Driving 12VDC relay with dropped rectified 250VAC - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: February 06, 2024, 05:58:26 pm
content: I need to control a relay (12V coil drawing 33.3mA), with 250VAC. Simplest and cheapest way, I was thinking a full wave diode bridge rectifier (e.g. 1N4007), a dropping resistor (9.34k 0.5W for 325VDC->12VDC), and a 100uF for max 3.33V ripple, then a 12-13V zener on the relay coil.Would 9.34k drop 325VDC to 12VDC @ 33.3mA be correct?Of course, any small fluctuations on the resistor value will greatly change the output.Closest value I can get is 9.31k which gives 14.977V. Would putting two in series be of any benefit (with other values of course)?And which order would it be best put the resistor/capacitor in?bridge rectifier -> cap -> resistor -> relay coil.bridge rectifier -> resistor -> cap -> relay coil.Large capacitors with high voltage ratings are big and expensive, resistors not. So I would prefer putting the resistor first, then cap in parallel with the relay coil.Last question, would there be any potential problems by just cutting one of the phases into the bridge rectifier when turning off (the on/off is controlled by a SPST switch).

author: jonpaul
author_rank: Super Contributor
timestamp: February 06, 2024, 06:03:11 pm
content: For safety, use LV trsf 120V >>12V>>diode.j

author: okw
author_rank: Frequent Contributor
timestamp: February 06, 2024, 06:08:48 pm
content: It's gonna be in a metal enclosure (with spacers, etc), so fire isn't a concern...

author: themadhippy
author_rank: Super Contributor
timestamp: February 06, 2024, 06:19:16 pm
content: Quotejust dont use 1/4 watt resistor,there be 10ish watts to get rid of,might want to consider a capacitor dropper instead

author: geggi1
author_rank: Frequent Contributor
timestamp: February 06, 2024, 06:25:57 pm
content: If you want to simplify things use a 12v relay with AC coil.You will get away with only the resistor for a voltage drop.Even better is to just use a relay with 230V coil becuse this can be driven directly without the power loss and heat form the resistor.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2024, 07:16:18 pm
content: The resistor would run rather hot: 215 V and 33 mA are more than 6 W. With a little less than norminal current one may get away with 5 W, but not much lower.With a fitler capacitor the actual current on the AC side is even a bit higher (e.g. the factor of 1.1 between rectifier values and RMS value).A capacitive droper would be the more suitable choice. Not need for a super large capacitor, though it should be a class X type and ideally some extra proction (e.g. fusible resistor) in series.If one has a choice a 24 V relay may be even simpler, though some skimp out on the coils and the current is not actually half.

author: KerimF
author_rank: Frequent Contributor
timestamp: February 07, 2024, 04:18:35 am
content: A high voltage non-polar capacitor of 0.5uF (or a bit higher) can provide the relay DC current of 33.3 mA at 220Vrms (50Hz).To limit the inrush current, 150R 1/2W resistor could be added in series with the capacitor.Also, a diode full bridge (4 diodes) is added in series with the capacitor to supply the relay with the DC current.A 12V zener could be added in parallel with the relay in case the mains voltage could become much higher than 250Vrms (because of a faulty network).For instance, the inductance of the relay’s coil smooths the rectified voltage on it.Do you need the calculations?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 07, 2024, 04:30:03 am
content: You can also get Solid State Relays, with 230V AC activation. They are not small, but they are simple to use.I used one recently, to slave additional LED lights to a low rated rangehood. Added 5.6k in series to ensure no inrush gremlins.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 07, 2024, 10:58:46 am
content: DO NOTconnect a 12V rated relay coil to mains, regardless of "dropper" resistors. Use a relay with a 230V AC coil, it will be rated for use with mains electricity.Mains is not to be taken lightly, not only do you have 230V RMS to deal with, but faults and lightning surges can create hazardous peak Voltages too.SJ

