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In need of a great power supply

Matheus Mello

May 23, 2018
2
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May 23, 2018
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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
 

kellys_eye

Jun 25, 2010
6,501
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Jun 25, 2010
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6,501
Can you get hold of the LT1083 device? This is a 3A (or 5A or 7A, depends on the sub-set device you choose) and pin-compatible with the LM317. Saves all the extra circuitry.

Current limiting down to 10mA seems a bit excessive but the LM317 (or LT1083) can be wired as a current limiter using just one component.

A built circuit board using the LT-device is only around $5

https://www.ebay.co.uk/itm/LT1083-A...e=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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25,510
If you want a linear regulator which you can vary over a large voltage range and which has a high current output, a problem is likely to be power dissipation at low voltage.

There are several options:
  1. Reduce the current capability at low voltages
  2. Have an enormous heatsink
  3. Reduction of the voltage range
  4. Use a switching preregulator
  5. Change the input voltage to better suit the output voltage.
Presumably the first three are not desirable.

The fourth option involves having a tracking regulator (a switching regulator) set to maybe 4V higher than the output voltage. This means that the linear regulator only had to dissipate a smaller amount of heat but always has enough input voltage to remain in regulation.

The fifth option is either simpler or more complex (depending on how you look at it). It assumes you have multiple taps on your mains transformer. A circuit monitors the output voltage and selects the correct tap on the transformer for the current output voltage. This is not quite as effective as a tracking preregulator in minimising dissipation, but is electrically quieter.

As for maintain a 10mA load on your LM317, the easiest way is to make the "top" resistor on your voltage divider equal to 120Ω. There is always 1.25V across this, so this value will maintain a constant minimum load. If you look carefully at the datasheet you'll see that they make recommendations about this resistor for this reason.
 

Matheus Mello

May 23, 2018
2
Joined
May 23, 2018
Messages
2
Can you get hold of the LT1083 device? This is a 3A (or 5A or 7A, depends on the sub-set device you choose) and pin-compatible with the LM317. Saves all the extra circuitry.

Current limiting down to 10mA seems a bit excessive but the LM317 (or LT1083) can be wired as a current limiter using just one component.

A built circuit board using the LT-device is only around $5

https://www.ebay.co.uk/itm/LT1083-Adjustable-Regulated-Power-Supply-Module-Parts-and-Components-DIY-Kit/401194277841?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

Actually, that one I can put my hands on hehe! Thanks for the advice! I'll also look into it!

If you want a linear regulator which you can vary over a large voltage range and which has a high current output, a problem is likely to be power dissipation at low voltage.

There are several options:
  1. Reduce the current capability at low voltages
  2. Have an enormous heatsink
  3. Reduction of the voltage range
  4. Use a switching preregulator
  5. Change the input voltage to better suit the output voltage.
Presumably the first three are not desirable.

The fourth option involves having a tracking regulator (a switching regulator) set to maybe 4V higher than the output voltage. This means that the linear regulator only had to dissipate a smaller amount of heat but always has enough input voltage to remain in regulation.

The fifth option is either simpler or more complex (depending on how you look at it). It assumes you have multiple taps on your mains transformer. A circuit monitors the output voltage and selects the correct tap on the transformer for the current output voltage. This is not quite as effective as a tracking preregulator in minimising dissipation, but is electrically quieter.

As for maintain a 10mA load on your LM317, the easiest way is to make the "top" resistor on your voltage divider equal to 120Ω. There is always 1.25V across this, so this value will maintain a constant minimum load. If you look carefully at the datasheet you'll see that they make recommendations about this resistor for this reason.

Wouldn't the fifth option turn off the power supply while shifting from one source to another tap and by that cut the feedback to the monitor circuit? And it's difficult to find a transformer with more than one output voltage/winding (except for center tap of course)...

The fourth option in my point of view is the best one by two main reasons: a sturdy switching power supply is somewhat cheaper than a transformer (and also fixable if something goes wrong) and I can hack it to maintain its output voltage something above the minimum requirement (I could use a 36V by 5A and reduce its output as required).

And about the minimum load, that's a great idea! I did not notice that on the datasheet (what a shame).

Thanks for the replies, people!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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25,510
No, the fifth option is reasonably commonly used.

The only time the input voltage needs to switch is when you're changing the voltage or when the power supply is in current limiting mode.

If you're using the power supply in current limited mode as a current source, and the voltage needs to change significantly and often, you may have some glitches. The simple fix is larger filter capacitors.
 
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