# Relays vs. thyristors: What is best for swiching 230 V lights on and off rapidly (~ 5 Hz)?

I am building my own disco/party lights. 230 V/60 W bulbs, and ideally I would like to switch them on and off at 5 Hz.

I was planning on a simple setup with an Arduino controlling the lights. My original plan was to use relays, more specifically SRD-05VDC-SL_C (datasheet).

It says "Max electrical ON/OFF switching" is 30/minute, which is too slow for what I had in mind. This seems like a common theme with relays, since after all there is a mechanical switch inside.

After some research triacs/gate turn-off thyristors seems like a possible alternative. I had a look at TIC226m (datasheet), but I could not find any max switching rate.

Are these usable for my setup? Any other tips and suggestions are greatly appreciated.

• Thyristors are good for switching on and off at 50/60 Hz so 5 Hz shouldn't be a problem. Oct 14, 2018 at 16:07
• You would be better off to use 12 or 24 V COB LED arrays as you can strobe them for very short durations and not have to deal with HV design at all. Oct 14, 2018 at 20:15
• Also take note of the frequencies of flashing lights that can induce epileptic fits in susceptible individuals. Design your controller to avoid such frequencies by dividing or multiplying flash rates to get past those zones. Cheaper to buy ready made budget or used equipment. Oct 15, 2018 at 10:43
• Please note that incandescent light bulb itself is pretty slow and could be too slow to give you the effect you mean at 5Hz. Unless of you're into this fade-in, fade-out effect, LEDs indeed are bit too brutal here. Oct 15, 2018 at 13:19
• 5 Hz is high enough to induce epileptic seizures. <3Hz is much safer. Oct 15, 2018 at 17:58

Mechanical relays won't last long if forced to switch at 5 Hz, but solid state devices (thyristors, triacs or transistors) are perfect for your task.

Instead of coming up with your own thyristor/transistor circuit, I suggest that you use a solid state relay. It's easier and safer. They consist of an optocoupler (for electrical isolation of the logic level input), a driver circuit and some kind of solid state switching element, all integrated into a single off-the-shelf package.

• And they are about $2 per piece. Totally worth it. Oct 14, 2018 at 16:24 • There were liquid-metal contact relays for frequent switching. I've seen one meant for about 2Hz, so I'd bet mechanical relay that lasts at 5Hz is possible. Just nitpicking : ) Oct 16, 2018 at 9:47 • @Agent_L Reed relays can switch quite fast too, often within a millisecond. – jms Oct 16, 2018 at 14:34 • @jms Hmm, my statement was unclear. What I meant is that a glass bubble with a pool of mercury large enough to reliably switch 60W is quite heavy. So you can have it down to ms fast or you can have it nearly indestructible and ticking for years - but getting both in one device would pose major challenge IMHO Oct 17, 2018 at 6:46 • @Agent_L It wasn't unclear. I knew that mercury displacement relays exist, although they have mostly been superseded by solid state relays. I just added that reed relays (another kind of electromechanical switching device) can switch fast too. Of course, reed relays aren't suitable for OPs application given their low current rating. – jms Oct 17, 2018 at 9:54 Three major issues here. The first is that you don't seem to be aware that you're effectively building a dimmer circuit when you start using electronic switching. If you Google "theatre light dimmer schematic" you'll find a few useful links, including an old ePanorama article about different types. You may find useful links on the Everyday Practical Electronics site too. This is almost historical now, since it was basically invented in the 1980s and couldn't be done any better/differently. The second major issue is that you aren't aware that your design is badly flawed. Regular domestic 60W bulbs aren't designed for decent stage illumination. The throw is all wrong, and besides they simply aren't bright enough to do sod all. Stage lighting needs crazy amounts of power if you're using incandescent lights. When you use that kind of power though, you get a lot of heat, and then you need the design of a stage par to keep that under control. So you might as well buy stage pars. Except that all that power is a nightmare to manage. It's a real job to make sure you aren't going to blow fuses during your gig. And worse, your switching circuit is going to have to dissipate high levels of heat too. As well as having to not interfere with the sound when it switches on and off, which is a perpetual problem for every mains dimmer ever made, even the best ones. Actually, never mind your sound - you also need to not block TV and radio reception for a couple of blocks around you. The solution which every stage and every gigging DJ and musician uses these days is LED pars. Fairly cheap, low power, decent brightness, controllable over DMX. Job done. So if you're going to build your own lights these days, you should be using LEDs. Do the job properly. Of course it'll cost you much more to make it yourself than to buy it off the shelf, but then you were going to have the same situation with your original suggestion, so I'm assuming you're doing it as a project to learn electronics, and not because the end result will be very useful or cost-effective. There's a third problem too, though. This involves mains voltage, and you very clearly are not yet highly skilled at electronics. This presents a clear hazard to you, your house, and everyone living in your house. I strongly advise you not to attempt this, because you don't know enough yet to be safe with mains. Edit to add: If you're planning on gigging as a DJ, this also presents a clear hazard to everyone at your gigs too. Your homebrew kit will not pass an electrical safety test. If anything goes wrong then you are directly to blame for gross negligence, and your insurance will not cover you. This means you and your family could also lose your house and every possession you have. If I've not made it clear enough, you really shouldn't be doing this... • If you are going to use consumer incandescents, the 500W linear halogen type are the ones to go for. They have a sort of floodlight pattern and a decent amount of power. They used to be a good cheap option for adding extra power when short of proper lanterns, and that can take being flashed without burning out expensive bulbs. Oct 15, 2018 at 12:06 • Plus, Disco lights are like, so last millennia, dude. Oct 16, 2018 at 13:22 Your best bet is probably to use SSRs (solid state relays) which include drive and isolation circuitry in one package. They also include a thyristor and usually a heat spreader to interface with a heat sink, if necessary, that is electrically isolated. The mechanical relays are unsuitable, they will be noisy and wear out very quickly. Keep in mind that SSRs will typically give complete cycles, so at 5Hz you may see undesirable beating between that and the mains frequency. You can't switch mains. First you shouldn't switch it because mains power is dangerous, and you should not be running it through a homebrew gadget (it violates the electrical code and will void your insurance if it causes a fire/injury). But the deal-killer is no mains lamps are fast enough. • Incandescents have too slow a rise/fall time; at 5Hz you wouldn't be flashing them so much as PWM dimming them. • Fluorescents are Right Out even if "instant start" was fast enough; startups are extremely wearing on a fluorescent and you'd burn up the usable life in the tube in about a half hour. • Mains lamps using LED technology are way too slow because they have an electronic driver module in front of the LEDs. That electronic module can't shift from "on" to "off" fast enough. Even if you find a driverless bulb that can, the driver module is likely to have a short life, and then how does your staff find another? So this deal will not happen in mains. Now you're in the 12/24 volt DC world. Here it is easy. Many low voltage DC LED packages use resistors to limit current, and those can be flashed at any frequency into the KHz. Any LED capable of being dimmed with a PWM controller will do exactly what you want. No problem. You don't even need to engineer high-power switching; you can just use an off-the-shelf "amplifier module" (typically made with 3-4 channels) and have your Arduino send a pilot signal to the amplifier. These things cost about$9.

