Author Topic: Quick/dirty adjustable PSU from 24V SMPS  (Read 1193 times)

0 Members and 1 Guest are viewing this topic.

Offline lordvader88Topic starter

  • Frequent Contributor
  • **
  • !
  • Posts: 935
  • Country: ca
Quick/dirty adjustable PSU from 24V SMPS
« on: April 04, 2019, 06:42:08 pm »
I have a nice 24V SMPS from a printer, that I want to use to power a pass BJT supply, for as wide a range as possible. I only want a few hundred milliamps, just for little quick breadboard use, and it will fit on a 5x7cm pref-board.

Should I heatsink a 2n3055 for say 0.5A at say 22V ?

So I could use a TL431 or overkill LM317 to drive the BJT. Should I use 2N3055 or the PNP version ?

And should I have a small resistor, from the regulator output, to the BJT base ? I assume I should put a small cap on the base. So that would make an RC filter.
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 10173
  • Country: gb
Re: Quick/dirty adjustable PSU from 24V SMPS
« Reply #1 on: April 04, 2019, 07:45:13 pm »
I have a nice 24V SMPS from a printer, that I want to use to power a pass BJT supply, for as wide a range as possible. I only want a few hundred milliamps, just for little quick breadboard use, and it will fit on a 5x7cm pref-board.

Should I heatsink a 2n3055 for say 0.5A at say 22V ?

So I could use a TL431 or overkill LM317 to drive the BJT. Should I use 2N3055 or the PNP version ?

And should I have a small resistor, from the regulator output, to the BJT base ? I assume I should put a small cap on the base. So that would make an RC filter.

Which is it?  :-\

2V drop at 0.5A is 1W, a bare TO3 pkg is probably sufficient.


EDIT: Plenty of simple options... https://duckduckgo.com/?q=2n3055+regulator&atb=v145-4__&iax=images&ia=images
« Last Edit: April 04, 2019, 07:58:57 pm by Gyro »
Best Regards, Chris
 

Offline lordvader88Topic starter

  • Frequent Contributor
  • **
  • !
  • Posts: 935
  • Country: ca
Re: Quick/dirty adjustable PSU from 24V SMPS
« Reply #2 on: April 04, 2019, 10:03:20 pm »
Oh right, it's Vce*Ic. So 10W if I had 4V@0.5A output with npn. Either way that won't matter much, I can put an old CPU heatsink on it if needed.
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20357
  • Country: gb
  • 0999
Re: Quick/dirty adjustable PSU from 24V SMPS
« Reply #3 on: April 05, 2019, 10:52:00 am »
I have a nice 24V SMPS from a printer, that I want to use to power a pass BJT supply, for as wide a range as possible. I only want a few hundred milliamps, just for little quick breadboard use, and it will fit on a 5x7cm pref-board.

Should I heatsink a 2n3055 for say 0.5A at say 22V ?

So I could use a TL431 or overkill LM317 to drive the BJT. Should I use 2N3055 or the PNP version ?

And should I have a small resistor, from the regulator output, to the BJT base ? I assume I should put a small cap on the base. So that would make an RC filter.

Which is it?  :-\

2V drop at 0.5A is 1W, a bare TO3 pkg is probably sufficient.
I thought he wanted it to be adjustable. The maximum power dissipation depends on the current, at the minimum output voltage.

The LM317 alone will probably do, as long as it's on a big heat sink.
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 10173
  • Country: gb
Re: Quick/dirty adjustable PSU from 24V SMPS
« Reply #4 on: April 05, 2019, 11:38:13 am »
That's why I highlighted the two conflicting passages and asked the question!
« Last Edit: April 05, 2019, 12:15:40 pm by Gyro »
Best Regards, Chris
 

Offline lordvader88Topic starter

  • Frequent Contributor
  • **
  • !
  • Posts: 935
  • Country: ca
Re: Quick/dirty adjustable PSU from 24V SMPS
« Reply #5 on: April 05, 2019, 03:44:06 pm »
Yes adjustable, and I know the LM317 could do it, but with the 2n3055/pnp for most little stuff, I'm guessing it's metal can would cool it enough.

Why is there usually a resistor feeding into the base on cases like this, is it related to some leakage current ? Or to slow 'in-rush' from the driver device ?
 

Online iMo

  • Super Contributor
  • ***
  • Posts: 5570
  • Country: va
Re: Quick/dirty adjustable PSU from 24V SMPS
« Reply #6 on: April 05, 2019, 04:21:24 pm »
LM317 includes a voltage reference, overcurrent and chip temperature control.
When planning with your 3055 as the pass transistor you have to add the circuitry.
The another version could be a beefy metal transistor on top of the 317, but again you need a circuitry at least for the current limit.
And 10W is too much for 317 as well as for 3055 without a larger heatsink (or smaller one with an active cooling).
Readers discretion is advised..
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf