A current limit and protection for NEGATIVE power - Page 1
Forum section: electronics / projects

author: shamooooot
author_rank: Regular Contributor
timestamp: February 18, 2024, 02:20:15 pm
content: HelloI need help in making a power supply that is:Adjustable with -18 V to -1.25 and 0/1.25 to 18 V separately controlled outputs.I found this circuit might be the best and simplest one to do the job:However I still need to limit the output current to around 500-750 mA and protect the power supply from short circuits or other faults. I was thinking the simplest way is to just to use an Efuse like the TPS25983, but I don't think there is an Efuse or solution that will work with the Negative rail.Someone proposed this circuit:But the transistors and the regulators each need to dissipate around 12W, which I think will need a HUGE heatsink (I don't know how big exactly), but 48W sounds a lot..Is there a way to work with the Efuse for the negative power, or is there a circuit that is more suitable to do the job?

author: coromonadalix
author_rank: Super Contributor
timestamp: February 18, 2024, 05:19:04 pm
content: why not using some buck regulators, you have some ic's who are current and voltage controlled ?

author: jbb
author_rank: Super Contributor
timestamp: February 18, 2024, 09:30:38 pm
content: Hmm. What would be nice is something like an LM317, but with a current limit pin.Analog Devices make LT??? parts with voltage set and current set pins, which are quite good but you pay a premium for em.Anyone else know of a kinda suitable part?

author: shamooooot
author_rank: Regular Contributor
timestamp: February 19, 2024, 04:56:58 am
content: Thank you guys..coromonadalix can you please suggest me an example of these regulators?Also jbb I tried looking into the selection table for LT ic's, but apparently nothin matches the required range, the min Vin is -12.5V, and the max Vin starts from 5.5V..

author: moffy
author_rank: Super Contributor
timestamp: February 19, 2024, 06:52:16 am
content: You can use the 'ADJ' pin of an LM317 with a couple of low power transistors to provide a settable current limit e.g.Unfortunately it can only drop to the minimum output voltage.

author: shamooooot
author_rank: Regular Contributor
timestamp: February 19, 2024, 11:34:15 am
content: Quote from: moffy on February 19, 2024, 06:52:16 amThank you, this circuit is only for the current limit part right ?I have tried simulation of the circuit:

author: moffy
author_rank: Super Contributor
timestamp: February 19, 2024, 11:51:29 am
content: Quote from: shamooooot on February 19, 2024, 11:34:15 amYes, only for the current limit, adjust resistor values to suit. The current limit is about 0.6/R2.If the current remains above the limit too high because 1.8V is not low enough it will tend to cook Q2. Maybe not the best.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 19, 2024, 02:45:15 pm
content: In commercial power supplies the negative (or the second, or third) channels are identical to the first. You can do that too, if you have or choose a transformer with two independent secondary winding.It would allow you to do modern current limiting tactics, like TPS25983 and decide how you want to use the independent supplies. Serial, parallel, or keep them isolated.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 19, 2024, 04:53:32 pm
content: Quote from: shamooooot on February 18, 2024, 02:20:15 pmDo you expect continual short Max current operating?You can add fold back to your circuit with a couple more resistors, but if you want continuous max then SMPS would slash the power loss.When you have both pos and neg supplies you can add -1.2 v / +1.2v bias points to allow your adjust to go to 0V, makes the supply far more usable.

author: shamooooot
author_rank: Regular Contributor
timestamp: February 20, 2024, 04:40:37 am
content: Quote from: dobsonr741 on February 19, 2024, 02:45:15 pmThank you, but I am not sure I got what you mean, but it's very interesting if it can work..You mean to use an indp. sec. winding transformer and change the rectifier orientation to get a negative voltage?, but then how can I use an IC such as the mentioned?.

author: shamooooot
author_rank: Regular Contributor
timestamp: February 20, 2024, 05:08:25 am
content: Quote from: PCB.Wiz on February 19, 2024, 04:53:32 pmThank you, do you mean something like this fromhttps://www.onsemi.com/pdf/datasheet/lm317-d.pdf:It's good and simple enough but the negative voltage required for the JFET's is a problem..Edit:Back to the first square these Regulators will need to dissipate a lot of heatPower = volts dropped across LM317 x current = (20 - 1.25)V x 1.25A = 23.4 watts.I think it's not the best idea..

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 21, 2024, 03:35:33 am
content: QuoteNo. You are to have two independent isolated secondaries. For each you build the exact same power supply circuit, with the pass element in the positive rail. When you need the +/- dual rail power, join the 2nd supply’s +ve to the GND of the first one.

author: shamooooot
author_rank: Regular Contributor
timestamp: February 22, 2024, 07:10:51 pm
content: I lost hope and I am looking for a regulator IC that can do the job.Can you guys suggest me an IC that's capable of inverting output, and one that's capable of non-inverting output with built in current limiting function?.Required:Adjustable output: -18 V to -1.25/0Adjustable output: 0/1.25 to 18 VCurrent limit: 1A

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 23, 2024, 06:39:25 pm
content: LT3081 and LT3091