DIY BJT SSR

[Axiomaticky]

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.html but 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: 2N3055 http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF
Q2: BD712 http://pdf.datasheetcatalog.com/datasheet/stmicroelectronics/4134.pdf

Other components unchanged.

I take it I did not read enought BJTs but could someone push me in the right direction? Thank you.

[Kevin.D]

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 Ic
so  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 it


In 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 .

[T3sl4co1l]

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

[digsys]

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?