DIY BJT SSR - Page 1
Forum section: electronics / beginners

author: Axiomaticky
author_rank: Newbie
timestamp: February 02, 2015, 08:29:38 pm
content: Hi,I found myself in need of SSRrelay for switching resistive heater load. Imput signal 12V output 12V 10A. Both DC. I found circuit:http://www.delabs-circuits.com/cirdir/power/static-relays/del30007.htmlbut the output is only for 5A, so I have read something about transistors and tried to do some changes, namely changing Q2 and Q3. My idea was that if I use transistor with highter CE amperage with other values similar to the original one transistor (what the shop had), it will work. And it does with low current, but not with 10A.I used:Q3: 2N3055http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDFQ2: BD712http://pdf.datasheetcatalog.com/datasheet/stmicroelectronics/4134.pdfOther components unchanged.I take it I did not read enought BJTs but could someone push me in the right direction? Thank you.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: February 02, 2015, 10:34:09 pm
content: Hi .You probably dont have enough base drive for 10 A .A quick look at data sheet :-2N3055 hfe ~ 5 @ 10A Ic.bd712 min is 30 @ 2A Icso the bd172 needs ~ 60 mA base to achieve the required 2A .A quick look at cny17-3 opto isolator data sheet shows a iC/If transfer ratio (like current gain ) of ~ 1 to 2 max.so you probably need ~40 mA into the opto isolator drive (it looks to be abs maxed out at 50mA so that would be pushing itIn your schema R2 sets the input diode current (Q1 will maintain the voltage across R2 = Vbe so ~.7V) thus it's currently set to .7/33 = 21mA . You could increase the diode current some by reducing R2 (and maybe some proportional drop in R1 depending on what input V your using).Alternatively (prob better since not so hard on the opto ) you could use higher gain for Q2 (use a darlington ) .(another option would be to make use of power mosfets (little base drive current required) .Good luck .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2015, 10:34:53 pm
content: Well, part of your problem right there is 2N3055... it's a bad transistor from an early era. On the upside, modern production parts are rarely as bad as the datasheet allows... but it's still a device best left forgotten.Second is supplying enough base current. In saturation, hFE ~ 10 is pretty typical. That means you need 1A of base current. 2N3904 isn't going to do that. You could use a TIP32 (rated for 3A), but it, in turn, needs about 0.1A. So you really need to add one more transistor to bring the opto's 10mA up to 0.1A for the next two. A 2N3904 would work there. To wire this in, you'll need to swap around the optotransistor's connections (collector to OUTPUT+, emitter to 2N3904 base; then 10k across B-E, and C-E where the opto is shown now).Expect about 1V of drop across this thing. Not exactly a good relay (why does it have to be an SSR, anyway?), but I suppose it'll be faster than a MOS type (which isn't very available in this kind of current rating anyway).Tim

author: digsys
author_rank: Unknown
timestamp: February 02, 2015, 10:42:04 pm
content: Seriously, I'd use an NFET. You can get a few mR at 60-80A easily. Easier to use, Literally NO drive current required.Any reason it has to be a BJT?