Simple voltage monitoring to turn off PSU ? - Page 1
Forum section: electronics / beginners

author: GG
author_rank: Contributor
timestamp: August 17, 2025, 07:39:39 am
content: Hello everyone,I have an off the shelf TDK miniature 300w PSU (RGC4W300W008A-001) with input voltage range of 9-52V and the same output voltage range, has a trim pin to set output voltage with a resistor - I set it to 12.2V out.Somehow it malfunctined and bursted out 30V kiling my devices that it feeds.There is a PSON pin on it, the PSU will be turned on when that is connected to ground and turned off if it isn't.What would be the easiest way to connect that to ground when the output voltage is 0-13V and float it when it goes above 13V?Would a MOSFET be able to do this?Any suggestions would be a HUGE help and greatly appreciated.

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 17, 2025, 08:47:34 am
content: Hi, when you say "it bursted out 30V" do you mean the output then returned to nominal ?

author: Gyro
author_rank: Super Contributor
timestamp: August 17, 2025, 09:14:26 am
content: Looking at the datasheet, I don't see any timing specification for the PSON signal response time. The output might still go to 30V for some time before it responds to the PSON signal, particularly if the load is small.In such a situation, I would use a crowbar circuit, where a voltage reference, a TL431 for instance, is used to trigger an SCR to directly short the output. This will work almost instantly. The PSU module includes current limiting, but it would be best to include a series fuse before the SCR to protect against catastrophic faults.If to web search 'crowbar circuit', you will find many examples.

author: GG
author_rank: Contributor
timestamp: August 17, 2025, 11:44:38 am
content: Thank you very much for checking the datasheet and for the 'crowbar circuit' suggestion @Gyro !!Could you please also suggest a circuit for the PSON method?Would love to explore both options.Load is around 50-100w.Replying to fourtytwo42's questions, sorry I used the wrong word, it didn't just burst, it is now giving out 28-30V fluctuating as if the PSU itself might have failed. I'll desolder the trim resistor to check it's value and see if maybe that failed.I'll let you know soon.

author: themadhippy
author_rank: Super Contributor
timestamp: August 17, 2025, 12:05:00 pm
content: so wot you want is a circuit tocomparethe output voltage and if its over 13V give a low output

author: GG
author_rank: Contributor
timestamp: August 17, 2025, 12:27:56 pm
content: Yes that's right.How would you do it?I was thinking of using a comparator but I don't understand how to supply it with a reference and what that reference should be, where it should come from.The input of this DCDC is 28v and the output is set to 12.2 with the trim resistor.

author: ledtester
author_rank: Super Contributor
timestamp: August 17, 2025, 02:14:50 pm
content: Quote from: GG on August 17, 2025, 12:27:56 pmHere is a video that was just released 11 days ago that goes through all the calculations:Window Comparator - 4k Video -- Old Hack EEhttps://youtu.be/C931hGtxgAIIt's a window comparator so you only need one of the comparators for your application.It specifies a +5V supply to power the comparators. However, if you use something like a LM393/LM339 you can power the comparators directly from your input power supply as long as its voltage is limited to 30V or so volts.It suggests using a LM4040 for the voltage reference but there are other possibilities.You may still have questions about the circuit, and if you do just ask.

author: themadhippy
author_rank: Super Contributor
timestamp: August 17, 2025, 02:45:08 pm
content: Quotetalk about overkill.2 resistors as a simple voltage divider across the 28v to generate the reference is much cheaper

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2025, 04:28:54 pm
content: Use a MAX809 driving an open-collector (common-emitter) NPN transistor, collector connected to PSON.Supply the MAX809 through a suitable voltage divider from the PSU output.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 17, 2025, 04:57:52 pm
content: I'd be trawling for voltage monitors, supervisor and reset ICs.Why make what you can buy?

author: GG
author_rank: Contributor
timestamp: August 17, 2025, 06:39:58 pm
content: Good point, but when I search for voltage supervisors on Mouser.com their output is either low or high - how can I take that and turn that into grounding the PSON or floating it?Sorry I don't understand how it works?

author: Gyro
author_rank: Super Contributor
timestamp: August 17, 2025, 07:04:20 pm
content: The TDK datasheet is again lacking in detail about the PSON signal input, only stating that it is negative logic and showing a switch contact in the application diagram. No voltage threshold information. In these circumstances, the only option is to have the power supply supervisor drive an open collector NPN into saturation (or N-Channel MOSFET). With luck, the right supervisor IC will include an open collector output.Ideally, you would have the overvoltage cutout latching as you don't want the output cycling rapidly on and off if the fault condition causes the output voltage to keep passing through the cutoff threshold. Sorry to repeat myself, but I really would go with a crowbar for this application.

author: xvr
author_rank: Frequent Contributor
timestamp: August 17, 2025, 07:58:14 pm
content: Let's imagine: your power supply will raise the output voltage to > 30V.Your control circuit will notice this (after some time) and bring the PSON down.The PSU will turn off (also after some time).The output voltage will drop to zero (again after some time).The load will experience a voltage spike > 30V continuously, from start to finish. Will it survive this spike?

author: GG
author_rank: Contributor
timestamp: August 17, 2025, 08:29:17 pm
content: Thank you eveyone!@Gyro's suggestions makes sense that the crowbar circuit is safer - thank you.I'll look up some schematics and whip up a small PCB for it.I'm sure I'll have some questions about this soon - I'll be back !

author: MathWizard
author_rank: Super Contributor
timestamp: August 17, 2025, 09:11:51 pm
content: How much current does the device thats uses 12V use ? How much input capacitance does it have ? As a backup for active shutdowns, IDK if you can get a 15V MOV or TVR, or even just some resistor, more capacitance, and some zeners.

author: ledtester
author_rank: Super Contributor
timestamp: August 18, 2025, 02:08:36 am
content: Quote from: GG on August 17, 2025, 06:39:58 pmFrom the datasheet:https://product.tdk.com/system/files/dam/doc/product/power/switching-power/dc-dc-converter/catalog/rgc_e.pdfand what you said in your first post, this should work:You can replace the the NPN with a N-channel MOSFET.If you use a comparator or voltage monitor with open collector outputs (such as the MC33161) then the transistor can be the one that is built into the chip.MC33161 datasheet:https://www.onsemi.com/pdf/datasheet/mc34161-d.pdfIMSAI Guy video on the MC33161:https://youtu.be/8-OHdeoej5E

author: GG
author_rank: Contributor
timestamp: August 22, 2025, 09:39:51 am
content: Thank y'all for all your help and suggestions, really appreciate you taking the time!!