Simple method for 240V from 240V line

[eeyore]

I am building a circuit at the moment that requires 240V @ 100mA (it is the Electra-print 6BX6 SE/PP designs). I live in Australia which is 240V. What is the simplest, cheapest and safest way of getting line voltage into the rectifier. I assume that it is not good to wire up the line voltage direct to the rectifiers, and there needs to be isolation. So, what are the ways of achieving this?

Thanks, David.

[pinkmouse]

It is indeed a bad idea to connect directly to the mains. Apart from safety, it will also not give you the 240V DC I assume you need, as when rectified, 240V AC will give you approx 330V DC, ( 240x1.4), so you will need a transformer with mains rated primary and around 170V on the secondary winding. I strongly recommend you read the HV safety thread in this forum if you're at all unsure of what you are doing

[zigzagflux]

You will be hard pressed to find anyone that does not strongly recommend an isolation transformer. Just a 1:1 turns ratio transformer that isolates you from ground. A 50 VA or higher would be my suggestion, if all you need is 100mA. You will therefore be free to ground the common node of your circuit to chassis frame.

As long as you're at it, I would suggest getting one with an electrostatic shield. Many of the straight isolation xfmrs come standard with one. This shield also gets grounded to chassis.

[astouffer]

You can also use the poor mans isolation transformer. Get two identical transformers and connect the secondaries back to back so the line goes in one primary and comes out the other primary. You might want to try Duncan's PSUD program because 240v full wave rectified will be over 350vdc.

[eeyore]

Thanks for all the replies. To clarify, I need before the recitifers 240V 100mA, which is about 24VA, so I guess the minimum 50VA sounds like a good start. Where would I go about finding one of these isolation transformers. The few that I have found are all-in-one units, with case, filtering, etc. Thanks again.

[Eli Duttman]

David,

Jack's circuits show a 240-0-240 rectifier winding working into a "classic" CT full wave rectifier. Perhaps someone who resides in a "240" V. zone can suggest a "1:1" isolation trafo, that could be combined with a full wave bridge rectifier to yield the B+ rail needed.

Another approach to the problem is to use a Triad N68-X, which has dual primaries and a "120" V. secondary. A "full wave" voltage doubler PSU, as found in "El Cheapo", would be employed.

FWIW, I checked the Hammond site and did not find a suitable model with "universal" primaries.

[croccodillo]

One can use two standard main transformer 240:6V side to side:

Connect the 240V primary of the first transformer to the main inlet, then use the secondary winding for the heaters and connect the second transformes inverted (6:240V) to the first one.

Look at the SCHEMATIC.

In this way using two cheap transformer you can have HV and heater supply.

Ciao,
Giovanni

[zigzagflux]

Allied Electronics has them.

Most are open frame, not having the cute little end bells, but they can be had for $75 or less.

[Shoog]

The only problem with back to back transformers is that regulation is increased, its either added or multiplied (don't know which), so you need the transformers to be a lot bigger than they otherwise would need to be.
I have used back to back a few times and it works well, but I would restrict its use to preamp circuits due to the regulation issues.

Shoog

[engels]

I would not recommend two "cheap" 6V transformers back to back. I also live in a 230V country and I've tried many setups like this. Two not the cheapest 1A 12V transformers wired back to back may supply 20-30mA of 210V. I've tried to make a power supply for a SE 6V6 + 6SJ7 amp out of small trannies but they buzz and overheat. I've tried the same with a beefier 12V@4A transformers intended for neon lamps but they also overheat when there's more than 40mA of HV current. I'm sure you will need something like two 10A low voltage transformers to supply 100mA and it will cost you a fortune.

Btw, I was using separate transformers for heaters in the above mentioned "experiments", so the overload came from the hi voltage load only.

This 230V in the socket is sort of that magic garden from Alice in Wonderland: you can see it but you cannot get there.