author: okw
author_rank: Frequent Contributor
timestamp: February 07, 2024, 11:00:17 am
content: Quote from: KerimF on February 07, 2024, 04:18:35 amThanks KerminF. I've come to this so far (see schematic). I think it's values are ok?I used the Microchip application note AN954 along with this calculator:https://www.nomad.ee/micros/transformerless/index.shtmlI put 13V zener to compensate for D2 drop (0.7V). So output will be 12.3V. Or maybe it's better to drive the coil with 11.3V (uses slightly less current, and prolong the relay life)?Do I need the rectifier bridge? AN954 seems to do without?And I used 47R since this will develop 0.22w and AN954 says double whatever value. 150R would be 0.77w so i need 1.5w. I was planning to use most SMD components with milling tracks where needed to separate L/N.I don't know how to read inrush current requirements of the relay datasheet. There must be some minimum inrush needed to close the contacts fast enough to not degrade the contacts.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 07, 2024, 11:22:14 am
content: same here use a 240vac relay with a small resistor drop and that's enoughplaying with a 12v relay is no good

author: Kleinstein
author_rank: Super Contributor
timestamp: February 07, 2024, 01:12:11 pm
content: One does not need a bridge rectifier, but it can help, as with a bridge rectifier one can use both signs of the current and can get away with half the capacitor size.

author: Ian.M
author_rank: Super Contributor
timestamp: February 07, 2024, 02:10:22 pm
content: Quote from: Solder_Junkie on February 07, 2024, 10:58:46 amSolder_Junkie may have a point - 12V coil relays may not have enough clearance and creepage distance between the coil and the yoke. I just grabbed a random 12V relay and found under 2mm creepage distance between the coil and the yoke. With a non-isolated mains derived coil supply, that would be a hazard. Even if its contacts are properly rated for switching mains, so have adequate insulation, ones with a metal or easily removable clip-on cover may be an unexpected live part hazard. OTOH if its got a proper datasheet with appropriately rated contacts, and an adequate coil to contact isolation voltage rating, no problem.

author: KerimF
author_rank: Frequent Contributor
timestamp: February 07, 2024, 02:13:10 pm
content: Quote from: okw on February 07, 2024, 11:00:17 amThe advantage of using a full bridge is that the value of the main capacitor could be halved. In this case, the relay will be supplied by the two mains half-cycles instead of one.To calculate the value of the capacitor approximately we may follow these steps:Let us assume:I_rly = the nominal DC current of the relayIc = the RMS current supplied by the capacitor.We know that in full rectification:I_rly = Ic*sqrt(2)*2/pi ~= 0.9*IcThe impedance of the capacitor is Zc=1/wC where:w = 2*pi*FSo,Ic = Vac/Zc = Vac*2*pi*F*CorC = Ic/(Vac*2*pi*F)C = I_rly/0.9/(Vac*2*pi*F)where:Vac = mains RMS voltage (assuming the voltage drop on the bridge is negligeable)F = mains frequencyFor instance, the inrush current is of the capacitor, not of the relay (which is an inductor that opposes the sudden voltage rise).I forgot to add earlier that it is better to add a resistor in parallel with the capacitor to discharge it when no mains. I usually use 1 Mega 1/2W resistor or two 470 K 1/2W in series. This has the advantage of discharging the capacitor soon after disconnecting Vac. And at the start, the discharged capacitor will not add more voltage sometimes (in case it is not discharged) and increase the inrush current.I_inrush = Vac*sqrt(2)/RsSo if Rs = 150R and Vac = 220V, I_inruch = 220*sqrt(2)/150 ~= 2A it is usually not too high.But we have to also calculate its dissipation power:P_rs = Ic^2 * RsNow, let us substitute the symbols with their values:C = I_rly/0.9/(Vac*2*pi*F) = 33.3e-3 / 0.9 / (220*2*3.14*50) ~= 0.536 uF which is close to 0.470 uFP_rs = Ic^2 * Rs = (Vac*2*pi*F*C)^2 * Rs = (220*2*3.14*50*470e-9)^2 * 150 ~= 0.158 WIt is also good to use the above formulas in reverse for a known C, like knowing the relay current for various values of Vac.Finally, in case you prefer the uploaded circuit:I_rly = Ic*sqrt(2)/pi ~= 0.45*IcTherefore, Ic will be doubled, also the inrush current in case Rs=150. And P_rs will be x4.Hope this helps,Kerim

author: coromonadalix
author_rank: Super Contributor
timestamp: February 07, 2024, 04:43:48 pm
content: while i do see tons of calculations, dont play god patching this and that, use the correct hardware for your needsuse the right stuff where it is needed, i would never patch a circuit like this, even for temporary usesit's just dumbmy 2 cents

