I was using a Vishay VO14624 solid state relay to shut down a power supply if too much current is drawn. The relay data sheet says the control circuit is CMOS and TTL compatible, so I hooked the control pin (an UNO digital pin) directly to the relay. That worked fine. The internals of the relay involve an optoisolation circuit, comprising a UV LED to operate a MOSFET phototransistor.
So I got around to putting this on a PCB. As I wire up PCB boards for the first time, I always incrementally add components, and incrementally test. (Ever wire up a whole PCB and then find it doesn't work? No fun debugging it.)
So, in the incremental testing, I tested the relay by applying 5 v. to the control pin. The ProMini was not yet on the board, so I couldn't test it from the ProMini. Well, that burned out the relay.
So with further experimentation, I discovered that that UV diode in the relay is not protected by any internal current limiting. If I had read the data sheet very carefully, I may have deduced that, but is not stated in the data sheet. It is CMOS and TTL compatible, provide those inputs do not deliver more than 50 ma. My direct touch with a 5 v. line was delivering well above 50 ma., so goodbye relay.
Anyway, I modified the circuit by adding a current limiting resistor. Lesson learned. Since I didn't read the data sheet carefully enough, I figure the same could happen to someone else.
jrdoner:
I tested the relay by applying 5 v. to the control pin. The ProMini was not yet on the board, so I couldn't test it from the ProMini. Well, that burned out the relay.
So with further experimentation, I discovered that that UV diode in the relay is not protected by any internal current limiting. If I had read the data sheet very carefully, I may have deduced that, but is not stated in the data sheet.
This is true of most (all?) optoisolators.
Inside the chip it's just a LED. The same rules apply to LEDs inside optoisolators as any other LED.
They can't really add a resistor because they don't know what voltage you're going to use to drive it.
Most SSRs do have a current limiting resistor, so I'm curious about the data sheet for this part. However, Google can't find it. Could you post a link to the data sheet?
Most "solid state relays" have a current limiting resistor, and the specs show a control voltage range. But I've seen parts like that one too. (Actually, I've seen the datasheets, not the actual parts.) Personally, I'd call it a "high power optoisolator" if it doesn't have a resistor... But the manufacturer gets to name the part!
Usually the clue is a voltage or current spec... If it says it's controlled by 5-15V, or something like that, it has an internal resistor. If they specify a current, you'll need to add the resistor.
Usually the control voltage and output voltage & current ratings are specified "up-front", on the datasheet since they are very important in selecting the right relay. If there's no resistor, the control current may be "buried" somewhere on a chart, since it's not quite as important in the selection process... Generally, you can select the resistor later.
So I just made a small tuition payment in the school of experience.
I like that! I'm going to try to remember that one!
jrdoner:
I was using a Vishay VO14624 solid state relay
According to Google, and Mouser, no such component!
No wonder you have had so much trouble with it!
jrdoner:
This is not a repairable component (6-pin DIP), but they're only about $2.00 each. So I just made a small tuition payment in the school of experience.
Now you have us very curious! A solid state relay in a 6-pin DIP. If they do not exist on the Internet, from where do you get these things?
I think you mean a IR led, not an UV. Thats the most common. Leds have higher voltage drops the shorter light wavelengths they produce. IR have about 1,2V red about 1,5, orange about 1,7 yellow about 2, green 2,4 and blue 3
I like that! I'm going to try to remember that one! 