I want to generate a brief pulse of 24 VAC (50 to 60 hz) to trigger a solenoid. I've found a number of circuits that will accomplish the VAC to VDC conversion, but they're designed to invert to much higher voltages.
I'll need to generate a pulse of a second or two a couple times a day.
I certainly can use a transformer / relay combination, but I need to get away from the wall socket.
I'm finding that I'm having a hard time with the math involved in scaling the components down to generate 24 VAC.
I really would appreciate any help anyone would care to share.
Are you absolutely sure that your solenoid needs AC, most AC solenoids will work quite happily off DC.
Most inverters use a transformer to generate the output, so to get less of an output you will need a transformer with a more equal turns ratio. Alternatively you can use a transformer with two secondary windings with one as 24V and the other say 12V. Ignore the primary and energise the 12V secondary and tap off the 24V from the other secondary.
It's interesting that you write that. I brought the solenoid home from Home Depot, took it out of the box, gave it 9 volts and it worked.
It was a bit flaky for a while, but now has stopped triggering altogether.
After my initial test with the battery, I put together a circuit that uses a TIP120 to feed it 9 VDC triggered by an output pin.
Power looked good on the circuit. But now I can't reproduce the behavior of the initial naked battery test.
When I discovered that the 24V notation for the solenoid meant AC, not DC I was a bit perplexed. I haven't synthesized enough information to even guess as to why it worked in the first place.
It was a bit flaky for a while, but now has stopped triggering altogether.
Solenoids take a lot of current, I guess that you flattened the battery. Also if it is rated at 24V then 9V might just be on the limit of working. It looks like 24V DC will be OK.
AC solenoids will generally NOT work of the same level of DC voltage. They are very inductive devices and as such use their inductive reactance to control the coil current, just as DC solenoids use their resistance to do likewise. If you measure the DC resistance of an AC solenoid coil you will find it has a low resistance and when you then do a Vsquared/R calculation to determine the coil wattage you will get an amazingly high value. As such they tend to burn out quite quickly when connected to a DC supply.
On the other hand if you use a DC solenoid on an AC circuit, whilst you wont have problems with over current you may well find it doesn't pull in due to the inductive reactance or it buzzes like mad because the magnetic core slug is not designed for AC use
Grumpy,
Indeed they will. I have some 24v AC heavy duty relays that operate quite satisfactorily at a maximum of 5 volts DC. Push them any harder and the coil current gets excessive. What I was getting at, is that you cannot simply substitute an AC relay or solenoid with a DC one of similar voltage.
jack
I love all the theory... really... but maybe the following Get The Job Done idea will be useful too?
Use the Arduino to drive a relay. Pedestrian... but idiot (me) proof. Be sure you put the necessary diode across the relay coil. (Search on "snubber" if you don't know about "the diode").
And wire the solenoid to a household-power-driven 24vAC transformer... make sure you get one with an adequate power rating... through the contacts controlled by the relay.