Hi,
I have a control unit which I want to use to switch a Hydraulic Solenoid. unfortunately the available switch in the control unit is momentary and I need the output to latch (so that I don't need to keep my finger on the button all the time) I had considered using a flip-flop to do this but I need to use a 24V supply. Are there any Flip flops which can take 24V? the highest I can find is 18V. I intend to use a relay to work the solenoid so the current rating is not important. I could use a volt reg to drop the voltage to a manageable level but I need this circuit to be as small (and simple) as possible. Does anyone have any ideas how to do this?
If the control lunit wasn't sealed, I'd change the switch but I don't want to break the seals.
If you search my posts, you will find that I designed a relay flip flop for someone a year or 2 ago. If you can't find it, let me know and I'll re-post.
A relay flip flop may be more convenient in your situation than an electronic one.
How about a normally closed push button in series with the coil?
Or a push to make in parallel with the SCR?
The latter is a pain as the switch needs to be rated the same as the SCR, but the relay isn't too bad - both switches only need to be rated for the coil current.
Hi Hero,
Yes I think that would work but I don't want to have to add more switches (originally I was going to just fit another (latching) switch to do the job but it would be much tidier if I could use the actual controller).
A bistable latching relay would be just what you want, as one impulse latches it, and then one impulse unlatches it. A typical one is the S89 series by TYCO(Potter and Brumfield).
If you want to use two switches one to latch and one to unlatch there are two approaches. As Nigel has mentioned a SCR. One normally closed switch between cathode and common of SCR. One normally open between gate and anode(with a resistor). Pressing the normally open will latch the SCR, Pressing the normally closed will unlatch the SCR.
The same technique can be used with a DPDT conventional relay, except a set of the NO contacts will latch the relay. If you need a circuit diagram I will be glad to provide one.
You did not say how you want to release the solenoid. If it is to be released by another button, then all you need is to latch the relay with one button and open the circuit with another.
This one will latch at the first button press and release at the second press.
Note that the operate and release times of the relays need to be at least 10 ms otherwise they may respond to contact bounce.
I use these chips a lot, they are touch sensor ic's that have an on/off switching mode, they are like $4. Very few components to build it, and you'll have a touch sensitive latch you can operate without a mechanical switch---maybe a piece of metal any size you want. Send the output to a mosfet on the low side of the solenoid. Maybe 5 bucks in parts excluding how fancy you get with the touch panel.
Thanks for all the ideas. I'm going to go with the Latching Relay because I've found somewhere I can buy on locally (it's called an Impulse Relay but it seems to be the same type of thing). Also it's the simplest option to install. If that doesn't work then I'll use the 2 relay circuit as it looks ideal, thanks Len. (it's a second press of the same button to release the solenoid).
The touch sensor ic is an interesting idea although I cannot use it for this as I want to use my existing momentary switch. I want to find something I can use this for though!
Mike, if you want to use the 2 relay circuit with a SPST button rather than a SPDT one as drawn, I can show you how to do it, but it needs a transistor and 2 or 3 resistors.
I agree, however a transistor and a few components may be cheaper and consume less PCB space than another relay. Another consideration is that the contact transit time of the third relay may be excessive if it has a diode across the coil to suppress the back EMF. So the diode may need a rresistor in series.