Replacing PSUs with + & - 5v Rails - Page 1
Forum section: electronics / projects

author: dexters_lab
author_rank: Unknown
timestamp: February 06, 2017, 10:54:05 am
content: I have a device here i need to replace the power supplies in, at the moment it has two PCB mount potted bricks that both have 240v mains input, one supplies +5v and the other supplies -5v to the device.The brick supplying the -5v looks like it's failed so i need to replace one or both of them.I had a look around and found i have two brand new Toko SWE15-05F 5v PSUs which would fit inside the original boxFor the keen eyed, lets ignore the current ratings for the moment but can i tie both of these Toko PSUs together to get + & - 5v?

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2017, 11:11:49 am
content: No reason why not if you can mount them safely. The SWE15-05F datasheet shows a fully floating output. However I would strongly recommend adding a 1A polyfuse to each rail as the new supplies have a far higher current capability. Another option would be to use one only and derive the negative rail from an inverting output DC-DC converter module but before you do that, you need to be sure there wont be any problems due to power sequencing if the negative rail comes up slightly slower.

author: dexters_lab
author_rank: Unknown
timestamp: February 06, 2017, 12:12:29 pm
content: thanks, i just removed the two old power brick/modules ready, the new ones will mount in the box underneath this PCB which is plenty big enoughThankfully the PCB already has fuses in place; FS1 & FS2 just next to where it says 'BAR CODE' in the pic, the replacement PSUs actually have 1/2 the current available than the original ones but i am really doubt the circuit uses much so i am going to power it from my bench PSU first to check what it actually consumesjust been reading the datasheet (i've attached the one i have), on page 6 i just spotted this:Quote

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2017, 12:46:41 pm
content: I misread the current rating in your photo of the original modules (I thought it was much less).The difference in current ratings is a concern, as is the prohibition of series connection, (though *IF* the diagram is accurate and the secondary side is floating, a reverse biassed Schottky diode across each output, rated to carry the maximum output current, will almost certainly avoid any problems).If you can verify the worst case current consumption is low enough, and the equipment doesn't draw a large current surge at startup (which might have forced a higher rating for the original modules), two modules wired for a split rail supply should be viable. I'd throw out my DC-DC converter idea - it was based on the assumption the replacement module had amps to spare.

author: dexters_lab
author_rank: Unknown
timestamp: February 06, 2017, 02:53:23 pm
content: thanks Ian, could you explain in a bit more detail about the diodes? i'd rather learn something new than just blindly do ittbh if there is a question over the suitability of these Toko PSUs i'd rather just find an alternative solution, my only desire to use them was because they are new and sat here unused

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2017, 03:28:30 pm
content: Its an issue with startup and with current limiting. The two supplies are unlikely to be exactly matched so if there is an overload (even transient) between the +5V and -5V rails, whichever supply that has a slightly higher current limit can continue to drive current through the weaker supply after its output collapses, reversing its polarity and damaging the secondary side voltage feedback circuit. By adding the Schottky diode, any polarity reversal is clamped at less that the typical forward voltage of a Silicon junction, and the bulk of the current is diverted.

author: dexters_lab
author_rank: Unknown
timestamp: February 06, 2017, 05:05:03 pm
content: thanks for the explanationi've actually just decided to do it another way, i pulled two Lambda PM10-24S05 5v DC-DC converters off a board i was scrapping where they were configured as + & - 5v, i should be able to dig up a small 24v laptop style power brick and run the Lambda modules off that. It means it takes up less space, will be easier to do, i don't have to worry about the series configuration and means there is only one mains supply in the box. The Lambda modules have better noise and regulation spec too.Both rails use 1.5amps or less so within spec for the PM10 modules.