TTL Breakouts

>>I'd like to control a relay for my mist coolant.
>>I can get a 5 volt relay (typical coil current 30-60 ma),
>>but I seem to remember that TTL level isn't designed
>>for the required current (?).

>>is there a better way?

Jeff,

The easiest way is to use just the relay you've got and switch the coil
using a transistor. An n-channel MOSFET would be great for that sort of
thing. You can get one that can be gated with 5 volts. They're reasonably
inexpensive and readily available. They require virtually zero gate current,
so you can drive at least several MOSFET-switched devices with the same TTL
output. The catch is that you have to be mindful of static electricity when
you handle them. But once they're all hooked up to their supplies/circuits,
static is generally no longer an issue.

To hook it up, connect the supply to the 'high' side of the coil and connect
the 'drain' of the MOSFET to the 'low' side of the coil. Connect the
'source' to the common (ground) from the coil's supply. Then connect your
TTL output to the 'gate.' It's important to switch the low side of the coil
because the gate voltage needs to 5 volts (or whatever the on-state
gate-source voltage is) greater than the level at the source. This will all
probably make no sense until you get a device and data sheet in your hands.

To pick one that's right for the job, you need to make sure the device has a
low enough 'on' resistence to allow the coil current you need. Also, the
continuous drain current rating for the device should be somewhat greater
than the coil current. If you're in doubt, just pick a really robust device.
It should be okay but if you really need more to go on, email me and I'll
help.

Carl

_________________________________________________________________
Fast, faster, fastest: Upgrade to Cable or DSL today!
https://broadband.msn.com