My first linear power supply using only discrete components - Page 1
Forum section: electronics / beginners

author: szabolcsx
author_rank: Contributor
timestamp: June 14, 2014, 09:54:24 pm
content: Hi everybody. I'm a beginner in electronics and as my first bigger project I decided to build a constant voltage/current power supply using only discrete components. The constant voltage part of my design is working reasonably well, it's adjustable from 0 volts to 25 volts, and it's stable enough with 2 amps load. My second purpose is to expand the schematics with current limiting capabilities. And that's where I faced the first major problem. I don't know how to design a variable current limit which goes down near 0 mA, using discrete components only. I tried some methods found on the internet, but at now the lower limit is near 500 mA. I made tried an other schematic that can limit down to 0 mA, but when I turn all the way down, the output voltage drops too but there is no load connected, or this is normal? I will attach two schematics. The first will be which could not limit below 500mA, and the second one will be that messes with the output voltage if I turn the current limiting pot all way down. I started to design my circuits in LTspice, but I haven't found any symbol for pots in software, so I replaced the pots with simple resistors. So in the first schematics R13 is a 10k pot, and R3, R4 forms together a 100 ohm pot. On the second schematics R13 has the same role, and R4 is a 10K potmeter. The part which is marked on the second circuit is a reference voltage generator. I know that there are specific chips for that purposes, but as a beginner, I want to learn how does these circuits works, so please help me to expand my circuit. And sorry, if I made some mistakes, English is not my native language. I would appreciate any help

author: rob77
author_rank: Super Contributor
timestamp: June 14, 2014, 10:03:36 pm
content: regarding your schematic - it's a messbut we're all learning all the time, so no worries - you'll learn to draw them better over time1. where are the output terminals ?2. is the 1R resistor the load or the current sensing ? (looks like a sensing)regarding the voltage drop @ 0mA - that's normal if you turn it down to zero, the voltage will drop with most of the circuits - try to turn it down to few miliAmps for example and measure the voltage.

author: szabolcsx
author_rank: Contributor
timestamp: June 14, 2014, 10:07:02 pm
content: I forgot to mention. The sensing resistor is R1 in the both cases, the output across the 1 ohm resistor. That resistor is the load, I simulated a high load, and I tried to adjust the current limit and see how low could go.Edit:If I adjust to 1 mA, with a 4,7k resistor as load, the output is near 4 volt, but doesn't stay sable, decreases with time. If I remove the load, and I turn the current limiting pot all way up, the output voltage is around 20 volts.With a 470 ohm load resistor at 10 volts I get around 20mA. If I adjust the current limiting pot to 10 mA, the output voltage decrases to near 5v but still drops with time and stops near 1.8V

author: rob77
author_rank: Super Contributor
timestamp: June 14, 2014, 11:33:00 pm
content: i had a closer look at your schematics... are those based on some IC design ?btw.. why not to use OP amps ? some cheapo jellybean ones would do better than the discrete transistor dif amp you have there for voltage regulation. and you could do the current sensing part with an op amp too. if you'll chose an op amp which can go down to ground rail on the inputs, then you'll not need the negative supply at all.

author: szabolcsx
author_rank: Contributor
timestamp: June 15, 2014, 06:35:23 am
content: I found similar designs on the internet and then I found an old book which contained a design like this. But at first I haven't understand how it works. Then I sesearched a lot with breadboard and I discovered the basics. I could replace the diff pair with an op amp, but I don't want to do that, because as I mentioned before I want to learn, I want to measure every node and determine why I got that value. The opamp would do better job, but this design also works well for me. So at first I want to use only discrete components then if I build a second power supply I will "increase the level" and I will use some monolithic chips too. So do you have anything to say about my original question, the current limiting. It's not a problem if we replace that part of psu:)

author: Stonent
author_rank: Super Contributor
timestamp: June 15, 2014, 07:17:47 am
content: There's also the video Dave did a year or two ago on power supply design where he went from an LM317 to some nicer regulators and then eliminated the second regulator.

author: szabolcsx
author_rank: Contributor
timestamp: June 15, 2014, 07:26:35 am
content: I want to use only discrete components.

