How it works?  Current Limiter and Output protector in PD Power Supplies - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: September 18, 2018, 05:05:57 am
content: How it works? Current Limiter and Output protector in PD Power SuppliesLooks simple but tricky - see schematic in attachmentHow to understand it to change current values?Thanx

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2018, 06:38:52 am
content: That is one strange apparatus. The series pass is a six way Darlington, if that's the right thing to call it.The final 4 stages draw current progressively from 4 unregulated rails of increasing voltage steps.At low output voltages, Q1/Q2 draw current from the rail filtered by C1.With increasing output voltage, a point will be reached where the C1 rail does not have enough voltage so Q2 takes over, drawing current from the C2 rail.The current from Q2 passes through the B-E junctions of Q1/Q2 to get to the output.And so on as the output voltage is increased.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2018, 06:47:33 am
content: I don't think it looks simple at all because of the horrible way the schematic is drawn. The upside down series pass transistors confused me for a while.The power supply is actually one of those floating style designs. It has plus and minus control rails which has their ground referenced to the regulated plus output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2018, 06:55:29 am
content: Seems like Q16 is the current limiter.Yeah, hard to tell with the awkward way those schematics were drawn. You'd think they went to school specifically to obfuscate them or something. Shows up too often through history to be mere stupidity...Tim

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2018, 07:06:05 am
content: Yes, Q16 controls current regulation by sensing the voltage across R30 while in "5 amp" mode. It can turn on Q17 which drags down the Base of the first transistor in the Darlington.Q13, Q14 and Q15 do hard limiting with different thresholds. Less maximum current is allowed for transistors drawing from the higher voltage unregulated rails.

author: 001
author_rank: Super Contributor
timestamp: September 18, 2018, 08:33:28 am
content: yeaThanx a lot!But it is works awesomeHow it do that?

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2018, 10:28:06 am
content: Quote from: 001 on September 18, 2018, 08:33:28 amDo you actually own it?

author: Wolfgang
author_rank: Super Contributor
timestamp: September 18, 2018, 10:28:33 am
content: Hi,all these darlintons plus diodes are there to provide automatic range switching for the different transformer taps. This was done to reduce dissipation at high load currents.This scheme was also used (in a slightly different style) by HP and successors. For the beginning of the seventies, this was not so dumb at all.

author: 001
author_rank: Super Contributor
timestamp: September 18, 2018, 11:07:29 am
content: YeaSee how meter current range switch worksIt looks like simple shunt switchNo real current limit, isnt it?i can`t understand it

author: mikerj
author_rank: Super Contributor
timestamp: September 18, 2018, 11:50:37 am
content: Quote from: 001 on September 18, 2018, 11:07:29 amThe output current passes through R42 and R43, creating a voltage proportional to current. Diode CR19 adds an extra ~0.6v to this voltage, and a proportion of the total (controlled by R32) is applied to the base emitter of Q16. The additional diode drop voltage added by CR19 means the current limit can pretty much go down to zero, and also helps to stabilise the current limit over temperature provided CR19 and Q16 are mounted close to each other.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 18, 2018, 12:22:29 pm
content: Quote from: 001 on September 18, 2018, 11:07:29 amThe switch shorts out R42/R43, but there's still R30.So in the 5A range, current limiting works by the voltage drop across R30, and in the 0.5A range by the drop across R30+R42+R43.

author: 001
author_rank: Super Contributor
timestamp: September 19, 2018, 05:13:29 am
content: Quote from: capt bullshot on September 18, 2018, 12:22:29 pmI`m not so sillyWhat about R73 circuit?

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2018, 08:15:28 am
content: Quote from: 001 on September 19, 2018, 05:13:29 amI have a possible reason for R73. It is rather involved starting with Q11 functioning as a constant current source for the zener reference.I need to know how much you already understand. how much current is Q11 set to source?Most of this current flows through CR18. I don't know what is the primary purpose of this diode besides providing a -0.7v source wrt to the ground reference.R73 draws some current from this -0.7v source which is able to flow through the 0.5 amp shunt resistance causing a small voltage drop with applies some reverse bias to the B-E of Q16. I think this is to ensure that the current regulation can be adjusted fully to zero.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2018, 01:28:06 am
content: Quote from: xavier60 on September 19, 2018, 08:15:28 amA more likely reason is to neutralize the shunt current that the regulator needs to supply to balance out the current supplied by the 6.23v reference to the minus rail via the voltage divider.0.7/82= about8.5ma , 6.23/700= about 8.8ma. I got the 700ohms by adding R10 to half of R9.But so far I can only make this idea work for the 60v range only.