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2024, 05:42:55 pm
content: It's a bad idea, as mentioned above.If you must do it, use a small mains transformer (you might need to go for 9V, rather than 12V, as it'll output a higher voltage, with such a light load) and bridge rectifier to drive the relay. It has the advantage of being fully isolated, so the reduced creepage and clearances in the 12V relay are non-issues.

author: KerimF
author_rank: Frequent Contributor
timestamp: February 07, 2024, 06:08:38 pm
content: Quote from: coromonadalix on February 07, 2024, 04:43:48 pmYou are totally right in general.But it seems to me that most engineers around the world didn't have the chance to discover that, in some regions on their planet Earth, an engineer has no choice but to use the hardware available around him because he can't get (please don't ask me why) the correct/optimum hardware for his needs.For example, I still use the very old AVR MCU, ATmega8, in my various products which I design for the local consumers because I can't get the more advanced ones. I also have to write my codes in assembly since about 4 decades ago (starting from the CPU Z80) because I had never the privilege to get a high-language compiler for CPUs or MCUs which is also needs its resources and its updates.To me in the least, doing something useful, even if it can't be done perfectly as it should be, is always much better than doing nothing.

author: okw
author_rank: Frequent Contributor
timestamp: February 07, 2024, 06:59:45 pm
content: When I come to think about it, I've often seen capacitive transformerless designs from well established companies. Latest was a repair of a heated floor smart thermostat from Schneider, which is a huge French company which supplies most of Europe with equipment for fixed installation. The thermostat had died, and just needed a new X2 capacitor. This transformerless design powered both the MCU, display, 433MHz radio and two 12V relays.I'm not convinced they would make a design which is a "bad idea"?

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2024, 07:33:03 pm
content: Quote from: okw on February 07, 2024, 06:59:45 pmTransformerless power supplies aren't a bad idea. What is a bad idea is using one to drive a 12V relay. A transformerless power supply is fine, if there's sufficient separation between everything it's connected to and the user, but a 12V relay might not have sufficient separation because it's designed for 12V, not mains.The proper way to do it is to use a relay with a mains coil. It will be much smaller and cheaper, than the additional parts required to make a power supply for it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 07, 2024, 07:41:53 pm
content: It's a coil, ac or dc shouldn't matter.If you say the relay will be safely placed inside an enclosure with no posible danger to anyone outside, a simple capacitor dropper will do it.33mA @ 12V makes a coil resistance of 363Ohms.33mA @ 220V 50Hz makes a capacitive impedance of 6.6K, that's 470nF.X2 MKP caps are common and cheap. You want X2 because it's a safety-certified capacitor, designed to fail open-circuit.Add a series 1W 220R resistor (Carbon, not metal-film as they tolerate much higher temperatures), so it acts as a fuse if anything shorts out.

author: Ian.M
author_rank: Super Contributor
timestamp: February 07, 2024, 07:43:31 pm
content: Its not an issue for Schneider's smart thermostat because its fully enclosed, wont have any external low voltage connections, and they will have selected a relay with appropriate safety approvals that is switching a mains circuit.Use a different relay with no or fake approvals to control a user-accessible low voltage circuit and its an accident waiting to happen.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2024, 08:31:01 pm
content: Quote from: DavidAlfa on February 07, 2024, 07:41:53 pmBut it does matter. AC in a coil designed for DC will cause it to chatter. An AC coil has a shading ring, a shorted turn, which stores some energy to keep the contacts closed, when the current passes through zero.https://en.wikipedia.org/wiki/Shading_coil?useskin=vectorQuoteIt's not that simple. The coil is inductive, not purely resistive, so it will cancel some of the capacitive reactance. If you get the wrong capacitor value, it will result in resonance and a much higher voltage across the coil, than expected.EDIT:If you have to use a transformerless PSU to drive a 12V coil, a fullwave rectifier is a must. A small smoothing capacitor might be required, although it's likely the coil will store enough energy, as the diodes will allow it to freewheel though the zero crossing.I've just had a quick look though TME for miniature relays with a similar coil power, as the one stated in the original post, but with a 230V coil. They're not hugely expensive and it's much easier and safer, than bodging it, so there's no reason not to.https://www.tme.eu/gb/details/rt424730/miniature-electromagnetic-relays/te-connectivity/7-1393243-9/https://www.tme.eu/gb/details/rm84-2012-35-5230/miniature-electromagnetic-relays/relpol/https://www.tme.eu/gb/details/hf115f-a_230-1zs3a/miniature-electromagnetic-relays/hongfa-relay/hf115f-a-230-1zs3af/

