Easiest way of getting +/-15V - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: October 27, 2013, 04:20:51 am
content: I'm working on a design that requires +/-15V at around an amp, however I'd like to build it into the most compact form factor I can. +/-15V SMPS's are pretty common, but they're relatively huge!So I'd prefer a solution where I could maybe have a single voltage input, anywhere from say 12-24V, AC or DC doesn't matter, and somehow get +/-15V out of it (and fairly cleanly, too).I'm not sure if a simple virtual ground setup would work, as the final device will be connected to other sources that will have a real ground, and from what I know that will cause issues.Any recommendations?

author: Psi
author_rank: Super Contributor
timestamp: October 27, 2013, 04:33:51 am
content: We need to know how much current you need?If it's just to run some opamps a small/easy way is to use a premade board mount module.33mAhttp://www.digikey.com/product-detail/en/PQMC1-D12-D15-S/102-2806-ND/4009729100mAhttp://www.digikey.com/product-detail/en/PQMC3-D12-D15-S/102-2825-ND/4009748Not the cheapest way but definitely very easy and quite small.

author: Things
author_rank: Regular Contributor
timestamp: October 27, 2013, 04:43:33 am
content: Quote from: Things on October 27, 2013, 04:20:51 amThe high current is the issue. I was thinking of using an AC plug pack to power it, and just using 2 diodes to get the rails from that, but I think 1A is even gonna be pushing it in terms of what size plugpacks I can use

author: Psi
author_rank: Super Contributor
timestamp: October 27, 2013, 05:03:43 am
content: oh sorry, i read amp as amplifier.Quote from: Things on October 27, 2013, 04:43:33 amSmall switchmode plugpacks rated at 2A+ are quite common.

author: Rufus
author_rank: Super Contributor
timestamp: October 27, 2013, 05:37:29 am
content: Quote from: Things on October 27, 2013, 04:20:51 am+/-15v at around an amp is around 30W and a 30W SMPS will be the size it needs to be, actual output voltage and if it is split being almost irrelevant. So claims of relatively huge are false. A single output power supply feeding a 'something' is going to be larger than than a power supply providing split rails in the first place.

author: Things
author_rank: Regular Contributor
timestamp: October 27, 2013, 06:52:35 am
content: The final project has to go in a custom enclosure, so the power supply would ideally be external, ie a plug pack. Incorporating the PSU in the device will mean a much bigger enclosure, and also possibly exposed mains, which I'd like to avoid.

author: fcb
author_rank: Super Contributor
timestamp: October 27, 2013, 07:04:25 am
content: You haven't stated how 'regulated' you need regulated +/-15, but assuming you are running some analog.Use an 18v adaptor, create the +15v using a linear regulator. Use an inverting buck regulator (simple switcher or equivalent) running at high frequency to create a -18v rail, then another linear regulator to give you a clean -15v. If you get the frequency high enough this could be tiny, and if you can live with a bit of noise then just use a 15v adaptor and ditch the linear regulators.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 27, 2013, 07:16:31 am
content: I dont know if people are trying to make this as complicated or as confusing as possible....None the less!If it doesn't need to be regulated, I'd be getting an 11volt AC plug pack, two ~3amp diodes, and two 1000uf, 25 volt capacitors...If it needs to be regulated, then go with a higher voltage plug pack, and do the same, with some linear 15volt regulators on the resulting voltagesOne end of the transformer becomes ground, the other end has the two diodes connected to it as a half wave rectifier, one forming the positive, the other negative

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 27, 2013, 07:47:23 am
content: I'm replying to my own post.... I probably wouldn't do it like my diagram, because if the load on each rail is different, the voltage will be different too... my bad!

author: wraper
author_rank: Unknown
timestamp: October 27, 2013, 08:48:06 am
content: Quote from: AmmoJammo on October 27, 2013, 07:47:23 amIt is also nice way how to overheat transformer if loads are not equivalent.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 27, 2013, 09:29:19 am
content: Quote from: wraper on October 27, 2013, 08:48:06 amah, how so?

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2013, 11:38:44 am
content: If the load on the positive and negative lines aren't balanced, there will be DC flux induced in the transformer, causing core saturation and smoke.Buy a 30V power supply and us a high current virtual earth circuit to give +/-15VDC.

author: donpetru
author_rank: Contributor
timestamp: October 27, 2013, 01:32:40 pm
content: Quote from: Things on October 27, 2013, 04:20:51 amPlease, post here the amplifier power supply schematic? Then, I will give you the solution.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2013, 04:51:05 pm
content: Quote from: wraper on October 27, 2013, 08:48:06 amsurprisingly, Agilent did exactly this in their "currently priced as ~$1K" diff probe power module. they used toroidal though not sure how that affect induced flux, and of course linear regulators thereafter, i guess the heat loss in regs will equalize one side to another. anyway if you care about the heat, sink a'lil bit of electrons for the cooling fan, works like a charm, i know.anyway also check mornsun series, apart from Psi's suggestion and the lack of OP specification, hence this is just a speculative suggestionhttp://www.mornsun-power.com/UploadFiles/pdf/A_S-2W%20&%20B_S-2W_EN.pdfhttp://www.mornsun-power.com/cn/product_list.aspx?typeID=1Quote from: Things on October 27, 2013, 04:20:51 amare you expecting smaller linear type with greater rating? go back to reality please!

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2013, 05:30:52 pm
content: Quote from: Mechatrommer on October 27, 2013, 04:51:05 pmThe induced DC flux depends on the DC current through the 0V rail. If it's small (under 10% of the transformer rating), it won't cause too much trouble.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 27, 2013, 07:13:39 pm
content: buy this, fixed, done, no more questions!http://www.jaycar.com.au/productView.asp?ID=MP3055you'll obviously need the rectifiers in your device itself, and maybe regulators, depending how accurate you need the voltages to be, but its a centre tapped transformer, in a plug pack, that will do exactly what you need

author: tszaboo
author_rank: Super Contributor
timestamp: October 27, 2013, 08:28:45 pm
content: Quote from: AmmoJammo on October 27, 2013, 07:47:23 amYour drawing works. There are products out there with this concept. Use two voltage regulator also. It is not that magic. I also wrote about this.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 28, 2013, 05:19:51 am
content: Quote from: NANDBlog on October 27, 2013, 08:28:45 pmIt will only work properly if the load on both rails is fairly even, which depending on the application, they might be anyway