My active AC surge limiter idea, tell me why it's stupid - Page 1
Forum section: electronics / projects

author: ambrozy
author_rank: Contributor
timestamp: April 29, 2025, 02:23:01 pm
content: I just came up with this circuit to limit any AC current surges, the only problem I see with it is the cost of high voltage low resistance mosfets, am I missing something?

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2025, 11:26:37 pm
content: Which AC voltage? Are we talking mains (120 or 230 V) or what?

author: temperance
author_rank: Super Contributor
timestamp: April 29, 2025, 11:37:31 pm
content: 50 Hz on the left, probably mains...Is there a reason why you aren't using varistors or other "normal" means as surge protection?This circuit will never work properly. After all, you will need surge protection to protect the MOSFET's. So across each MOSFET you must place some clamps to short out the MOSFET's when a surge arrives. You are now back at square one because with the MOSFET's shorted the surge will appear across the load you are trying to protect.

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2025, 11:44:39 pm
content: Quote from: temperance on April 29, 2025, 11:37:31 pmI think you missed the termcurrentsurge. You're into voltage spikes.

author: temperance
author_rank: Super Contributor
timestamp: April 30, 2025, 12:22:55 am
content: QuoteNo I didn't. The current surge can be the result of voltage surge on the mains or the current surge is generated by the load upon applying power.To cover the first case you will need something to protect the MOSFET's when they open. Also, modern MOSFET's will go up in smoke in no time when operated in the linear region because of current crowding caused by small differences in VGth.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 30, 2025, 12:35:04 am
content: Like any problem, I would start by defining it. This includes details about the transients, line impedance, loads...

author: ambrozy
author_rank: Contributor
timestamp: April 30, 2025, 02:19:58 am
content: Right I was too vague, it is for 230VAC and is suppose to limit the current for things like turning on ATX PSU which has a rectifier and a large capacitor right on the mains.

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2025, 03:08:23 am
content: Quote from: ambrozy on April 30, 2025, 02:19:58 amIt could work and you are on the right track. Tektronix did something very similar in their 2213/2215 oscilloscope power supply, but they used the MOSFET switch to short a 20 ohm power resistor which actually limited the current during startup, as shown below.ATX power supplies usually have a power NTC thermister which limits inrush current. As the resistor heats up, its resistance drops keeping efficiency high.

author: Benta
author_rank: Super Contributor
timestamp: April 30, 2025, 10:35:34 am
content: I had the issue after buying a 1 kVA isolation transformer. It would trip the breaker 9 times out of 10 when turned on (core saturation).I thought about using a NTC, but didn't like the fact that once heated, it takes time to recover, which makes it unsuitable for unstable mains (eg, someone playing with the power switch).I concocted this little circuit, which works extremely well.I chose a relay for robustness, but it could easily be replaced with a power TRIAC.And yes, D3 is necessary.

author: ambrozy
author_rank: Contributor
timestamp: April 30, 2025, 02:09:04 pm
content: This has to be fitted after the particular device on-off switch, the point of my idea is to put it before the device and it should react in real time to current surges.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 30, 2025, 02:38:41 pm
content: Quote from: David Hess on April 30, 2025, 03:08:23 amCommon to see a relay in parallel with thermistor to bypass it after bank is charged.Quote from: Benta on April 30, 2025, 10:35:34 amWe have some systems where the transformers have to be soft started, and shut down to avoid saturation. Nothing like pulling a 4X the rated current from the service.

author: IconicPCB
author_rank: Super Contributor
timestamp: April 30, 2025, 08:15:45 pm
content: Where is Your minimum load current path?Think of a resistor in parallel with the switches.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 04, 2025, 11:37:01 am
content: Using two MOSfets in "anti" series to switch AC can work, but where are you getting the gate voltage from?There are photovoltaic opto couplers that are designed to generate a 15V or so output to drive a FET.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2025, 02:29:22 pm
content: Just use a timer relay. Either buy your own, or roll your own from a capacitive dropper, bridge rectifier, 555 timer & DC relay.

author: Benta
author_rank: Super Contributor
timestamp: May 04, 2025, 03:48:20 pm
content: That's basically what the circuit I posted does. And NOT with a 555!

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2025, 12:40:23 pm
content: Quote from: Benta on May 04, 2025, 03:48:20 pmA 555 timer will work. Here's a timer relay circuit, using a 555 timer. The relay modelled is a 12V relay.I would just buy an off the shelf timer relay, rather than make one.

author: ambrozy
author_rank: Contributor
timestamp: May 05, 2025, 05:57:58 pm
content: Quote from: Doctorandus_P on May 04, 2025, 11:37:01 amFrom the other side of the load, just look at schematicFor the other posters: This is suppose to go before the particular device on-off switch and react in real time, not after like all those delay based soft-start circuits

author: Benta
author_rank: Super Contributor
timestamp: May 05, 2025, 08:55:45 pm
content: Point taken, your reply #9 was not really clear on that.I'll step out.

author: jonpaul
author_rank: Super Contributor
timestamp: May 06, 2025, 06:48:25 am
content: see Series PTC one part, $0.50https://product.tdk.com/en/techlibrary/applicationnote/howto_ptc-limiter.htmlj

author: Benta
author_rank: Super Contributor
timestamp: May 06, 2025, 09:21:15 pm
content: @ambrozySorry to return again, but I've looked a bit more deeply into your circuit and find the approach interesting.First a comment to optimizing: if you use an AC optocoupler (anti-parallel LEDs), the bridge rectifier is redundant.I get your idea fully, but see potential difficulties.1: Optocoupler. CTR varies wildly and will not give you a well-defined transistor current.2: MOSFETs. VGSalso has a wide spread, plus matching the two parts may be necessary for symmetrical response.I'm not certain if you intend a "soft" current limiting or simply "clipping" the current peaks?Depending on your load, you may experience very odd voltage vaweform.Anyway, a tip for working with AC mains current/voltage manipulation: transform it into the DC (or pulsed DC) domain. This is quite easy, see the attached pic. And power bridge recitifiers are cheap.Cheers.

author: MariuszD
author_rank: Regular Contributor
timestamp: May 08, 2025, 04:27:39 pm
content: Transistors have very limited SOA at high voltage, you must know what parameters your surges will have, if you don't want to burn the MOSFETThermistors and ceramic composition resistors can withstand high energy pulses, because their resistive element has high mass, and can withstand high temperatures. Transistors are not that rugged.