author: szabolcsx
author_rank: Contributor
timestamp: June 17, 2014, 08:30:22 am
content: I got it to work. I made an other differential amplifier(in fact it's a non inverting amplifier) which amplifies the voltage drop of shunt resistor. This amplifier has an adjustable gain with a pot. The output of this amplifier is fed with a series resistor to an other transistor's base. If current is drawn, the amplifier amplifies the voltage drop on shunt resistor depending the state of pot and if it's enough to turn on the transistor that will shunt the second differential amplifier's (which controls the output voltage) to ground.

author: Willy.nebula
author_rank: Newbie
timestamp: June 21, 2014, 12:07:01 pm
content: hi szabolcsx i would like to see your new schematic plz. As i to am learning

author: szabolcsx
author_rank: Contributor
timestamp: June 22, 2014, 10:01:30 am
content: This is the last version. It's also built on prototyping board and works. I haven't checked how noisy is, but I will. R4 sets the maximum output current, R6 sets the minimum current which is required to turn on the current limiting. The R5 sets the actual current limit, R16 sets the output voltage, and R9 sets the reference voltage. The R9, R10, R11, R13, R14, Q13 and Q14 could be substituted with a 5.1V zener diode. In that case, R12 needs to be changed to 3.3K. If you have other questions, just ask them and I try to answer.Edit: The current limiting still can't go down to 0mA, the minimum is around 70mA, but is still lower than 500mA.

author: Jebnor
author_rank: Regular Contributor
timestamp: June 22, 2014, 01:13:00 pm
content: Have you read this thread? The OP there is doing a good job explaining what is going on. Perhaps you could use it as more reference for your design.https://www.eevblog.com/forum/projects/another-kit-all-discrete-power-supply/

author: Paul Price
author_rank: Super Contributor
timestamp: June 22, 2014, 01:30:23 pm
content: What kind of spec do you have for line/load regulation at different output voltages (max output V, or V close to zero) under load?What happens if you set current to max and voltage to max and short-circuit the output?What happens if you set output voltage to midrange and short the output momentarily? What is the transient response on a scope?What if you remove a short-circuit condition on the output after a few seocnds. How does the output voltage change?How does the output voltage change with temperature rise after being loaded for full-output current for several minutes?How does the power supply react to a load like a brush-type DC motor with a paralleled electrolytic capacitor across it?

author: szabolcsx
author_rank: Contributor
timestamp: June 22, 2014, 02:35:08 pm
content: I have only multimeters so I could play with scope only when I'm at school. The maximum output voltage is set to 25V and the minimum is 34mV. If I set the output voltage to 1 volts with a 4.7 ohm resistor as load, the voltage drops to 0.998V. At 4.7V with the same load the voltage is 4.68V. At 10V with a 10 ohm resistor as load the voltage is 9.98V. At 15V with the same load the voltage is 14.93V. At 20V if I connect the 4.7 ohm resistor the current limit turns on and the voltage decreases to 10 volts. At 25 volts if I short the leads, at maximum current limit the voltage decreases near 0 and the current near 2.5A. If I turn down the current limiter pot, the current decreases. At 20V after 5 second of short circuit the voltage goes back to 20V instantly. At 15.17V when I connect the DC motor with a paralleled capacitor, the voltage decreases while the capacitor charges due to current limiting circuit then recovers, it is 15.16V. I can't test how does the output voltage change at full load after several minutes because I don't have proper heat sinks (I think. I need to calculate the parameters of the heat sink). All of these information are based on my measurements with a pair of UNI-T UT33D.

author: szabolcsx
author_rank: Contributor
timestamp: June 22, 2014, 04:16:47 pm
content: Jebnor, thank you for showing me that thread, it's really useful, but I think I should wait for the complete documentation, because it's (or it seems) a little bit complex than mine.

author: szabolcsx
author_rank: Contributor
timestamp: June 26, 2014, 09:11:40 pm
content: Hi everybody, I have a new schematic. The new version has one change. It measures the current on high side, so the shorts to earth will be detected too. I haven't tested yet on breadboard, but I will when I have some free time.