Simple 120V AC protection for 24V AC circuit? - Page 1
Forum section: electronics / beginners

author: vv_eev
author_rank: Contributor
timestamp: July 26, 2023, 07:30:29 pm
content: Power electronics isn't really my thing. What's the best way to protect a circuit from accidental 120V AC in a 5V DC MCU circuit that's converting 24V AC through a full bridge and linear regulator? I'm talking about some idiot accidentally connecting 120V AC mains to terminals that should take 24V AC... and leaving it there for a while. Without starting on fire. Bonus points if it's no more difficult/expensive to also protect from 240V AC, and if it could drive an LED or buzzer.What's the best way regardless of complexity or cost?What's the cheap, easy, quick and dirty, practical way?I don't need UL cert or anything, but it needs to be reliable

author: sparkydog
author_rank: Regular Contributor
timestamp: July 26, 2023, 08:10:25 pm
content: Cheap/easy/quick/dirty? Zener diode and a fuse?Especially if your rectifier can handle high voltage, a Zener between the rectifier output and ground will turn into a near-short if you feed in too much voltage. That should both protect the regulator from seeing too much voltage (especially as you should be using one that's rated for 48V or more) and blow the fuse in short order, at which point you've effectively unplugged the doohickey. (The fuse is both because you should have one anyway, and to protect the diode from having to drop 96V continuously.)You might be able to get away with using a "resettable fuse" if you want the user to be able to use the thing again after messing up. OTOH, having to replace a fuse would be a good incentive to your user to not do that again.(Disclaimer: I'm a n00b. Test and verify before relying on the above advice!)

author: srb1954
author_rank: Super Contributor
timestamp: July 26, 2023, 10:18:22 pm
content: Quote from: vv_eev on July 26, 2023, 07:30:29 pmOn the AC input start with a high quality, fast acting fuse followed by a thyristor type TVS (LittelFuse trade name SIDACtor).These devices are effective at clamping over-voltages from accidental contact with mains power (including 240V AC) and will do the bulk of the work clamping the excess voltage down to a lower level. As the SIDACtor latches on it will draw maximum current to ensure that the fuse blows quickly. Providing the fuse blows quickly enough the SIDACtor isn't damaged by this over-voltage and will be restored to normal operation the next time the power is restored.Since SIDACtors have a fairly wide range between their safe operating voltage and their triggering voltage you may still need some supplementary over-voltage protection i.e. a moderately large Zener on the input to your regulator to temporarily clamp the voltage until the SIDACtor triggers. Since this Zener doesn't have to clamp the voltage for an extended period its total energy absorbed won't be too high and it won't short out. A little series resistance on the feed to the Zener to limit its peak current wouldn't go amiss. You could possibly use an NTC surge current limiter on the bridge rectifier input to provide this resistance. This would also have the beneficial side effect of protecting your bridge rectifier and smoothing capacitor against excessive power-on surge currents.Adding an LED or a buzzer would be more difficult since the power will be removed from the circuit very quickly and there would be no useful indication from either if they were powered downstream from the fuse. You could put a low current LED (with suitable current limiter) or a neon indicator across the fuse to indicate that the fuse has blown. However, this would continue to supply a small current to the circuitry so you would definitely need a supplementary Zener to protect the rest of the circuitry. The SIDACtor won't protect the circuit in this case as it needs a minimum operating current to stay latched on and clamp its terminal voltage.Overall, you are going to have to a fairly delicate balance between effective, low-cost protection and overly sensitive protection that triggers spuriously on minor, short duration voltage transients. Doing some simulations in SPICE might be a good idea. Littlefuse seem to offer SPICE models for their SIDACtors but I haven't ever used them and can't vouch for the accuracy or usefulness.

author: David Aurora
author_rank: Frequent Contributor
timestamp: July 26, 2023, 11:03:29 pm
content: What is the current connection setup? Is it actually possible to easily connect mains to the 24V terminals? If so, why, and can you fix that?

author: tooki
author_rank: Super Contributor
timestamp: July 26, 2023, 11:36:56 pm
content: Take a look at the Bourns TBU series.Here’s an example of how it’s used to protect low-voltage circuitry from 230V mains:(I’ve built that circuit and can confirm it works: connecting it to mains causes no issues at all.)

