Help. Powersupply for +-6V and up to 6A - Page 1
Forum section: electronics / beginners

author: Maltitol
author_rank: Newbie
timestamp: July 10, 2018, 08:33:00 am
content: Hey Guy´s,i wanna build a +-6V Powersupply and up to 6A output current. I have found an very old schematic and i try to understand how it works. I´ve already simulated with LTspice and it works.But i will understand this.I have simulated only the +6V part with other Transistors because i didnt have the old Spice Models.In the Original Schematic:Q1 = 2N3903Q2 = 2N3054 (4A 40W)Q3 = MHT7603 (10A 60W Vco 100V)Q4 = BC171BMaybe you can help me or give a tip on how I could do it better.

author: JS
author_rank: Frequent Contributor
timestamp: July 10, 2018, 08:44:10 am
content: The transistor you don't know what is for is limiting the current, for protection.The major issue I see with this is the very very low efficency and high power dissipation, as your input is 11V you need to dissipate almost half the power, or almost the same going to the load, that's 36W!! you will need quite a chunk of aluminum and a fan to do that!Do you need to be efficient? Can you have such heatsink?Is your input voltage fixed? Still with this design you will need at least 8V and still dissipating 12W, different topologies could do better, using maybe 7V input and be fine, just dissipating 6W. If not you should consider a SMPS. Still low voltage high current supplies aren't very efficient by nature.JS

author: Maltitol
author_rank: Newbie
timestamp: July 10, 2018, 08:58:38 am
content: Quote from: JS on July 10, 2018, 08:44:10 amIt does not have to be efficient. I can use a relatively large heat sink. In addition, I have seen in the old documents that I can switch several Q3 parallel. i only want a low output ripple. In no case a switching power supplyBut if there is a solution that is more efficient, that would be fine too. Because then I do not have to worry so much about the power loss.

author: JS
author_rank: Frequent Contributor
timestamp: July 10, 2018, 09:56:43 am
content: As I said, lowering the input voltage would be the first step, ensuring 8V considering all previous variations to the transistor collectors shouldbe good enough for that circuit. Then, if you can get away with one less transistor at the darlington better, you could get away with less than 8V. Then if you change the bjts for a mosfet you could get even closer to the output.JS

author: Maltitol
author_rank: Newbie
timestamp: July 10, 2018, 10:53:31 am
content: Quote from: JS on July 10, 2018, 08:44:10 amI have seen that I have an error in the current monitoring circuit.At Q4 is still a resistor to ground and the emitter has only 33 ohms against the 6V.At the moment the smallest input voltage is at 11V. So I have at 3A load 6V at the output. But if I reduce the input voltage to 10 then my voltage drops to 5.5V after about 50ms. Only tested in the simulation. I think that's because of the collector emitter voltages from the transistors.If I build up the power supply then I can choose the voltage supply. A Transformer between 8 to 14 volt with bridge rectifier and filter capacitor.i have attachted the new Schematic and a Traceplot for Input Voltage at 10V

author: JS
author_rank: Frequent Contributor
timestamp: July 10, 2018, 04:45:20 pm
content: The 33 isn't against 6V, look that the base emmiter and resistor is connected to the output resistor, when the voltage at that resistor is enough to turn on the transistor this transistor turns off the pass transistors and limits the current. Even in a short circuit this transistor will only see about 2V, from collector to emmiter and the 33r.To get lower input voltages to work you should increase the voltage of your opamp, feed it with 12V or something and let the power rail ve 9 or 10V, see what happens. Later on your design you can replace this second source with a voltage multiplier, from the ac directly, just few cents in components.JS