Permanent Latch Circuit
« Reply #2 on: March 23, 2020, 10:19:13 pm »
That can't work for my application as it would allow the load to run even if there is no power going to the circuit or the circuit suffers some kind of failure.
 

Offline Prehistoricman

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Re: Permanent Latch Circuit
« Reply #3 on: March 23, 2020, 11:58:58 pm »
Here's something I just threw together in simulation.

Here's what the circuit does, in chronologic order:
This generator on the left is set up to deliver a power-up type slope over 5ms (to make sure the system performs with a realistic turn-on).
R1 initially turns T1 on, so that the relay turns on at power-up.
C1 makes sure that T2 doesn't turn on before T1 turns on
SW1 is set up to close after 10ms to simulate some external input going high
R3 limits the current into the base of T2 (this resistor must be smaller than R2)
T2 pulls the base of T1 low to turn off the relay
SW1 opens at 15ms to simulate the external signal turning off
R2 provides feedback to keep T2 turned on (the current flows through the relay coil)

I've attached a zip of the TINA-TI (that's the simulator I use) schematic, if you want to play with it.

Improvements that should be considered:
A resistor to discharge C1
Diode on the input, so that a low state cannot be asserted by the external signal
« Last Edit: March 24, 2020, 12:16:39 am by Prehistoricman »
 
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Offline Zero999

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Re: Permanent Latch Circuit
« Reply #4 on: March 24, 2020, 10:39:40 am »
The circuit posted above is an example of a bistable multivibrator.
https://en.wikipedia.org/wiki/Bistable_circuit

C1 should discharge via the transistor's base when the power is remove, so it shouldn't be an issue.

The circuit can be made less sensitive to the input being forced low, by increasing the value of R3.
 

Offline PentoadTopic starter

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Re: Permanent Latch Circuit
« Reply #5 on: March 24, 2020, 08:32:22 pm »
Here's something I just threw together in simulation.

Here's what the circuit does, in chronologic order:
This generator on the left is set up to deliver a power-up type slope over 5ms (to make sure the system performs with a realistic turn-on).
R1 initially turns T1 on, so that the relay turns on at power-up.
C1 makes sure that T2 doesn't turn on before T1 turns on
SW1 is set up to close after 10ms to simulate some external input going high
R3 limits the current into the base of T2 (this resistor must be smaller than R2)
T2 pulls the base of T1 low to turn off the relay
SW1 opens at 15ms to simulate the external signal turning off
R2 provides feedback to keep T2 turned on (the current flows through the relay coil)

I've attached a zip of the TINA-TI (that's the simulator I use) schematic, if you want to play with it.

Improvements that should be considered:
A resistor to discharge C1
Diode on the input, so that a low state cannot be asserted by the external signal



Your solution was brilliantly simple and did exactly what I wanted. I built it on a breadboard and it worked first time. Many thanks!
 
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Offline PentoadTopic starter

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Re: Permanent Latch Circuit
« Reply #6 on: March 30, 2020, 08:27:36 pm »
I know I said this was working perfectly and that was using the BC547 transistors I had lying around. I tried 2N3904 when they arrived and the circuit just oscillates and a tone comes from the relay no matter what state the input is at which is strange as it worked in the simulation.
 

Offline Prehistoricman

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Re: Permanent Latch Circuit
« Reply #7 on: March 31, 2020, 11:08:03 am »
I don't really know how that could happen.
Try increasing C1 and changing the value of R2. Or perhaps it's transistor-to-transistor variance?

Offline Mechatrommer

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Re: Permanent Latch Circuit
« Reply #8 on: March 31, 2020, 12:06:54 pm »
i guess solenoid is an inductor (L), R2 and C1 forms resonant circuit, hence oscillation. from the original circuit in OP, try adding capacitor (maybe 0.1uF) from 9V to input trigger, and resistor (maybe 1Kohm) from input trigger to ground. reduce R1 to say 1Kohm or 100 ohm. during startup, the capacitor should momentarily giving initiating pulse to turn the thing ON and applying trigger LO later on to deactivate relay (OFF state). it may work it may not depending on the leakage. and maybe also reduce R4 to between 1-10Kohm. ymmv.
« Last Edit: March 31, 2020, 12:26:33 pm by Mechatrommer »
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