Solid state relay question.

Balok · Post by **Balok** » Mon Feb 25, 2008 9:55 pm

Does a solid state relay have an indefinite life if you don't abuse it, or is it rated for so many cycles? I'm thinking about using one or two to flash my FMMF lights. Before with the blue lights I built, the brightness, while pretty good was definitely limited by the 556 chip used to flash them. I used the 556 because it seemed to let more current through than the 7556. I'd like to use the same flasher circuit now with a 7556, but controlling more current by way of the solid state relay. What say gurus? Is it a plan or is it trash?

macfrank · Post by **macfrank** » Mon Feb 25, 2008 11:10 pm

I hate mechanical relays, so I've replaced all the mechanical relays with solid state relays in our equipment at work. They're rugged, easy to drive and last forever. They tend to be expensive, though.

For what you're trying to do, why not use a transistor or a MOSFET to drive the LED from the output of the 555? It'll be a lot cheaper, and simpler.

Balok · Post by **Balok** » Mon Feb 25, 2008 11:32 pm

That's good news about the solid state relays. Transistor eh? Don't like to admit it Frank but those suckers have never worked for me.

Can you suggest anything to get me over my fear of them? Anything like the javacript LED series resistor calculator for transistors or mosfets out there in your bag of tricks?

jwrjr · Post by **jwrjr** » Mon Feb 25, 2008 11:40 pm

Macfrank is correct. SSRs will last ... indefinitely, if not abused. The nice thing about power mosfets (I am rather fond of the bs170, 60 volts, 500ma, and CHEAP) is that you need no base resistor or anything like it. For model work you won't get much closer to 'bulletproof'. You can get much higher voltage ratings, much higher current ratings, or both.

Balok · Post by **Balok** » Tue Feb 26, 2008 2:53 am

Okay guys, I'm scraping my wits together and learning about transistors and mosfets. Thanks for the tip about the bs170 jwrjr.

One question Frank: Remember the freeware schematic program that comes up with a printout of a board design? What was that? I had it on my long dead laptop and now I can't remember what it was. I want to investigate getting some boards made to speed up production. I also want to show my tested circuit for perusal and improvement.

Nate

macfrank · Post by **macfrank** » Tue Feb 26, 2008 10:01 am

Balok wrote: One question Frank: Remember the freeware schematic program that comes up with a printout of a board design? What was that?

It's the Eagle light edition from CADsoft. It's a little clunky, but it works well and the price is hard to beat!

As for the MOSFET drive circuit, this page has an example, driven from a digital line.

And this page has both transistor and MOSFET examples. You should treat MOSFETs with the same care as you would any other static sensitive device while out of circuit. Transistors don't require special handling. Look at the specs for the BS170 (or other MOSFETs) to make sure that your output voltage will be enough to drive them. If it's over 3 volts or so, it should be OK.

Frank

jwrjr · Post by **jwrjr** » Tue Feb 26, 2008 10:16 am

I use a freeware CAD program called WinQcad. I don't know how it compares to Eagle. It is just the one that I am used to.
As Macdrank says, mosfets are sensitive to static. But that has not been a problem for me. The controller chips that I use run on 5 volts. This is plenty to drive the bs170. I just connect the 'gate' of the transistor directly to the controller output pin. In other applications this wouldn't be good enough. But for controlling lights in a model, it does fine.

RossW · Post by **RossW** » Tue Feb 26, 2008 2:59 pm

James - if the gate is connected to the output pin of the PIC, what is the base/emitter connected to? One of the circuit diagrams posted by macfrank has '1/2/3' nomenclature, but that still doesn't tell me which is which.

macfrank · Post by **macfrank** » Tue Feb 26, 2008 3:53 pm

RossW wrote:James - if the gate is connected to the output pin of the PIC, what is the base/emitter connected to? One of the circuit diagrams posted by macfrank has '1/2/3' nomenclature, but that still doesn't tell me which is which.

FETs are not bipolar transistors, so there's no emitter or collector. They have a Gate a Source and a Drain. In theory there's no real difference between the source and the drain, since FETs act as a switch. In practice, due to the way they're built internally, an n-channel FET (or an n channel MOSFET like the BS 170) has a slight preference; the drain pin should usually be more positive than the source. In the schematic, the 1/2/3 indicates the package pins.

jwrjr · Post by **jwrjr** » Tue Feb 26, 2008 5:12 pm

The way I do it is like this: gate goes to the controller output, source goes to ground, and the load goes to the positive power supply on one side and the drain pin on the other. The load can be light bulbs, led(s)+resistor, or whatever. Some mosfets (the bs170 is one of them) have a reversed diode between the drain and source pins. So hooking it up backwards isn't catastrophic, just not helpful. If the load stays on all of the time, try reversing the 'D' and 'S' pins.

Paulbo · Post by **Paulbo** » Tue Feb 26, 2008 11:04 pm

jwrjr wrote:Macfrank is correct. SSRs will last ... indefinitely, if not abused.

What, you mean like using them to switch the legs 240 VAC power against the neutral leg so you can swap between 120 and 240 ... but not taking into account the speed of the switching so you get a dead short running straight out of high power mobile generator?

On the downside, we totally fried the relays.

On the plus side, the 5 foot jet of flame that burned right through the 12ga. steel control box was really cool to watch!

(Adding a delay into the switching system took care of the problem and only took a half hour to wire in. No more flame jets, though

)

jwrjr · Post by **jwrjr** » Tue Feb 26, 2008 11:14 pm

Even though the relays were fried, becaused you used them, the control circuitry wasn't fried. Kind of expensive fuses, but it could have been worse.

Paulbo · Post by **Paulbo** » Wed Feb 27, 2008 5:05 pm

jwrjr wrote:Even though the relays were fried, becaused you used them, the control circuitry wasn't fried. Kind of expensive fuses, but it could have been worse.

There were fuses (actually circuit breakers) in the circuit, but the relays fried faster than the response time of the breakers!

As I say, very cool flame jet and thank goodness nobody was standing in the wrong place when the sucker went off.

Balok · Post by **Balok** » Wed Feb 27, 2008 9:03 pm

Thanks Frank,

I'm getting parts for the circuit and struggling with Eagle a bit. I downloaded a couple other schematic programs, but Eagle appears to have all the devices in it I need to use in it.

Nate

Balok · Post by **Balok** » Mon Mar 24, 2008 12:06 am

Okay, Ive finally gotten the chance to play around with the BS170 somewhat.
I got one to work well in the solderless breadboard. Correct me if I'm wrong, but the gate must be negative. Given that the output of the 7557 is positive, what do I need to put in between to get the signal to the gate to be negative? Looking through the CMOS Cookbook I see the 4066 might work. What do you think Frank? Or maybe my setup on the board is wrong.

macfrank · Post by **macfrank** » Mon Mar 24, 2008 4:31 pm

Balok wrote:Correct me if I'm wrong, but the gate must be negative. Given that the output of the 7557 is positive, what do I need to put in between to get the signal to the gate to be negative? Looking through the CMOS Cookbook I see the 4066 might work. What do you think Frank? Or maybe my setup on the board is wrong.

Nate, the 4066 probably isn't a good choice - it won't let you draw that much current through it.

You can always put your LED load on the drain or source side. That way you control what gets switched (GND or V+) when the gate pin goes high.

jwrjr · Post by **jwrjr** » Mon Mar 24, 2008 10:18 pm

The BS170 is an N-channel mosfet. You drive the gate positive to turn it on. I just connect it directly to the output of the controller that I am using.