In need of a great power supply

Thread Starter

Bortmoun

Joined May 23, 2018
3
Hi there. I need a good bench power supply, which i want to build myself, for my hobby lab. The thing is, i have been thinking about LM-317 as my linear voltage regulator (and because here where i live i don't have no access to better ones). The design must have these features:

a)Current control from few mA to 5A

b)Voltage control from 0V to 30V.

For this purpose I've built this schematic, which i attached here. Please, tell me what you think about this project or if does work or does not, for i am a beginner in these things. R8 is just a dummy load.

The things i need help:

1) How can i add a minimum load (10 mA as by LM-317 datasheet) to keep the power supply operating in different potentials?

2) I need a replacement for 2n3822, i can't find it in my country too.

3) Can I run it from a fixed switched power supply (way more cheaper than buying a transformer here)?

4) In the future, I'm thinking about adding digital control with PWM to this circuit with Arduino or something like that. Do you think this is feasible?

Thanks in advance and sorry for my English.

BS.png
 

ian field

Joined Oct 27, 2012
6,536
Hi there. I need a good bench power supply, which i want to build myself, for my hobby lab. The thing is, i have been thinking about LM-317 as my linear voltage regulator (and because here where i live i don't have no access to better ones). The design must have these features:

a)Current control from few mA to 5A

b)Voltage control from 0V to 30V.

For this purpose I've built this schematic, which i attached here. Please, tell me what you think about this project or if does work or does not, for i am a beginner in these things. R8 is just a dummy load.

The things i need help:

1) How can i add a minimum load (10 mA as by LM-317 datasheet) to keep the power supply operating in different potentials?

2) I need a replacement for 2n3822, i can't find it in my country too.

3) Can I run it from a fixed switched power supply (way more cheaper than buying a transformer here)?

4) In the future, I'm thinking about adding digital control with PWM to this circuit with Arduino or something like that. Do you think this is feasible?

Thanks in advance and sorry for my English.

View attachment 152979
Looks like you've been at the application notes - not many bother doing that before asking us to do it for them.

Personally I'd hijack an old ATX PSU. Its hard to make it continuously variable and you need to remember a few tricks of the trade - but its much more efficient and you get much more current rating with most of the build already done.
 

Thread Starter

Bortmoun

Joined May 23, 2018
3
Looks like you've been at the application notes - not many bother doing that before asking us to do it for them.

Personally I'd hijack an old ATX PSU. Its hard to make it continuously variable and you need to remember a few tricks of the trade - but its much more efficient and you get much more current rating with most of the build already done.
Yeah, I have already thought about hacking some of my old ATX... Indeed it's a good idea, but it's not so rewarding as building myself one hehehe and it does not go up till 30V (well, at least not in the videos I saw). I'll stick with my linear voltage regulator but will try this in parallel too =)
 

wayneh

Joined Sep 9, 2010
17,496
Another vote for using an old PC power supply. I haven’t hacked mine at all because I’ve never needed to: I never need a voltage over 12V and if I did I’d just use the -12V leg of the supply. It doesn’t have much of a current rating but it’s enough. For voltages below 12V, and when 5V won’t do, I just use a linear regulator such as LM317. The thing is free and has a cord, a switch, a fuse, all sorts of built-in protection and so on. Really tough to beat.
 

AnalogKid

Joined Aug 1, 2013
10,989
Getting a switching power supply to deliver an output that is 250% over its design point is ... difficult. Step 1: dismantle and rewind the power transformer without knowing anything about its operating characteristics.

Separate from that, what is *your* opinion of the purpose of the two LM317's in series?

al
 

Thread Starter

Bortmoun

Joined May 23, 2018
3
Getting a switching power supply to deliver an output that is 250% over its design point is ... difficult. Step 1: dismantle and rewind the power transformer without knowing anything about its operating characteristics.

Separate from that, what is *your* opinion of the purpose of the two LM317's in series?

al
The first one is limiting the output current (at least in the simulation) but not quite controlling it (something like an over-current protection - I need to work around that if possible) while the second works as the voltage regulator. Correct me if I am wrong =)

And yeah, hacking the ATX would demand too much work disabling safety precautions to be able to go high voltage... But it's a nice idea if used near it's nominal values.
 

atferrari

Joined Jan 6, 2004
4,764
The first one is limiting the output current (at least in the simulation) but not quite controlling it (something like an over-current protection - I need to work around that if possible) while the second works as the voltage regulator. Correct me if I am wrong =)

And yeah, hacking the ATX would demand too much work disabling safety precautions to be able to go high voltage... But it's a nice idea if used near it's nominal values.
IIRC, in the ANs from National Semiconductors, there is one showing a double control current/voltage using the LM 317. Just in case, look one for the LM 301 (op amp). Fuzzy recolections.
 

AnalogKid

Joined Aug 1, 2013
10,989
The onset of current limiting will be "soft" because of the interaction between R1 and Q4. The current limit trip point set by U1 is then amplified by Q1 after the voltage drop across Q1 exceeds the turn-on voltage of Q4's Vbe. Both Q4's Vbe value and R1's resistance value will change as they warm and cool. Also, Q4 starts to conduct as low as 0.4 V - 0.5 V, and its Vbe can be greater than 0.8 V at higher base currents. Finally, the gains of Q1, Q2, and Q3 (and how they vary) are another source of error in the overall current control loop. So the overall current limiting performance is not nearly as good as that of the LM317 by itself. Way way better than nothing, but not "lab supply" quality.

ak
 
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