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2023, 12:47:23 pm
content: Avoid idiots!

author: vv_eev
author_rank: Contributor
timestamp: July 27, 2023, 02:37:49 pm
content: Quote from: David Aurora on July 26, 2023, 11:03:29 pmPigtail to a connector, necessary d/t the application. Eventually some knucklehead will put 120 VAC through one of these, and I don't really see a way around it. RMAs are a pain, and clearly a consumer never does anything wrong.Currently using an MBL series bridge rectifier package. It can handle 100 to 1000 V depending on model, no problem.I run that through a 240 uF bulk filtering cap to a 5V linear reg (RS3007-5.0YE3) that has 1uF in/out caps. That bulk cap can't really be sized up to 120V AC (170V Pk?), but is it even necessary for the linear regulator?The regulator can't handle more than 38 or 40 Vin.Protection at the moment is just a TVS (SMBJ36CA) before the rect and resettable fuse (0ZCJ0010FF2E) from rect to regulator.Quote from: Terry Bites on July 27, 2023, 12:47:23 pmGreat idea! Why didn't I think of that? 😏The TBU + MOV looks interesting. I'll read more about that.

author: vv_eev
author_rank: Contributor
timestamp: July 27, 2023, 04:01:10 pm
content: This may be the way I go.

author: srb1954
author_rank: Super Contributor
timestamp: July 27, 2023, 10:19:30 pm
content: Quote from: vv_eev on July 27, 2023, 02:37:49 pmIt can often be really hard to avoid idiots. There are just everywhere these days!QuoteThe TBUs are best suited for protecting data lines with relatively low operating current. Using them on power input lines may be more difficult and require more design attention:They will probably trigger on the initial power-on surge current into the main reservoir capacitor cutting short each pulse of current into the capacitor. This will cause the power supply voltage to dribble up relatively slowly as the main reservoir cap is charged by these short pulses of current. This may have implications on the startup performance of the rest of your circuitry.They have significant series resistance so there may be power dissipation issues if your PSU draws any significant current.A detailed SPICE simulation of your complete circuit would be advisable.One other disadvantage I see using these devices for circuit interruption as opposed to a real fuse is that they are self-resetting. Consequently, the idiot who wires up the wrong supply voltage doesn't suffer any great inconvenience from his actions and doesn't modify his future behaviour. Having to replace a blown fuse is much more of an inconvenience and is a better lesson for taking more care in the future.

author: sparkydog
author_rank: Regular Contributor
timestamp: July 28, 2023, 06:31:06 pm
content: Quote from: srb1954 on July 27, 2023, 10:19:30 pm...and fuses are cheap. You could probably afford to include 1-2 replacements with your product, and/or you might be able to use one for which replacements are readily available at local brick-and-mortar stores where you expect your product to be used. (Or, if your users are fellow EE hobbyists, they can deal with ordering from the usual sources.)It's good to have a fuse anyway. It's good to have a bi-directional TVS anyway to guard against ESD. Given that those shouldalsoblow the fuse when someone applies too much continuous voltage, I'm really not convinced you need anything fancier.You indicated a desire to drive a buzzer/LED. I can't think of any way to do sosafelythat doesn't significantly increase the complexity of your gadget. What I would do is wire a LED across your rectifier output and ground, so that you have an idiot-light when your devicehaspower. That way, you can tell users that, if the LEDisn'tlit, either it's toast (unlikely) or the power delivery circuit isn't working; did they check the fuse? There's no way around false-positives or false-negatives due to the indicator itself failing, and IIUC your rectifier is unlikely to fail, so 99% of the time this will have the same effect as an error light, just with the lit/unlit states switched.