power supply design  - Page 1
Forum section: electronics / projects

author: old gregg
author_rank: Regular Contributor
timestamp: August 18, 2013, 04:53:29 pm
content: hi everyone ,I'm new here so I hope I'm doing things right...I decided to build a bench power supply . Nothing new I guess . I want it as simple as possible but I came across few design issues (issues come quicker with simple designs). I'll will use the famous 317T/357 couple regulators.the power supply will be built mainly for audio circuitry breadboard project . I need a variable V- as well . I got the power transformer already. It's a 2x15v 16VA. I had the idea of creating two power supllies in one box . both with V+/0V/V- . But I'm not quite sure about this , is a dual power supply that handy ? It increases the cost and i'm not sure that I will use the second power supply. So I think you guys are more experienced than I'm with this helpfull tool so maybe you can give me inputs ?voltages should go from 3V3 to 18V with 1A max (if a simple power supply) or 500mA max for each channels (if dual power supply). This will be acheived with a rotary switch which will connect trimmers in the circuit for the proper voltages (3V3 , 5V , 9V , 12V , 15V , 18V ) . Potentiometer has been an option but the fact that the voltage won't be displayed an error can occur badly . So voltages presets (labelled with Dymo) should be better. Beside the rotary switches are harder to manipulate to minimize accidents. Of course , this lead to the fact that the voltage isn't really variable and voltages between presets aren't available.I'm also thinking of creating another circuit for the low voltages in order to keep the regulator cool. 22V input and 3V3 output isn't really a safe condition for the regulator I guess , especially with such big currentThe other issue , the big one, is the variable current limiter . I'd like a current limiter but once again with such power, implanting a simple limiter is pretty hard . I thought about a power mosfet used as a variable resistor but I don't know about its linearity or even if it's doable. I know that JFET can be used like that (but with really annoying added circuitry and trimmers). Obviously , i could use massive 20W resistors and another rotary switch but it's not an elegant way of doing (even though it's functionable)To not forget I've also desinged a simple short circuit protection , just an idea and didn't have the component yet. The switch mosfet should be a beefy one .http://smokingtip.files.wordpress.com/2013/08/mosfet-protect11.jpg(link because the image is huge)the idea is to control the pass mosfet like a switch. If the control mosfet sees 0V on its gate its drain/sources conducts , putting the gate of the switch mosfet on ground thus it stop conducting (high resistance). The PNP transistor is here to light the LED when this happen . Mainly to avoid me getting pissed off with my messy breadboard. The additional components (caps , resistors ) are here to prevent inrush current because of the capacitive load (inspired by a nice application note about mosfet switches)So, I'd like few comments , ideas , advices about my project . Dual or simple power supply ? variable current limiter ?thanks for reading .best regards

author: Dave
author_rank: Super Contributor
timestamp: August 18, 2013, 06:39:18 pm
content: Q1 - that should be an NPN transistor, not PNPQ2 - flip it horizontally (switch drain and source connections), otherwise you just have a diode thereQ3 - that should be an N-channel MOSFETI don't really see how this circuit would start, because Q2 would never get turned on. You need a button (or something like that) to reset the latching mechanism of your circuit.Are you really fixed on the idea of making an LM317 based supply? You could try making a copy of the famous HP E3610A power supply. It's going to be much more reliable and stable than the LM317.If you are interested,hereis the service manual for these supplies (you can find the schematic on page 60; the descriptions of the principles of operation start on page 51).

author: old gregg
author_rank: Regular Contributor
timestamp: August 19, 2013, 04:17:20 pm
content: thanks for the answer !Q1 must be PNP if I want it to trun on only with 0V at the gate. In the doubt I'm right now having a PNP transistor hooked up with a led , and it turns on only with 0V the base.for the other Q pointed , yes I've been messy with the schematic . My bad. That was more a visual idea than a final schematic.I'm not fixed with the lm317 actually. I've still have to dig a bit about this regulator if I go for a single power supply 1A . The main goal remains simplicify and avoid a complex circuit Thank you for the pdf I'll read it. Not sure that will copy it but there's certainly things that I can borrow.Actually I didn't dig deep enough because it exits some voltmeter display :this allow me to add a continous variable voltage potentiometer ! (still have to check the datasheet and the current needed)

author: Dave
author_rank: Super Contributor
timestamp: August 19, 2013, 05:28:22 pm
content: You are right about it needing to be a PNP, but you still have to flip the collector and emitter.Also, a PNP transistor opens at Vbe = -0.6V, not 0V.I have also tried simulating your circuit, and it does work, but it seriously screws the (already poor) regulation. And you are still relying on the voltage dropping below the Vgs threshold of Q3 to activate the overload protection. If your load doesn't load the supply that much, you components will still get overloaded.I suggest you put a shunt into the circuit and use an opamp connected as a differential amplifier to read that voltage. You then compare that with a reference voltage, and if it exceeds the reference, it trips a latching protection circuit. You then add a pushbutton to reset that latching mechanism.

author: old gregg
author_rank: Regular Contributor
timestamp: September 07, 2013, 05:36:08 pm
content: Hi ,sorry for the delay. Thanks for your time and the fact that you simulated the circuit. No doubts that the circuit sin't that safe.After thoughts and thoughts I think I'll redesign or make something simpler . The huge voltage drop across the regulator (in low voltage mode) is ridicoulus considering the current. I think a poprer transformer designed for such application should be better , with serveral winding on the secondary and for the display and controls electronics.I'll keep all the basic fonctions and think really clearly what I want with this.