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 07, 2024, 08:58:19 pm
content: You're right! How the hell did I miss this, I've seen this so many times, relays going bzzzz because of dry caps causing 100Hz pulsed DC.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2024, 10:27:11 pm
content: Quote from: DavidAlfa on February 07, 2024, 08:58:19 pmThere are no capacitors in a relay with an AC coil. It's the shading ring which stores the energy to keep it closed. Read the Wikipedia article I linked to above.There should be a discharge resistor across C1.C1 is far too big. 390nF is the best value to use, but it 330nF could be enough.C2 might not be needed, as it's possible the coil will store enough energy to hold the contacts closed.I used the energised coil inductance and resistance from the 12V Omron relay G2RL, for the simulation. It's a 400mW coil so should be similar to the one described in the original post.Note how the current always remains what would flow due to the holding voltage given on the data sheet. The bridge rectifier is acting as a flyback diode. This wouldn't happen if the coil were just run off AC because the voltage changing direction would force the current through zero.https://docs.rs-online.com/6544/0900766b814209a1.pdf

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 07, 2024, 10:48:17 pm
content: I meant those were DC relays, the caps in the PSU were gone, so only receving pulsed DC from the rectifier, buzzings/witching at 100Hz

author: KerimF
author_rank: Frequent Contributor
timestamp: February 08, 2024, 03:38:05 pm
content: Once I had no more 12V DC relays for my controllers. And only the 12V AC relays were available.As you know, the MCU usually drives a BJT (transistor) which, in turn, turns on and off a DC relay.I had only the 12V acid battery to supply the controller and the relay.Fortunately, the solution wasn't complicated as I thought first. In the contrary, it was simple:By using a PWM pin to drive the transistor, I switched it at 122 Hz (squarewave) and the 12V AC relay turned on!This wasn't magic. The inductance of the relay’s coil with the fly back diode did it.Truth be said, it was impossible for me to believe that this solution could work till I did it and I was able to continue, for a period of time (till the 12V DC relays were available again), selling my products with 12V AC relays instead of 12V DC relays.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2024, 05:58:58 pm
content: Quote from: KerimF on February 08, 2024, 03:38:05 pmYes, driving an AC relay with DC will work, so long as either the voltage or duty cycle is reduced to compensate for the fact the impedance is lower. About half the voltage or 50% duty cycle will do. You could have used 24VAC relays on 12VDC and they would have worked. Obviously a 12VAC relay on 12VDC will smoke. We made that mistake at work driving a 24AC contactor from 24VDC. It worked for a short length of time, before releasing magic smoke.

author: Eraldo
author_rank: Regular Contributor
timestamp: February 09, 2024, 11:58:07 am
content: Honestly why choose the hard and unsafe path. Just use a 1W transformer that is small and that'll do the job. With that you can also safely power the other circuitry.All you need is a transformer, a diode a capacitor and optionally an lm7812 or zener diode (if the voltage is not that high)

author: soldar
author_rank: Super Contributor
timestamp: February 09, 2024, 04:04:14 pm
content: Quote from: Eraldo on February 09, 2024, 11:58:07 amWhat if you don't have those things and cannot get them?Not everybody has access to everything.

