Activity of transistor with LM317 - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 07, 2016, 02:53:16 am
content: Looking for some advise on using an LM317 with a series pass transistor. I'm unsure how the transistor will behave at lower voltages.Say 1.25 Volts up to a maximum of 12 Volts, how will the transistor handle currents at lower voltages ? Is there a point where the transistor is no longer conducting ? I know current as load is important to keep it supplying the load. But is there a point at which it stops conducting and its just the regulator supplying the current needed. Could use some help please.

author: StillTrying
author_rank: Super Contributor
timestamp: December 07, 2016, 04:01:02 am
content: Quote from: davelectronic on December 07, 2016, 02:53:16 amYes, when the base-emitter voltage drops below about 0.6V the PNP transistor will be off.This is about the most sensible description I could find.http://sound.whsites.net/articles/vi-regulators.html#s40

author: Audioguru
author_rank: Super Contributor
timestamp: December 07, 2016, 04:05:42 am
content: The PNP transistor simply boosts the maximum output current and it does nothing when the output current can be passed only by the LM317. The resistors that set the output voltage are the minimum load (about 10mA) for the LM317. the minimum load current for a more expensive LM117 is 5mA.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 07, 2016, 10:53:53 am
content: Thank you for your replys and the link, i will have a good read of this. As i say i was just unsure at what point the transistor will no longer supply the current as the voltage drops away. Thanks again !

author: Ian.M
author_rank: Super Contributor
timestamp: December 07, 2016, 11:41:12 am
content: That's fairly simple - the LM317 dropout voltage graph in its datasheet tells you what the minimum headroom for the regulator is, and you can approximate the current by assuming a 0.7V drop across the b-e resistor, then add that 0.7V to the dropout voltage (assuming the fuse in fig.7 of the linked site has negligible resistance). Once there isn't enough headroom to maintain regulation, if the load current drops (i.e if the load is resistive) the current through the b-e resistor will drop and by the time the voltage across it is down to 0.5V there will be negligable current through the external pass transistor

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 07, 2016, 07:27:50 pm
content: Thanks for all your help and replys. I think i can use a series pass transistor in a variable regulator circuit, given some modest constraints.

author: Audioguru
author_rank: Super Contributor
timestamp: December 08, 2016, 03:59:28 am
content: The voltage should not drop away since the LM317 is regulating it.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 08, 2016, 10:33:36 am
content: So for argument sake, lets say the regulator does the work up to 0.600mA and 3 volts. at a voltage of say 2 volts, and a current load of 1 Amp, would the trasistor still be supplying the extra current to the load above 0.600mA at 2 volts ? Just a hypothetical senario.

author: Brutte
author_rank: Frequent Contributor
timestamp: December 08, 2016, 12:01:52 pm
content: Quote from: davelectronic on December 07, 2016, 02:53:16 amNot sure how you imagine splitting current between those two. Besides, LM317 has a bunch of protection circuitry inside (overtemperature, overcurrent). The raw transistors do not come with protection.

author: Ian.M
author_rank: Super Contributor
timestamp: December 08, 2016, 12:45:52 pm
content: Doing 'what if' experiments is a lot easier in a simulator than it is in real life. Also you wont let out any 'magic' smoke or melt any breadboards. It can be a big help towards understanding the operation of the circuit, as long as you remember that a simulation is only as good as the models used, the parts are all ideal with no stray capacitance, lead inductance or resistance, and 0% tolerance. Some circuits dont simulate well and some that do are impractical in real life.However you want to gain understanding of a LM317 + external pass transistor circuit under low supply conditions. That's certainly easier to simulate than to investigate in real life, unless your bench is set up with a programmable PSU, programmable load and a few bench DMMs that support datalogging, + the PC software to control it all.I posted a LTSPICE sim of a LM317 + external pass transistor a while back.https://www.eevblog.com/forum/beginners/variable-regulator-pass-transistor/msg1059349/#msg1059349Its a bit more complex than the one you've been experimenting with (as it uses a Sziklai pair to avoid the need for a PNP power transistor and have half the Vbe of a PNP Darlington, + has over-current protection for the pass transistor, and over-heating protection if its on the same heatsink), but once you have it up and running, it would be trivial to rip out the extra parts and substitute a PNP power transistor so you can simulate your circuit.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 08, 2016, 01:05:20 pm
content: As a Linux user I've no idea what simulation program will run on it. I don't use windows, gave it up years ago. I can experiment up to about 5 Amps on a breadboard, although i know that's pushing it some what. I would like to get it right in a small model, before building a 20 Amp psu with an LM317K and 4 x MJ4502 transistors, although that one's for fixed voltage for radio use. I'm currently using a 5 Amp restored Alti psu for a cb radio and 25 watt linear amplifier .Its working very well, using an LM317K and a single MJ4502 pnp transistor. Before i move on to the LM723, i plan on building this LM317K and 4 x MJ4502 psu unit. But I've only got Linux available to me for software simulation. Thanks for your help everyone.

author: Ian.M
author_rank: Super Contributor
timestamp: December 08, 2016, 01:29:03 pm
content: LTSPICE generally runs OK under WINE.

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2016, 01:59:51 pm
content: Quote from: davelectronic on December 08, 2016, 10:33:36 amI think you've got the decimal wrong. You probably mean 600mA.Quote from: davelectronic on December 08, 2016, 01:05:20 pmQuote from: Ian.M on December 08, 2016, 01:29:03 pmI'm a Linux user myself.Yes, LTSpice does run under WINE but it's not perfect. I use XVII and there are some minor issues with the component list not scrolling properly. It can be worked around by using the keyboard to select different components but it isn't ideal. I believe IV has better WINE support and runs perfectly but I've not tried it under WINE for a few years.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 08, 2016, 06:04:16 pm
content: The LM317 + PNP combination might get unstable with certain loads (like low esr cap + current sink). Also some output capacitance might be needed - rather similar to low drop regulators.The circuit linked down below is not good in sharing the current over more power transistors (the resistor on the collector side does not help there. One would need resistors on the emitter side - this might be just fuses (and maybe some wire) for each transistor separately if the resistance is large enough (e.g. 0.1-0.3 Ohms range). So add another 200 mV to the minimum voltage drop for this.For currents up to about 3-5A I would consider just beefier regulators like LT1085/LT1084.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 08, 2016, 07:32:28 pm
content: Quote from: StillTrying on December 07, 2016, 04:01:02 amI've had no problems to date with instability issues, i use 470uf 35 volt electrolytic on the final output. Only time i had instability problems was when i tried a high power darlington MJ11015 transistor. I was just trying to work out what voltage output the series pass transistor will still handle current at lower voltages, given the fixed base resistor.I will look into LTspice with wine on Linux, thanks. The " 0.600 mA " was just the way i wrote it down, it still accurately denotes 600mA EG: replace the 0 with a 1 would be 1.6 Amps. But maybe i should have labled it 0.600 A.