Low current 240V 50Hz supply - Page 1
Forum section: electronics / projects

author: AdamSparky
author_rank: Newbie
timestamp: April 12, 2014, 08:50:24 am
content: I want to make a circuit that creates a 240VAC 50Hz output, but only needs to deliver 0.5-1mA. My input supply would be about 6VDC.I was thinking of creating a sine wave using an op amp oscillator circuit and then upping the voltage with a transformer. Would a small flyback converter transformer work at such a low frequency? My output current is very low as I said.My other option would be to use a boost converter and then switch the high voltage DC to AC.Which way would be best? Is there a better way?

author: IanB
author_rank: Super Contributor
timestamp: April 12, 2014, 09:10:22 am
content: You could simply take a regular 6 V step down transformer and reverse it. Such a transformer normally accepts 240 V AC on the primary and produces 6 V AC on the secondary. If you turn it round and feed 6 V AC into the secondary you will generate 240 V AC on the primary side.The question then becomes how to make the 6 V AC to feed the transformer with.

author: fcb
author_rank: Super Contributor
timestamp: April 12, 2014, 10:25:13 am
content: IanB hit the nail on the head. And it works, I've done it before (1986 to be precise!), I've still got the prototype and it still works.There's a couple of things to bear-in-mind:1. Single ended amplifier with a 6V rail will at best give you 2.12Vrms - good luck finding a mains transformer with a 240:2 ratio.2. Bridge configuration amplifier will give you +/-6V swing, giving you 4.24Vrms - a bit better but rare.To get round this, either boost your 6V, or use a 115:6 transformer (twin primary perhaps) and over drive it - very unlikely to have a problem with this as your load is so small.If you need the frequency and amplitude to be accurate, then you'll be using a micro. If you don't need that degree of precision, then I'd throw together a phase-shift oscillator.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 12, 2014, 10:36:04 am
content: Does it really have to be sine? You may get away with simple square wave...Making good sinewaves at 50 Hz using simple circuits is not so easy (capacitors and inductors get really big).Look atAN-263from TI for some techniques of generating sine waves.

author: SeanB
author_rank: Super Contributor
timestamp: April 12, 2014, 02:27:24 pm
content: Probably does not have to be a good sine, just good enough, and use the secondary and primary as a filter, using a simple sine like oscillator driving the transformer, probably a 6V3 centre tapped 0.5A unit using only the one side to the centre tap, and driving it with a H bridge or a simple car amplifier that uses a H bridge.Going to have a low efficiency, and as you are likely going to be using this to drive a clock mech I would suggest using a 12V CT transformer, driving it with a simple power stage using a SG3524 and some power transistors, and then using a crystal oscillator or other stable method to drive the converter clock with 100Hz. That way most of the logic is in a single cheap IC, and then the output can be shaped to something closer to a sine wave using a parallel capacitor of around 1uF 400V to filter the drive.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2014, 05:16:59 pm
content: At this frequency, I'd recommend a basic triangle wave generator, followed by an integrator to covert the triangle wave to a sine. Put it through a power amplifier driving a transformer and you have 230VAC 50Hz.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 12, 2014, 05:50:06 pm
content: One way is to use an audio power amplifier to feed a backwards mains transformer.What's it for ?This is in the neighbourhood of larger EL panel drive - if you don't need sine, there may be a driver solution targetted at that market that may be worth a look.

author: dannyf
author_rank: Super Contributor
timestamp: April 12, 2014, 05:56:06 pm
content: Depending on what power source you already have: if you have access to 50Hz ac, using a couple center-tap'd transformers would do it.Otherwise, oscillator + step-up transformer -> some can be combined into one.Depending on exactly what you have and what you want to have.

author: AdamSparky
author_rank: Newbie
timestamp: April 13, 2014, 11:36:47 am
content: Thanks for the replies, I'll give a few of the ideas a try. The device is going to be battery powered and as small as possible so efficiency and a compact transformer is important.

author: Zero999
author_rank: Super Contributor
timestamp: April 13, 2014, 07:08:16 pm
content: Unfortunately, the designs discussed above are not efficient. At best, you're looking at an efficiency of 25% but your output power requirement is only 0.23W so it shouldn't matter too much, as long as you're prepared for that.As far as size is concerned. You can buy very small mains transformers <3VA is tiny but the smaller the transformer, the less efficient it will be.

author: fcb
author_rank: Super Contributor
timestamp: April 13, 2014, 10:29:31 pm
content: AdamSparky, perhaps you could tell us more about what you are doing with the 240V @ 1mA?

author: 2N2222
author_rank: Newbie
timestamp: April 27, 2014, 08:47:41 pm
content: Yes, knowing about the load could help to sort out ideas.

author: David Hess
author_rank: Super Contributor
timestamp: April 29, 2014, 10:28:45 am
content: Quote from: fcb on April 12, 2014, 10:25:13 amYou can always use two or more of the same transformer with the primaries in parallel but the secondaries in series.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 29, 2014, 11:02:04 am
content: You'd want a tiny transformer so magnetizing current was small.