Electronics project. DC to AC

[Retro6]

I want to do a fun little project turning DC from a battery to AC so that I can connect a mains transformer in reverse to get a higher voltage out of the other end to see what things I can power from it as an experiment. I have a relay and a mains transformer I'm also thinking about making a doorbell from an old GPO telephone bell set. That requires at least 50 odd volts AC before it moves, it will almost run on 50 volts AC but when voltage is increased to about 70 volts AC it starts to work properly.

With a mains transformer, they only work with alternating current which is 240 volts in and 12 volts out but if I reverse it and feed 12 volts in I get a higher voltage out the other end but the only problem is my power source is DC at 12 volts... well 13.8 volts. What I need to do is convert the direct current to alternating current and I am wondering if I could do that with a relay?

 

[decto]

I want to do a fun little project turning DC from a battery to AC so that I can connect a mains transformer in reverse to get a higher voltage out of the other end to see what things I can power from it as an experiment. I have a relay and a mains transformer I'm also thinking about making a doorbell from an old GPO telephone bell set. That requires at least 50 odd volts AC before it moves, it will almost run on 50 volts AC but when voltage is increased to about 70 volts AC it starts to work properly.

With a mains transformer, they only work with alternating current which is 240 volts in and 12 volts out but if I reverse it and feed 12 volts in I get a higher voltage out the other end but the only problem is my power source is DC at 12 volts... well 13.8 volts. What I need to do is convert the direct current to alternating current and I am wondering if I could do that with a relay?

I don't think that is going to be possible, you can't switch the relay fast enough, and you'll burn out the contacts if you try with any significant current.

You need a low frequency oscillator, something based on a 555 timer might do.
Then you need to boost the voltage and current to switch the primary side of the transformer using transistors(s) or mosfet(s) depending on how much power you need.

Or perhaps an output from an Arduino which again you amplify to a decent voltage. You could then use code to create different waveforms, a bit like a signal generator.

 

[kitfit1]

Just buy an Inverter, no need to re-invent the wheel.

 

[hornetstinger]

Just buy an Inverter, no need to re-invent the wheel.

Next you'll be telling me you take your vehicle to the petrol station to fill up, rather than how I do it with filling a cup of petrol, walking home to pour it into my fuel tank, then walking back to petrol station for another cup, and so on and so forth.

 

[Jonny69]

Just buy an Inverter, no need to re-invent the wheel.

BORING

To the OP, there are a couple of ways to do it DIY. One is as stated above with a 555. I did one a few years ago to take 6V from 4x AAs in series up to around 300V DC for a portable valve amplifier. It’s a pretty nasty AC to deal with because the 555 puts out a square wave which transformers hate. A more analogue way is with a Wien bridge on an op amp followed by transistors to supply the current, then transformer.

It is technically possible to do it with a relay, but it won’t last long. In the really olden days, you would use a trembler circuit to make a dirty AC from DC from batteries, which you could then step up for the HT feed for valves. Old valve car radios used to do this. The trembler itself was mechanical (just like a relay) but had a fairly short lifetime.

Don’t forget your DC battery will need to be able to supply quite a lot of current.

 

[Jonny69]

Another maybe easier way to generate a sine wave is to use a dedicated function generator IC like an ICL8038 or a CEM3340. It’s a bit overkill as both of those also have triangle and square wave outputs as well, but you can get cheap kits on eBay to play with.

 

[Retro6]

Some good ideas to try. I'll build something, the other thing is my door bell project which will be made from a telephone bell set and an old Victorian door bell button and rather than driving the door bell button with high voltage I'd want to have that on the lower voltage side, it will also be earthed as well but the bell ringer will need about 70 volts at 25Hz but doesn't have to be strictly 25Hz it will still run on 50Hz AC. It doesn't have to be a clean AC output just as long as it does the job with getting the bell set moving then its all good.

Its two projects so the door bell and then a mobile device that is battery powered from one of those dry lead acid batteries which is around 7 amps or a bit more from a larger battery that can give me 240volts AC output.