driving solid state relays with the BS2

nhbrewer

I've struggled to answer this question searching the web - maybe I'm missing something. I am building a controller with a BS2 BOE board and want to fire some 3V SSRs to intutn control line voltage circuits. If the signal inputs on the SSRs have 1K of resistance am I safe to run them directly off the output pins and if not whats a simple way to drive them. I'd like to drive some 2v LEds in parallel as indicators of which SSR is firing but don't want to add to the load on the pins if I'm already pushing the limit.

Having fun but a bit over my head

Mike Green

Most SSRs can be driven directly from BS2 I/O pins. Most have some kind of current limiter for the internal LED in the optocoupler in the SSR. The BS2 I/O pins can supply on the order of 20mA although there's an I/O pin group limit of 50mA for the 8 pins (0-7 and 8-15) and an overall current limit for the BS2 itself. If the SSRs have a 1K input resistance, they would draw about 5mA. You could easily add an LED if you used a 1K resistor on the LED and didn't try to drive more than five pairs of SSRs/LEDs at a time.

nhbrewer

thanks Mike,
I had calced the 5ma but didn't trust myself, seemed so low compared to the limit on the pin. I had not factored the issue of combined load so thanks for pointing that out. So I understand, if you had two seperate loads of say 30 ma, would you still be OK if you simply put one on pin 0-7 and the other on 8-15? Not an issue for me now but just wanting to understand. IS there a SIMPLE way to fire higher draw devices that exceed the pin limits without getting into building and involved amplifier circuit?

Mike Green

There is no such thing as "pin 0-7" and "pin 8-15". I referred to groups of individual pins. There is a separate current limit on the total current supplied to I/O pins 0-7 as a group and I/O pins 8-15 as a group. This current limit has to do mostly with the capacity of the on-chip wiring.

See Nuts and Volts Column #6 downloadable from Parallax. Go to the main webpage and click on the Resources tab. You'll see a link to the Nuts and Volts Columns index.