Any high voltage input 3-terminal 12V 1A regulators? - Page 1
Forum section: electronics / general

author: peter-h
author_rank: Super Contributor
timestamp: September 30, 2025, 02:12:49 pm
content: Strangely, I cannot find anything.The old LM2940CT-12 seems to be a standard for 12V automotive systems and the spikes you get there. There is some ISO spec for these spikes.I am looking for a device which can withstand something like 80V.Obviously it can be built with discretes; there are plenty of 100-200V transistors.One could also clamp the spike, but in this case they can come from quite a low source impedance so would need a hefty clamp diode.

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2025, 05:26:54 pm
content: How long does the pulse last for?Choose the LM2940-N which is rated to withstand 60V and a chunky TVS diode.

author: peter-h
author_rank: Super Contributor
timestamp: October 01, 2025, 07:03:30 am
content: Probably 100-200ms.

author: Psi
author_rank: Super Contributor
timestamp: October 01, 2025, 07:19:28 am
content: TL783CKCSE3 is rated to 700mA and max 125V input.The current limit doesnt kick in until higher than 700mA, depending on a few factors.

author: Psi
author_rank: Super Contributor
timestamp: October 01, 2025, 07:32:08 am
content: If you look hard enough at actual datasheets you might find a regulator where Digikey is giving the normal max voltage and not the max transient protected voltage which will be higher but only for a short time.Some Automotive regulators have that, eg, the 500mA LM2937 falls into this class where digikey shows it as a 26V max regulator but datasheet shows it has 60-V input transient protection for load dump for up to 100ms.But normally the regulator transient protection only needs to get you up to the TVS Vclamp in your circuit, so finding one higher than 60V might be hard.

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2025, 11:46:38 pm
content: Having dealt with this issue in the past, I think you are better off with a normal regulator combined with a discrete circuit to limit the regulator's input voltage and implement shutdown if the input voltage becomes excessive.

author: peter-h
author_rank: Super Contributor
timestamp: October 02, 2025, 06:09:07 am
content: I agree; it is the way I am going. A TIP121 does 80V max and I can clamp above that. A resistor and a zener feeding the base is all extra one needs. And a series 1N4006 to block reverse spikes.I am after an approx 12V output so don't need the accuracy of a 7812.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2025, 06:38:40 am
content: Quote from: peter-h on October 02, 2025, 06:09:07 amYou could put the TL431 in the feedback loop, if you want greater precision.What about current limiting and thermal protection?If you're going to use a BJT, then pay attention to the safe operating area.

author: peter-h
author_rank: Super Contributor
timestamp: October 02, 2025, 01:03:48 pm
content: Current limit just needs 3 diodes and a resistor to drop 0.6V. Yes indeed one can make a nice zener with a TL431.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2025, 05:08:06 pm
content: Quote from: peter-h on October 02, 2025, 01:03:48 pmI was thinking about something like this.TL431 BJT overcurrent.asc(3.85 kB - downloaded 4 times.)Note, I've not tested the transient response. You'll probably need a big capacitor on the output and a zener to make up for the transistor's slow turn-off time, when subjected to the spike.I still recommend using an IC regulator, because it provides more robust over temperature and safe operating area protection.

author: Psi
author_rank: Super Contributor
timestamp: October 03, 2025, 12:20:45 am
content: You might be able to find a 1A high side load switch that has transient protection and overvoltage/current shutdown protection built in.Stick that before the 12V reg with its enable pin held on.Just make sure its not a latch off one, otherwise it will need power cycle to come back after tripping. Which may/may-not be ok depending on application. You don't want it tripping off on transient but maybe you do on overcurrent.It really depends if you want discrete/cheap + lots of testing vs less testing but more expensive.

author: peter-h
author_rank: Super Contributor
timestamp: October 03, 2025, 08:05:09 am
content: QuoteWith a cap, say 10uF, at the base of Q1, the transient response will be just fineIn fact the TL431 DS has a spec on the cap size, for stability. It's a trick... the value is actually package dependent!BTW the Darlington I am using is a 2STP535FP. Higher voltage than TIP121.

author: Zero999
author_rank: Super Contributor
timestamp: October 03, 2025, 04:49:23 pm
content: Quote from: peter-h on October 03, 2025, 08:05:09 amNo, put the capacitor on the output, not across the U1, which would make it worse. The U1 is really an op-amp and a voltage reference. Adding a capacitor to the TL431 will slow it down, ruining the transient response. A capacitor on the output would be much better because it will reduce the output impedance at high frequencies.EDIT:I've simulated the transient response. A load which draws a 1A pulse for 10ms was connected to the output. I tried it with no capacitors and a 10µF capacitor on across U1 and on the output.Note the scale differs on each plot.Conclusion: the one with the capacitor on the output clearly has the best response.

author: peter-h
author_rank: Super Contributor
timestamp: October 03, 2025, 07:18:51 pm
content: Oh yes; I forgot the TL431 is taking its feedback from the outputNormally when using a 431 as a reference one has a cap across the 431.