Any ideas on what this transformer can handle? (and making power supplies) - Page 1
Forum section: electronics / projects

author: XOIIO
author_rank: Super Contributor
timestamp: May 11, 2014, 06:38:56 am
content: Hey all, I'm currently somewhat interested in power supplies, and possibly making my own variable one, I have a 30v 5 amp one, and want to get a dual channel one of those off ebay at some point but I am kind of interested in making my own, I plan on making a 3.7v lithium battery powered one with an ebay step up board (the kind you adjust with a 10k pot), I just need to figure out how I can get the voltage down to 0 and then go up using the transformer using one potentiometer if I can.Anyways AC ones of course are more diverse due to the higher current they can put out, and I have some small power supplies from large routers that put out 12v and 5v at 1 and 1.5 amps each, so I could probably rig up a dual channel one, again I'd need to make a circuit to both be able to drop the voltage down and boost it past 12v.I think starting from scratch would be easier, but I'm curious as to how people go about finding/choosing a transformer for a power supply.I have a few from UPS and other device teardowns, there's an interesting vitec one here but I can't find any info on it, it has the following info:58P6075430-5007_0It's the kind that just solders into the PCB on pins.Anyways on one side it has some decently thick wires, and on the other side it has two separate sets made of several smaller wires (all are enamel coated).I was wondering if there is a way with this one or transformers in general you can get a rough estimate of what they can handle/what they were for if you can't find any info, maybe based on winding resistance or something like that? I've never really done much with transformers so it's fairly new it me. I can post a picture of it tomorrow.

author: mariush
author_rank: Super Contributor
timestamp: May 11, 2014, 07:30:43 am
content: A transformer takes AC on one side, outputs AC on the other side.If your power source is a 3.7v lithium battery, there's no reason why you would need a transformer for such a power supply, because you already have isolated dc voltage.The transformers in those linear power supplies are there to provide isolation from the mains 110-240v (depending on country), and they're also the classic type (large, heavy) because this way the power supplies can achieve low noise, low ripple, very good precision when setting the output voltage if a 10-15 turn potentiometer is used and so on.The battery is already isolated from the mains (it's a battery, duh).The boost dc-dc converters from eBay don't work the way you think they work. Yes, they say input voltage 2v - 30v and output voltage 3v -35v at 3A or something like that, but that doesn't mean all the extremes are possible in any condition. I mean you can't get 30V at 3A from 3.7v... you probably can't even get 15v at 1A with just 3-4v at the input with such dc-dc converters.To get 30v at 1A (30w) using such a dc-dc converter, the converter would have to "suck" about 40w from the battery (40w or more, because the efficiency of such converters is only about 65-70% at those extreme input/output voltages) ... so that means drawing 10A from the lithium battery. Those eBay converters aren't designed for 10A currents, they won't work right.The transformers from UPSes aren't ideal for linear power supplies. The 110/240v -> 12-24v transformers are often designed with the application in mind... charging batteries. A ups battery will not be discharged often, so that transformer will only be forced to output a lot of current only for short amount of times (while battery charges up to a level, after that battery will now draw that much current) so they're designed to potentially overheat for short periods knowing they'll be able to cool down afterwards.As for questions about transformers and how to choose, I've explained stuff about transformers in several threads to several people here... see the following threads, maybe it helps you:https://www.eevblog.com/forum/projects/bench-power-supply-3-30v-2a/https://www.eevblog.com/forum/beginners/transformers-30310/https://www.eevblog.com/forum/beginners/linear-psu-design-30207/etc etc etc ... just click search button and type there "transformer" and at search for user enter my nickname and you'll find some other threads. Or just use the search button.

author: XOIIO
author_rank: Super Contributor
timestamp: May 11, 2014, 04:09:11 pm
content: I'm not using the transformer for the battery powered supply.