transformer for multi output psu - Page 1
Forum section: electronics / beginners

author: Deathwish
author_rank: Unknown
timestamp: July 08, 2015, 11:42:29 am
content: Is a 6VA 2x 20V transformer enough to make a dual 12v rail and a 5v rail good enough ?. Its only to run one of those chinese DDS function Gen boards.

author: Simon
author_rank: Unknown
timestamp: July 08, 2015, 11:46:20 am
content: Do you know how much current you need, that is only 150mA per secondary and if your using a bridge rectifier and smoothing capacitor you need to be careful not to overload the transformer so I'd go 100mA max

author: Deathwish
author_rank: Unknown
timestamp: July 08, 2015, 11:56:59 am
content: No idea Simon. Its for this thing I got.http://www.ebay.co.uk/itm/291399023398?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

author: Simon
author_rank: Unknown
timestamp: July 08, 2015, 12:01:46 pm
content: you need to measure the current consumption, I guess it won't be much Try it and see, a short overload won't hurt too much.

author: Deathwish
author_rank: Unknown
timestamp: July 08, 2015, 12:02:59 pm
content: Problem is I have nothing to power it up with at present. Hence the need to build a psu for it and then box it.

author: Simon
author_rank: Unknown
timestamp: July 08, 2015, 12:05:11 pm
content: Build the psu and test it, if it draws too much disconnect it quick, or get some batteries to test with.

author: Deathwish
author_rank: Unknown
timestamp: July 08, 2015, 12:09:01 pm
content: Ok. I might just go OTT and either get a 100VA one which should put out about 5A, or just hedge it and shove a 12VA one on it for around 300mA. I hate trying to work such things out.Thanks for the help Simon , it is appreciated.

author: Simon
author_rank: Unknown
timestamp: July 08, 2015, 12:24:12 pm
content: I can't imagine it needing more than 500mA

author: Deathwish
author_rank: Unknown
timestamp: July 15, 2015, 09:55:32 am
content: referring back to this, I want to draw +12v -12v +5v and possibly +3.3v from it would 18V secondarys be ok.

author: Simon
author_rank: Unknown
timestamp: July 15, 2015, 09:58:24 am
content: Do you mean change to a transformer with 18V secondaries, yes that is fine. You can go down to 15V for a 12V output.

author: Deathwish
author_rank: Unknown
timestamp: July 15, 2015, 10:03:34 am
content: Thank you Simon

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2015, 10:11:17 am
content: If you find you are marginal on the +V current capability, consider using switching regulator modules from the unreg +V rail to get 5V and 3.3V as they will need three time less input current for the same output current compared to linear regulators (or add a separate 8V transformer to feed the low voltage linear regulators).

author: Simon
author_rank: Unknown
timestamp: July 15, 2015, 10:13:07 am
content: Yes switchmode regulators will make much more sense, you will gain so much more power as efficiency will be at least 70% and probably 80%, linear regulators will just waste lots of power by chucking it out as heat.

author: rolycat
author_rank: Super Contributor
timestamp: July 15, 2015, 10:48:25 am
content: I believe the original design of which your gadget is a clone can be foundhere.The power supply uses a 7.5VA 2x15V transformer with linear regulators.

author: Deathwish
author_rank: Unknown
timestamp: July 15, 2015, 11:45:46 am
content: Thanks Roly and everyone, I can only find a 6va or a 12Va transformer at 2x15v / 2x18v anything else is more than I can currently afford.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2015, 03:35:28 pm
content: If you use a centre tapped winding into a full wave bridge to provide a + and - rail, the 6VA 18V transformer will only be good for 105mA from the +V rail. The 6VA 16V transformer is good for 120mA, (using the usual full wave bridge de-rating factor of Iac=1.6*Idc). The transformer will run somewhat cooler than it would at its max rating due to the low load on the -V rail.However if you parallel the two windings and use a capacitively coupled bridge rectifier for the -ve rail, you can utilise the transformer more efficiently and you can get nearly 200mA of -ve rail current + 40mA of -ve rail current from the same 6VA 16V transformer, which is more than the original design's PSU could have delivered without overloading its 7.5VA 15V transformer.

author: Deathwish
author_rank: Unknown
timestamp: July 15, 2015, 04:17:30 pm
content: Thank you Ian.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2015, 04:54:31 pm
content: Note correction above: 200mA of +ve rail current not 240mA.Also ithe LTSPICE simulation is overloaded by about 3% as I wanted to be sure the rails would hold up at the maximum output the transformer can provide.Seehttp://hammondmfg.com/pdf/5c007.pdffor derating factors for various transformer/rectifier topologies.Be careful of the polarity of C3 and C4. They also need to be low ESR with a ripple current rating of 1.6 times the negative rail load current.You should use a switching regulator for the 5V rail, direct off the unreg. postive rail, especially if the LCD is backlit.

author: Deathwish
author_rank: Unknown
timestamp: July 21, 2015, 12:51:01 am
content: Bah, Garry snafu'd again. I ordered the transformer thinking as it stated CT at the start of the number it was a centre tapped one, it has 2 separate secondaries which means I have either 2 single 18V lines out, or a single 36V if the two are in series. guess I am going to have to order a dual 9v output one now. RTFW page properly next time !.

author: rolycat
author_rank: Super Contributor
timestamp: July 21, 2015, 01:26:30 am
content: If it is a 12VA model, the transformer you have will be fine. Link the two secondaries in series and lead out from the link as your centre tap.If it's only 6VA, you can parallel the windings and adopt Ian's suggestion for a larger current from the positive rail.

author: Ian.M
author_rank: Super Contributor
timestamp: July 21, 2015, 01:42:04 am
content: Don't forget to check the phasing when linking secondaries. Make ONE link only then measure the voltage across the other two winding ends. If its near zero they are phased for paralleling, if its double the single winding voltage they are phased for series use. If the phasing is wrong for the intended use, reverse the connections to ONE winding.NEVER parallel winding without confirming correct phasing first!

author: Deathwish
author_rank: Unknown
timestamp: July 21, 2015, 06:24:06 am
content: Rolycat .. it's a dual 18v 50Va .Ian.M... Thank you