And bonus, you've avoided the hazard of mains.

The other option is to use manufactured "smart" LED light products that allow you to command them to various color/dim levels. Fair chance those put the dimming on the DC side, and are able to respond instantaneously. So you could command them to bright/dim at 5 Hz, and they could do it.

Relays are bad for that sort of thing because every opening of the circuit will arc which will burn the contacts and wear them out. OK, so you are switching A/C so sometimes you'll catch it at the null current so no arc, but mostly you'll have a lot of arcing going on. This will both subject the relay to high wear and create a lot of heat.

Thyristors will work better because there will be no arcing. But, switching will generate more heat than steady-state "on" (if not at a zero-crossing of the A/C). When switching at a low rate, say at least a few seconds between cycles, it probably won't amount to anything, but at 5 Hz, there may be some additional heat build up. You may need to pay a little extra attention to how the devices are mounted and cooled to ensure that they won't overheat.

Lightbulbs are terrible lightsource for your application. They are ineffective and they are slow. Much better alternative is switching to LEDs.

1. Lower waste heat (they need to be cooled although)
2. Lower voltages needed
3. Lower currents needed
4. Smaller dimensions
5. Default colours (red, green, amber, blue, white) plus RGB unit
6. Rapid changes of brightness
7. High tolerance to rapid switching
8. Easy brightness regulation via PWM

And regarding your main question: As others already suggested, you can buy solid-state relays. This way all possible issues with switching you would have to face were faced years ago and since they are capable of switching frequencies on order of kHz you can simply regulate the brightness in the arduino as well.

There is nothing faster than a 230v relay. The contacts are designed for the load and repetitions and the safest method.

And someone mentioned Led's. Christmas tree light controllers are the most dangerous fire and heat generating electronics on earth. Even at only 2 to 6mv. Fires !

And just a waste of heat to use solid state electronic controlled on/off purges to a secondary relay then out to a main relay.

Square D has 230v with second or millisecond solid state timers you can pre set and plug directly on the front or beside the relay. Your switches to the coil from it can range from 12 to 120v internally with hardly any or no wires. Hand switch to timer to 230v to lights.

The larger the Relay the less heat generated.

Use case is every Bottling company in the world on the filler lines use a push drop return can packager and or 3 or 2 liter bottle case packers. These run 24/7 as well. Where packing lines have to repetitiously and very quickly switch motors from forward, stop to reverse faster than disco lights.