author: Eraldo
author_rank: Regular Contributor
timestamp: February 09, 2024, 04:14:35 pm
content: Quote from: soldar on February 09, 2024, 04:04:14 pmAny hobbyists who has ever owned a broken old radio/any other devise with a transformer would have saved at least 1.I have around 15 or so transformers from various devices that i keep just in case. And it's never wrong to choose the simplest/ safest route around a project

author: soldar
author_rank: Super Contributor
timestamp: February 09, 2024, 06:35:56 pm
content: Quote from: Eraldo on February 09, 2024, 04:14:35 pmYou are sure that anyone anywhere in the world will have access to electronic components? At low or no cost? North Korea? Cuba? Eritrea?Are you sure? Do you want to rethink your answer? Or do you believe the entire world is similar to your little corner of the world?

author: Eraldo
author_rank: Regular Contributor
timestamp: February 09, 2024, 07:02:56 pm
content: Quote from: soldar on February 09, 2024, 06:35:56 pmI didn't know Norway was a poor country. You might be the one to rethink your answer here. Transformers are one of the most common components in "vintage" electronic devices, from radios to outlet powered clocks etc. They are still used today and you can buy them in every electronic distributors website (or even local).You are more than welcomed to fiddle with 230V mains for your projects though.From now on i won't be responding anymore to your provoking replies since it doesn't do any good to this community so peace

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 09, 2024, 10:09:26 pm
content: People is so obsesed with safety here.What's the danger of an enclosed device? Same as the mains wires going to a "safe transformer".I'd say 90% of home appliances use capacitor droppers. Washing machines, ovens... They're everywhere.As long as it's properly isolated, it'll be safe.

author: okw
author_rank: Frequent Contributor
timestamp: February 09, 2024, 10:13:41 pm
content: Quote from: DavidAlfa on February 09, 2024, 10:09:26 pmThank.... YOU.... (!)

author: coromonadalix
author_rank: Super Contributor
timestamp: February 10, 2024, 12:06:01 am
content: @OKWyour response was stupidobsession with safety yesif by mistake you do listen some bad advises and YOU DO make some mistake, some may be minor, some could be majorAnd the problems could get to you, say electrocussion, fire, etc ... you could be held acountable for it / themso do take some time to think ... i've received shocks in my life, few things exploded near me, i even done mistakes, received some burns, i could have died etc ..so play with intelligence or play dumb

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 10, 2024, 12:38:18 am
content: Oh, here we go again.Your argument is like saying screwdrivers are unsafe because children could get hurt.If someone has no clue about electricity/electronics, the only safe option is to completely stay away from HV, simple as that.Fire risk? You only need few volts, a wrong resistor or a reversed diode, plastic housing there it is, fire! No dangerous ac needed!Even a toothbrush is dangerous in the wrong hands.He clearly stated ”Closed metal enclosure”. Which should be grounded. So hardly any fire/shock risk, else that lights going out when something blows!

author: coromonadalix
author_rank: Super Contributor
timestamp: February 10, 2024, 02:42:30 am
content: @davidalfa you're not his advocate ....i'm not doing the "here we go again", and you're the one who react the most instead of the OPi have seen so badly designed enclosures ... i'm doing certifications in some of them, some come from india, some form other countries, and i may assure you they are dangerous to some extent ... and they can catch fire internallyWhyguess ........... i wont go furtheryou should stay out, the same way i'll docomment(s) have been saidthe OP now should be mature enough to decide by himself ... but there are 2 sides of a coin ...some threads will be voiced and commented positively and negatively upon people(s) point of views / experiences and other factorsthat's the way it goes here

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2024, 05:48:04 pm
content: Quote from: okw on February 09, 2024, 10:13:41 pmSafety is an issue, but isn't the only reason why I wouldn't do this. It's more expensive, bulky and less efficient, than simply ordering a relay with the correct coil voltage. I understand you want to get this done with what you have in your parts inventory, but this is a foolish way do it. The only time it's acceptable is, if there's a tight deadline to work to i.e. there isn't enough time to get the correct part.

author: okw
author_rank: Frequent Contributor
timestamp: February 10, 2024, 06:36:07 pm
content: Thanks for all the input. Seems 230V coil relays are generally more expensive than <=24V. But it shouldn't compromise safety.If I extend the use case; What if this relay will be controlled by an MCU?Is it then safe enough to capacitor drop the 250VAC to 12V to control the relay and a LDO for MCUs 3.3V? Seems many products are designed this way.They of course select a relay with coil/contract insulation that can handle failures.But if it's such a bad idea, why are so many products designed (and approved) this way?

