Variable regulator, pass transistor. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 29, 2016, 06:08:17 pm
content: I've looked about a bit for this, and it seems viable. I'm looking to add an emitter follower pass transistor to an LM317 circuit. I've done this with 78XX regulators, but never a variable regulator.What I'm after is still a fixed voltage using 1% tolerance resistors for a precise voltage, but wanting a bit more current than 1.5 Amps. The current I need is around 3 to 5 Amps, and I'm thinking an MJ2955 could supply this after I've fixed the voltage of the LM317 regulator.Would I be right in thinking this will work, as above, if the LM317 has a fixed voltage output. Thanks for reading, any help much appreciated.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: October 29, 2016, 07:39:39 pm
content: http://www.brighthubengineering.com/diy-electronics-devices/119294-interesting-small-lm317-enhanced-power-supply-circuits-explained/

author: edavid
author_rank: Super Contributor
timestamp: October 29, 2016, 08:25:19 pm
content: An emitter follower doesn't provide regulation.There are various LM317 current booster circuits around, but you should consider just buying an LM338 or LM1084.

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2016, 08:43:09 pm
content: If you put an emitter follower after a LM317 it will seriously degrade the regulation so there's no point in using 1% resistors. Also there wont be any overload protection.There is a fairly simple circuit that can do it though, without needing unobtanium LM195 pass transistors.The ratio of R1:R4 sets how much extra current Q1 passes. Its protected by the LM317's current and thermal limits. Q2 and Q1 are aSziklai pairso you don't have to find a high power high gain PNP transistor. D1 advances turn-on of the pass transistor so it shares the load from a very low current.Don't forget decoupling at the input (R1,R4 junction) and load.However this circuit needs about 5V headroom for good regulation at max. output so its rather wasteful compared to a more modern design and needs a honking big heatsink, possibly fan cooled.LTSPICE simulation and LM317 model attached.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2016, 11:01:11 pm
content: I think a single PNP pass transistor will do for only 3A. Here's a schematic, with the external PNP pass transistor, showing the internal workings of the LM317. The unlabled resistor senses the current, turning on the pass transistor when the voltage across it exceeds VBE, about 0.6V. A 1R resistor will start to turn the pass transistor on when the output current exceeds around 600mA.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 29, 2016, 11:09:33 pm
content: Thank you for all your replys. I understand not wanting to affect the saftey aspect of the LM317. I orded two lots of LM338k regulators from china, there all fake )-: so I thought I would go with the LM317. I have some in TO3 packages, so thought the pass transistor with the Lm317, as its something ive never tried. Many thanks again. I will post up on how it goes, after I've built it.Thanks all.

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2016, 11:35:47 pm
content: The problem with the simpler circuit is what happens if the output is shorted. A TO220 package genuine LM317 can pass as much as 3.4A for a short time if it starts from near ambient temperature. Once the drop across the the b-e resistor exceeds 1V, the bulk of this current will pass through the b-e junction of the transistor, possibly damaging it, and certainly turning it hard on. If the supply has a low enough output impedance the transistor will expire explosively before it can heat the heatsink up enough for trhe LM317's thermal SOA protection to cut in. The cure is to add a series emitter resistor, to reduce Vbe in proportion to the current through the transistor. If properly chosen it can prevent the transistor's Ic_max being exceeded. The excess dissipation will heat up the heatsink and trigger the LM317's thermal SOA protection, reducing the current through the b-e resistor, removing most of the base drive.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 30, 2016, 12:40:45 am
content: I was looking at two circuits, (added protection crowbar on the output) It was simply to set up the output voltage, and add a single pnp transistor for the added current. My resistor values for base and ballast resistor where going to remain the same.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2016, 08:58:30 am
content: The circuit with the added external transistor have a limited protection and are also prone to oscillation with a complex load. Also the dropout voltage increases by about 0.5-1 V. So if you want an external transistor to dissipate the heat for more current, I would consider to go from the LM317 to an LM723.The 723 is made for an external transistor and has provisions for current limiting. With a sufficiently large transistor one may not need extra SOA limit, except maybe foldback current limiting. The circuit is slightly more complicated, but not that much if you stay with a fixed current limit and don't need voltage adjustment all the way down to 0.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 30, 2016, 10:27:51 am
content: Thanks, the 723 is one I've looked at before, its one I've seen in many purchased units. I was hopping to stay minimalist with components. I would have just used the LM338k or LM338T, but they turned up from the far east after 4 weeks, only to find out there fake. So I have LM317k's and 7812k's and 7815k's. I wanted as close to 12.50 Volts as possible, so will try the LM317 with a pass transistor, see how it goes. A while back I did try the pass transistor TIP2955 and it worked very well. But then I tried a more powerful darlington wdith a 7812 and that suffered oscillations.

author: Ian.M
author_rank: Super Contributor
timestamp: October 30, 2016, 11:54:37 am
content: Even on its own, if you ignore the datasheet 's decoupling advice a 78xx regulator can act as a power oscillator at frequencies as high as tens of MHz. Add an external pass transistor and unless you carefully limit its gain, problems aren't unusual. The phrase 'sings like a canary' comes to mind.The LM723 is designed to support external pass transistor applications, and the 'Figure 19. Positive Voltage Regulator (External NPN Pass Transistor)' circuit from the T.I. datasheet is no more complex than an adjustable 3 terminal regulator + external pass transistor. Its the easy way to go . . .

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 30, 2016, 12:14:06 pm
content: My early 78xx and TIP2955 circuits didn't oscillate, and I used decoupling. Just to see, I will put together the LM317 and a single MJ2955 just to see what happens. But thank you for the heads up on possible issues that might arise.

author: edavid
author_rank: Super Contributor
timestamp: October 30, 2016, 08:12:36 pm
content: Quote from: davelectronic on October 30, 2016, 10:27:51 amLM338T and LM1084IT-ADJ are still in production and readily available from distributors.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 30, 2016, 08:42:16 pm
content: I fancy trying it first with the LM317, reference is made to it here as well.http://www.sentex.ca/~mec1995/circ/ampboost.html