Twisted transistors! overamperage concern for BS2

firestorm.v1

Ok, working with transistors again (yes this has something to do with my other post about serial ports)
I have come up with a simple design that I wanted to run by more educated and smarter people than myself to see if I have left anything out.

I know that the BS-2 can't source more than 20mA, is that a per-port limitation? Does going through the transistor alleviate this?

What I am thinking of doing is creating a relay-array that is software controlled by a host computer (hence the serial port post). In order to do that, I am wanting to make sure that with all 8 indicator LEDs fired up (imagine 8 copies of the schematic below) that the amperage requirement is not exceeded. according to my math each LED/470 ohm combo will use 7.23 mA. Assuming that is a per-port limit, then I should be able to build many of these without fear of destroying the I/O pins. (the regulator I'm not worried about as they Vcc for the transistor/relays will be fed off of a 5v, 1a regulator and I don't anticipate even with 16 relays of over-drawing the amperage on that.

Any insight would be helpful as I'm still trying to learn. [noparse]:)[/noparse]

Mike Green

Download a datasheet for Microchip's PIC 16F57 microcontroller. This is the one use in the BS2 Stamp. It would have the specific current limits defined. There's a per pin, per port, and overall chip current limit involved. When you have a driver transistor as in your schematic, the base current is what the I/O pin has to supply and that's set mostly by the series resistor.

The diode needs to be across the relay to dissipate the reverse EMF when the transistor shuts off. It needs to be a power diode (typically 50V at 1A) and is connected in reverse with the cathode to the positive supply. You don't need a diode coming from the I/O pin.

You could easily use a 1K resistor in the base lead which would cut the per pin current in half, then put the LED and a separate current limiting resistor in parallel with the relay coil. You can choose whatever resistor value you want for brightness.

Post Edited (Mike Green) : 2/10/2008 11:22:11 PM GMT

firestorm.v1

Excellent! Your suggestions were very helpful, I appreciate your assistance.

The LED was more for status indication than anything, but using that resistor will ensure I don't pull too much amperage from the BS2.

To make sure I follow, instead of the arrow and the line on the schematic (--|>|---) pointing towards GND or in this case the transistor emitter, it should point towards, Vcc correct?

Mike Green

I'm not sure what your last question is about, but the schematic looks correct.

firestorm.v1

I was concerned about the orientation about the power diode you recommended going across the relay coil. Even though I had a hard time phrasing the question, you answered it anyways.

Thank you for your help, praying I don't blow it up, this will be a really really nice project for others as well. Assuming I can fix the serial port problem, I will release all schematics and code in the completed projects forum.

Thanks again!