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2024, 06:57:37 pm
content: Its *not* a safety problem with the right relay, but if it controls an externally accessible low voltage circuit, the relay *MUST* have an adequate isolation voltage rating between its coil and contacts, which rules out all the cheap s--t from far east suppliers that change their trading name as often as their underpants!

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2024, 11:03:50 pm
content: Quote from: okw on February 10, 2024, 06:36:07 pmDoes it really work out cheaper once the cost and board space of the additional components are taken into account? I very much doubt there's that bigger difference in the total cost.QuoteThat's a different use case. As mentioned above, the relay needs to be designed with sufficient separation to be safe for mains.The products I've seen which do that don't use 12V relays, but a higher DC voltage such as 24V or even 48V. I would aim for as higher DC voltage rating as possible, so the transformerless PSU can be as small as possible. Another thing to bear in mind is the voltage required to hold the relay is much lower than that required to turn it on, so the power supply can be designed so the voltage drops once the relay is on, or the MCU can PWM the relay, to keep the current down. I would also consider switching a 230V relay with a TRIAC, or just using a big TRIAC and going completely solid state.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 10, 2024, 11:23:21 pm
content: Let's put this to an end.For the cost of the required parts (Capacitors, bridge rectifier...), for a little more you can do it properly:https://www.aliexpress.com/item/1005005552969433.htmlhttps://www.aliexpress.com/item/1005004765902480.html

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2024, 10:50:57 am
content: Quote from: DavidAlfa on February 10, 2024, 11:23:21 pmBut those power supplies are too cheap to be sure they're safe.It's still cheaper just to buy the correct relay and safer, since a relay is much less complicated and easier to design, than a switched mode power supply.Here are some 220V to 240V coil relays, in clear cases, which appear to be half decent safety-wise, as the coil is wound on a double insulated former. The creepage between the coil and contacts might not be good enough, but it depends on the tracking index of the material and the environment. I suspect if it's installed in a dry area, it'll be fine.https://www.aliexpress.com/item/32874867614.html?spm=a2g0o.productlist.main.1.615e2270vZ9YZi&algo_pvid=17a1f97d-d9ae-480d-a3cb-e755e90841df&algo_exp_id=17a1f97d-d9ae-480d-a3cb-e755e90841df-0&pdp_npi=4%40dis%21EUR%211.90%210.47%21%21%212.00%210.50%21%40211b617b17076474460423950e5552%2165539953884%21sea%21PT%210%21AB&curPageLogUid=Udjn4wzlJuVo&utparam-url=scene%3Asearch%7Cquery_from%3Ahttps://www.aliexpress.com/item/32827100876.html?spm=a2g0o.productlist.main.3.615e2270vZ9YZi&algo_pvid=17a1f97d-d9ae-480d-a3cb-e755e90841df&algo_exp_id=17a1f97d-d9ae-480d-a3cb-e755e90841df-1&pdp_npi=4%40dis%21EUR%212.27%212.11%21%21%212.39%212.22%21%40211b617b17076474460423950e5552%2165609551961%21sea%21PT%210%21AB&curPageLogUid=HebIFXMOV4vj&utparam-url=scene%3Asearch%7Cquery_from%3AI would rather just spend a bit more and buy from a distributor such as RS, Farnell, TME etc. for peace of mind. A piece of cake, at a nice restaurant costs the same as a decent 230V coil relay. It's not worth skimping on.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2024, 11:02:22 am
content: +1 to just get a relay with AC coil. Non-isolated resistive or capacitive dropping is acceptable if done correctly, but it's a lot more work and definitely costs more, not to mention all the extra heat generated. I don't see the point except if you have to have it working tomorrow and have no local store who carries AC coil relays.