Using TRIACs for turning transformer ON and OFF - Page 1
Forum section: electronics / beginners

author: spyro
author_rank: Newbie
timestamp: October 03, 2023, 10:28:10 am
content: Hi!I'm new to the forum. Greetings to you all from Mexico.I was looking for help with some topics of electronics. Here's this one:I'm building a little spot welder for welding battery packs and thin sheet metal goodies. I modified a microwave oven transformer (separated the core, took out the secondary, made a new secondary with a few turns of 10ga copper cable, and welded the core again), so it gives me 10volts output at the secondary.I need to just turn the transformer ON or OFF using triacs (i have a bunch of BTA24's). I don't want to try doing phase angle control over it, 'cause it's a very bulky inductive load, and i know is really tricky. I'm planning to use a NE555 in monostable to create a short pulse (from 0.1 to 2s) to turn on a phototriac (MOC type) that will drive the gates of some BTA24 triacs in parallel, so the transformer is turned ON for the duration of the pulse, and my spot weld gets done.The transformer pulls around 15 amps with no load, but the primary current climbs up to some nasty 60 amps when i short circuit the secondary. My idea is to use 5 BTA24 triacs in parallel to switch the primary of the transformer, and use the microwave oven capacitor (1uf 2000v) and a beefy 4.7ohm resistor to improvise a snubber to protect the triacs from the inductive discharge when turned OFF. My BTA24's are rated to 800v.Could anyone give me some advice or directions for this project? Could my idea actually work when put together, or is it a potentially disastrous monstruosity? I'm fairly a beginner on electronics. I'm a physics student. Will be really thankfull for any advice or support.Best regards!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 03, 2023, 10:36:06 am
content: Quote from: spyro on October 03, 2023, 10:28:10 amUse a suitable rated off-the-shelf solid-state relay - someone else has dealt with the safety side of things.Follow the manufacturer's recommendations for inductive loads ( snubbers etc.)

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2023, 11:36:50 am
content: Howeverdon'tuse a 'zero cross' switching solid state relay. You will most likely get a very nasty current surge at power-on due to core saturation (which will probably make your 60A look like peanuts). Use a 'random' switching one. This won't guarantee that you won't get the occasional saturation surge but at least it won't happen oneveryturn-on. Ideally, you would sense the mains waveform (opto isolator) and energise the SSR at some point before mains peak voltage.

author: jonpaul
author_rank: Super Contributor
timestamp: October 04, 2023, 05:24:05 am
content: Use properly rated relay or contactor. See magnecraft catalog. $25.Avoid triacs, or solid state relays in inductive load/welding/ power applications.Jon

author: MrAl
author_rank: Super Contributor
timestamp: October 04, 2023, 08:23:33 am
content: Quote from: spyro on October 03, 2023, 10:28:10 amHi,You should know that there is no way to force a triac to turn off exactly when you want it too. Same with an SCR.The only way a triac turns off is when the current through the main terminals goes to (or through) zero. Unfortunately this means that you can not use a pulse on the gate unless you sync it to the current waveform, and turn on just before the end of one of the half cycles. Another unfortunate consequence of doing this too is the sine wave current will not be a pulse it will be a part sine wave, so the current may not be as large as you expected it to be.There is the possibility that you can use an entire half cycle, or an integer number of half cycles. In this case, you can use a "pulse" but it wont really be a pulse it will be a train of half sine cycles. The current then ends when the last half cycle is allowed to conduct through the triac.You will have to figure out what the duration of your "pulse" has to be, then figure out how many half cycles you need to pass. Since each half cycle of 50Hz is 10ms and each half cycle of 60Hz is around 8ms, that means that the shortest pulse will be either 8ms or 10ms depending on what the line frequency is where you are going to run this. This also means that the next shortest pulse will be either 16ms or 20ms, and the next after that 24ms or 30ms, and the next after that 32ms or 40ms, you get the idea. This means you can't get 4ms or 43ms for example.If you use IGBT's or MOSFETs you can get any pulse width you want, but it will still not be a true pulse it will be either a portion of a half cycle (short pulse) or a train of half cycles, where now the last one can be cut short. If the last one is cut short though, there will be a lot of inductive kickback to worry about snubbing.There is a chance your pulses will be long enough where you don't have to worry about this too much, but you'll have to figure that out.

author: Zero999
author_rank: Super Contributor
timestamp: October 06, 2023, 05:38:05 pm
content: Quote from: Gyro on October 03, 2023, 11:36:50 amTrue. If anything, you want to turn the TRIAC on when the voltage, which would be 325V, with 230VAC in. In reality you want to fire it at a slightly lower voltage to take into account the mains voltage might be on the low side, so about 280V..