Bench supply from a PC PSU? - Page 1
Forum section: electronics / beginners

author: Morlan
author_rank: Newbie
timestamp: October 23, 2012, 12:03:19 pm
content: hi,I'm thinking about making a bench supply from a PSU supply.I'm thinking of using 4 channels, 2 using the 5v and 2 using the 12v.Thats pretty easy by itself, just need to use the soft-on button to start/stop the supply.Except that i would like to be able to range using a PIC.Would I be better off using just the 12v lines and rate from those (persumably giving me a Max12v)could I use some kind of jFET to limit the current down and some kind of Vdivider. If so how would I make a divider that isnt just 2 resistors (need to be digital)Any ideas?

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2012, 12:28:19 pm
content: Modern PC power supplies have the most power on the 12v rails, so yes, they would be best.To have a variable voltage controllable by a pic you will need to build a switchmode buck converter to take the 12V and produce your output volts.The PIC can output a voltage (filtered pwm) to the feedback system on the switchmode IC. If you pick the right switchmode IC you can probably do high side current sensing to get current control from the pic in the same way.Another way is to trace the PC powersupplies own feedback circuit and try to control it directly to give different output voltages on the 12v rail.However that can be a bit complicated (but less design work needed).

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 23, 2012, 01:01:58 pm
content: If you are going to get a PC PSU and put a buck converter on to it, you should get a server powersupply - they usually only have 12v rails but are generally far more reliable and well built for the money, plus they usually don't need cross loading to regulate well, as they are designed to use 12v only. There is also a lot on information on them, things like how to isolate the outputs and much more.

author: mariush
author_rank: Super Contributor
timestamp: October 23, 2012, 02:33:58 pm
content: You don't need a server power supply.Yes, the cheaper power supplies are usually group regulated, meaning the output voltage on 12v won't be quite steady when you don't have any load on 5v, or the other way around. The solution is either to use a couple of 10w resistors on the 3.3v and 5v rails to give it a minimum load, or to simply to go for a power supply based on a modern design where the 3.3v and 5v are generated from 12v using dc-dc converters.With the later, the power supply won't care about 3.3v and 5v.You can use a PIC or an Atmel (whatever is used in Arduino) and if you don't know how to do pwm to generate a voltage for the adjusting the regulator, you can use digital potentiometers.For a few amps, you can simply use a linear regulator to go from 12v to 5v, provided you'll put the linear regulator on a good heatsink and in the flow of the fan to keep it cool.If you go the linear regulator route, you'd probably want a switching regulator to generate about 9.5v out of 12v and switch the linear regulator between the 12v and 9.5v input depending on what the output voltage is desired - you simply want the voltage dropped on the regulator to be as kept as possible. It's not a good idea to get 6v 1A output out of 12v as that would be (12v-6v) x 1A = 6 watts dissipated as heat on a tiny heatsink.I guess you could do a tracking switching regulator, which would stay about 1v above the linear regulator but that's a bit more complicated.PS. On a lot of power supplies, you can adjust the feedback on the controller so that the 12v output goes up to about 18v and the psu is still stable. It's just replacing a few resistors and capacitors. However, if you go over about 14v you'd need to replace the output capacitors, changing the 16v rated ones to 25v rated ones, and that can be a pain - it's often hard to find 25v rated caps to fit in such small space.

author: Morlan
author_rank: Newbie
timestamp: October 24, 2012, 10:17:53 am
content: Thanks guys, some great info already here.I have used PWM before, i did'nt think about using PWM to control the output, i thought there was only possibility for liniar type controllers.So, it would be reccomended to start from scratch? Dave had some great videos i would like to try. again im still an electronics beginner by far lol.I'm not bothered about cred and "its all my own design", im purely resaults based on this project.I will post again when im back from work, and look into some of this.Mors

author: Psi
author_rank: Super Contributor
timestamp: October 24, 2012, 10:30:02 am
content: Quote from: Morlan on October 24, 2012, 10:17:53 amYou use the PWM into a capacitor as a 'poor mans Digital to Analog converter'. This gets you a software controllable voltage you can use as the reference (desired output voltage) You then use the switchmode IC's built in comparators to compare this reference to the actual output voltage. The switchmode IC will automatically pull back if the voltage is higher than the reference and push more current if its lower. Thus you have regulation to the voltage your PIC is putting out.Current limit control can use another one of the built in comparators but using the opposite inputs. So if the current gets above the reference the output is reduced creating a current limit.The switchmode IC with 'OR' the two comparator outputs internally, so if either comparator says "pull back" the output is reduced.