Driving a relay - do i need amplification?

Patrick Stave

I have a BS2 with Board of Education, and i need to drive eight miniature 5V-relays with it.
I have alrealy soldered the relays to a PCB, with diodes to protect from reverse voltage.
None of the relays will be active at the same time (only 1 will be active at a time).

The question here is; can i connect the relays directly to the pinouts on BOE, or do i need to amplify them with transistors?

Regards,
Patrick.

Chris Savage

Patrick,

Relays generate back EMF when the coil de-energizes. This is not good for the BASIC Stamp pins. Sometimes the coil current is near or above the current rating of the I/O pin. For this reason we often recommend using a transistor to drive the relay and use diodes on the coils. For multiple low current relays a single ULN2803 chip will provide both items in one package. Take care.

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Chris Savage
Parallax Tech Support

Mike Green

The answer depends on the specifications of the relays. Stamp pins will generally provide currents up to about 20-30mA. Some miniature 5V relays will "pull in" with coil currents like that and some will not. If your relay requires more current than that, you'll need some kind of amplification. You can use transistors or a transistor array like the ULN2803.

cjs

I'm in a similar pickle. If the coil consumption is 200mW then it doesn't appear that I am "pulling in" at 5V times 20-30 mA. See this relay:

http://www.mcminone.com/product.asp?catalog_name=MCMProducts&product_id=26-539

can anyone recommend a relay that will pull-in?

If not, how do I configure a transistor, and can anyone recommedn one. I have a low voltage application that I want a simple relay to switch via BOE.

thanks in advance

Rory Storm