Linear or SMPS for 5 Voltage Rails? - Page 1
Forum section: electronics / projects

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 04, 2012, 04:01:53 am
content: Hi. I am designing a curve tracer that operates on +/- 30V (for output sweeps), +/-15V for internal circuitry, and +5V for some digital stuff. We're looking at about 0.75A for the 30V, 0.1A for the 15V, and 50mA for the 5V.I am planning on using 2 24V transformers in series, to effectively make a center tapped transformer. I don't know much at all about power systems, as most of my design experience has been small signal with 50mA max draw from the supply. As such, I've never done an SMPS before.What do you think? Would dropping 30V down to 5V be reasonable through a linear regulator, or should I go for switched mode? Noise isn't much of an issue for this circuit.Thanks!

author: Psi
author_rank: Super Contributor
timestamp: December 04, 2012, 04:21:54 am
content: If you only need 50mA then yes you can use a typical linear reg, 7805 etc.The wasted power at max current will be (30V - 5V) * 0.05A = 1.25WYou will need a small/medium sized heatsink on it to handle that power and it will get very warm, but 1.25W is well within spec for a 7805+heatsink.If vcc noise doesn't really matter in your application then I guess it comes down to cost and linear regs are cheaper.You can always upgrade to a switchmode drop-in 7805 replacement in the future if you want. That would give you much more available current.The only potential issue i can think of is your supply voltage. You need to be sure its never going to be above the 7805 maximum of 35V. If the 30V is RMS and unregulated then this might be an issue.You could move the 7805 to your 15V rail, that would fix the issue.Or you could get a linear regulator that can handle 40 or 50V input (there are lots of regulators available, 7805 is just one of them)

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 04, 2012, 08:32:37 am
content: I would use the 15V as the source for the 5V. At 50mA the maximum dissipation is half a watt. I'd not bother with a switcher if you have no experience with them. Just selecting one can be a PITA because of the overwhelming amount of available devicesAs Psi says, you could upgrade later with those plug-in 7805 switcher modules.The nicest solution would be a transformer that has all the required windings; +/-30V, +/-15V and +6.3V but I think that would be custom transformer territory. An extra 6.3V transformer is likely not more efficient in the end.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 04, 2012, 04:25:31 pm
content: Thanks for the replies so far. I have a huge stash of 24V transformers that I picked up surplus for free, so I use those for everything. They're 75VA, and I'm not sure that I could cram 5 windings into one of those should I choose to rewind it.I was thinking about running the +5V regulator from the +15V as well, though this increases the power through the 15V regulator to 2.25W. Still shouldn't be a problem with small heatsinks. The chassis is pretty large and the entire device runs on ~60W, so a fan should keep ambient low enough.

author: SeanB
author_rank: Super Contributor
timestamp: December 04, 2012, 05:58:23 pm
content: If you have a lot then it is easy to strip one totally to a core and former and wind the secondary back on as the primary, then wind on additional windings to give multiple outputs. Just takes a little time, but you can go for a custom voltage for each output.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 04, 2012, 07:57:29 pm
content: Sean, I actually picked these transformers up with the intention in rewinding them for various purposes. I've just never bothered to try.

author: SeanB
author_rank: Super Contributor
timestamp: December 05, 2012, 04:53:09 am
content: Not difficult depending on construction.........