Can IGBT´s be used instead of relays - Page 1
Forum section: electronics / beginners

author: paul23
author_rank: Regular Contributor
timestamp: September 05, 2012, 09:41:34 pm
content: I am working on a project that needs to be able switch 230V AC appliances. The maximum load is 20A. It is a retrofit to an existing design which utilises 10A relays. The problem is that 10A relays are a lot smaller then 25A relays, so when I have come to layout the board I physically cannot fit bigger relays on the board.In looking for a solution I came across IGBT´s, which to be honest I have never heard of before. I found examples of the application and seems they are generally used for high load applications, like industrial motor control.Can they be used like a relay?Thanks for any advice

author: gxti
author_rank: Frequent Contributor
timestamp: September 05, 2012, 10:03:06 pm
content: I would use a TRIAC, but there are a lot of considerations you don't have to worry about when you use the relay. The TRIAC needs heatsinking, and if the load isn't purely resistive (e.g. a refrigerator or other motor) you need a snubber. A relay should have a snubber too, but while the relay wears out prematurely without one the TRIAC will simply get stuck in the "on" position. Also TRIACs (and any other solid-state switch) don't have built-in isolation so you need to isolate the switching circuit from the logic side to keep unsafe voltages away.The safest and easiest thing to do is to use a premade "solid state relay" module, with its own isolation and snubber. These are available pretty cheaply from both reputable and less-than-reputable sources. You will still need a heatsink, though, so it will probably end up bigger than the 20A relay anyway.

author: Psi
author_rank: Super Contributor
timestamp: September 06, 2012, 01:55:24 am
content: Quote from: paul23 on September 05, 2012, 09:41:34 pmRelays are rated for the current through one contact set.You might have more luck looking for 10A or 15A relays with multiple contacts.Then you can parallel up the contacts to get your 25A current capacity.They will probably be smaller in size than a 25A relay.Note: When putting relay contacts in parallel you need to be aware that one will make contact first and take all the load for a microsecond until the other contact touches. So you may want to over-design it, eg two 15A relays for 20A.Of course it does depend on your load.If your load doesn't draw the full current until sometime after switch-on then there isn't much of an issue.It might require someone to press another switch to engage the high current part of the device etc..Or on the other hand, your load might draw peak current at switch-on, so that wouldn't be suitable.If you cant figure out a way to do that, and need your large 25A relay, maybe look at putting 4 holes in the pcb and having an elevated 2nd small pcb for the large relay.

author: TerminalJack505
author_rank: Super Contributor
timestamp: September 06, 2012, 02:45:00 am
content: IGBTs are polarized devices. They have a P-N junction that will prohibit current from flowing in one particular direction. Because of this, they aren't suitable for switching AC.

author: David_AVD
author_rank: Super Contributor
timestamp: September 06, 2012, 03:24:08 am
content: Maybe stage lighting dimmers that use IGBTs use them in pairs?

author: krish2487
author_rank: Frequent Contributor
timestamp: September 06, 2012, 03:56:34 am
content: @terminaljackNot completely true, Static bypass switches use anti-parallel IGBTs or full bridges for switching AC. as a matter of fact in the bypasses IGBTs are used to replace the function of relays for high power ratings.@OPYou can use a Triac to switch AC,Is your application designed to be used using discrete logic or with a micro?I am attaching a small subsection of a project i worked on earlier, it is a discrete, single pot controlled solid state relay with undervoltage and overvoltage cutoff.see if that can be of some help.

author: paul23
author_rank: Regular Contributor
timestamp: September 06, 2012, 08:21:54 am
content: Thanks for the replies.The load can vary, sometimes there will be a couple of minutes between switching the relay and the load consuming anything over a few mA, other times there is a possibility that full load could be consumed as soon as the relay switches.There are two PCBs stacked, the lower one contains the relays, transformer, fuses and bridge rectifier, all mounted on a 10 x 5cm PCB. The relays can have a board space of just under 4cm x 4cm, which clearly is not much at all.The board above has all the digital stuff (microcontroller, switches, LCD, LEDs, etc) A ULN2003 for driving the Relays and LEDs, a 7805 and 7812. The two boards are stacked quite close together (25mm), so whatever I use has to be lower than 25mm.I did look in to Triacs, but thought it was going to be a really messy solution, based on what I read. It seems IGBTs could be equally messy.Quote from: Psi on September 06, 2012, 01:55:24 amDo you mean using a 15A DPST relay instead of a 20A SPST? That might just work, I´ll look in to that. I was looking at DPDT and DPST relays trying to do the whole thing of 1 relay instead of 2, but I was looking at 20A and above and couldn´t find anything that would fit. However, I never thought of looking at smaller relays and paralleling the contacts. I will definitely check this out. Thanks

author: T4P
author_rank: Super Contributor
timestamp: September 06, 2012, 03:52:04 pm
content: I have used triacs as isolated SSR's beforeWhen i couldn't find any SSRs anywhere