latchingrelay
I've been told that this relay circuit should latch.
I built it and it does not.
I want the LED light to stay on after the N.O dP switch is is closed and then released back to its N.O. state.
Using 24 VAC power with a 24 VAC LED signal light.
I'm wondering if the LED is somehow interfering with the relay coil action. Otherwise I don't see any thing wrong with the circuit design.
Any ideas?
A very simple way to do this involves two diodes and a regular DPDT relay. The relay could also be a DPST relay with normally open contacts.
The idea is that the input 12V pulse comes and powers the relay coil for the duration of the pulse. During that time the relay closes and supplies power to your load. At the same time the other pole closes and gates power through the second diode to maintain the relay coil in the energized state as long as the NC unlatch switch is closed. A benefit of the circuit is that if the 12V supply goes off the circuit automatically resets itself.
This topology can be made somewhat simpler if the whole thing is powered from the same voltages and suitable relay coil voltage is selected.
Just to be sure we're on the same page, relay coils are between pins 1-12 and 6-7, not between 3-10. Also note that Q2 has some voltage drop even when turned on, so if you set the PSU to the latching voltage, the relay will see a smaller voltage and may not activate. Set the PSU to "rated" voltage, i.e. 5V. You could even leave it at 9V, since the relay is guaranteed to withstand up to 10.9V, though that's getting pretty close to the limit which is OK for a prototype but not advised for long-term usage.
Best Answer
The error is in wiring the lamp in series with the relay coil thereby reducing the current through both. Added to that would be the clipping effect of the LED.
That circuit would only function if it used a 12 V AC lamp and a 12 V AC relay, both having the same current rating.
Here's how the AC version ought to be. An RC snubber is to be connected across the relay coil to suppress voltage transients during turn-off.
Two LEDs in parallel protect themselves from the reverse voltage of the AC supply.
Here's the DC version. A freewheeling diode is to be connected across the relay coil.