Negative voltage regulator - 2 transistor - Page 1
Forum section: electronics / beginners

author: floobydust
author_rank: Super Contributor
timestamp: December 26, 2016, 11:43:12 pm
content: I made this negative voltage regulator years ago and recently noticed it doesn't really work.Originally I had the 470k resistor across C-B of Q1 and it appeared to work- I could dial any voltage. But I used a variac and found it doesn't regulate at all.Moving the 470k to E-B now the output is stuck at -12.5VI can't figure it out. It's confusing because everything is reversed polarity.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2016, 12:07:10 am
content: The 470k resistor was in the correct place before: collector to base. Current needs to flow into the base, in order for it to turn on. With the resistor between the base and emitter, the transistor will never turn on.It's not surprising the regulation isn't very good, as is often the case with zener diodes.

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2016, 01:48:29 am
content: Moved the resistor to C-B and lowered it to 33k and it works fine.I see- the pass transistor must be biased normally on and then Q2 turns it off. Thanks for restoring my sanity.I get 2% line regulation over 40-90VDC input, the zener is not so bad.One alternative I considered is a high-voltage 3-terminal VregTL783but its min. output current is 15mA, which is awful.

author: BrianHG
author_rank: Super Contributor
timestamp: December 27, 2016, 02:06:12 am
content: Quote from: floobydust on December 27, 2016, 01:48:29 amYou can always feed the TL783's output into and emitter-follower configuration. Your output voltage will drop around 0.8 to 0.5v depending on the temp of the transistor for approximately the first 0.5 amps of load.As for the transistor, I would use a KSA1220AYS, or KSB546YTU. (1A, 2A depending on your current needs.)

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2016, 01:04:16 pm
content: Quote from: floobydust on December 27, 2016, 01:48:29 amDoes it have to be a negative regulator?Can't you use a positive regulator and treat the output as the 0V reference or is the transformer & rectifier powering other things?You can add other components to the usual LM317 to increase the output range. If it really needs to be negative then use PNP transistors and the LM337 and reverse the power supply polarity.

author: guenthert
author_rank: Frequent Contributor
timestamp: January 02, 2017, 01:08:02 am
content: You might also want to review your rectifier.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2017, 02:38:15 pm
content: With one more resistor, one could get much of the zener current from the output side instead of the unregulated voltage. This way the zener voltage would be more stable, as the current is essentially constant. Also the regulator would have a minimum load and thus can show slightly better regulation.If only voltages above 40 V are needed, one could also use a 33 V "zener". There are relatively good 33 V stabilization chips available for use in older TV and radio circuits. Sometimes also two 12 V or three 8 V zeners in series are better than on 24 V type.