# Logic Level Relays

Discussion in 'Electronic Basics' started by Jonathan Leppert, Jan 7, 2004.

1. ### Jonathan LeppertGuest

Hi,

Can anyone recommend a good power relay that can be triggered via typical
low current low voltage logic levels (5 VDC). I need one that can switch a
12VDC 10 amp circuit and also one that can switch 120 VAC, around 2 amps. I
am confused about what coil voltage means, etc. so if anyone could give me
any pointers I would greatly appreciate it. Basically all I need to do is
switch the source on and off.

Thanks!

2. ### Peter BennettGuest

The coil voltage is the voltage you apply to the relay coil to operate
it. If you intend to operate the relay from TTL-ish logic levels,
then you need a relay with a 5 volt coil that will operate with fairly
low coil currents - 10 - 20 mA, or so. It is likely that you will
need to use this relay to operate a higher power relay that will
actually switch your 12V/10A or 120V/2A.

3. ### Jonathan LeppertGuest

I'm looking at a tyco made T90 series coil that has maximum contact ratings
of 30A @ 240 VAC, with a nominal coil voltage of 5 VDC, 185 mA. So am I
correct in saying I could use a simple transistor to switch this relay on
directly from a microcontroller?

Here's the digikey page for the relay (\$2.33 ea):

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=209889&Row=3174
70&Site=US

4. ### CFoley1064Guest

Can anyone recommend a good power relay that can be triggered via typical
This type of relay used to be fairly common, but I'm not sure anyone makes them
any more. Driving a relay from a logic output is fairly simple -- you just
need a transistor, two resistors and a diode to do the job well. (View in
fixed font as Courier):

VCC VCC
+ +
| |
1N4002 - C|
^ C| RY1
| C|
| |
| |
Logic Pin | |
___ | |/
o--|___|---o-o-| 2N3904
4.7 K | |>
| |
.-. |
| | |
4.7 K| | |
'-' |
| |
| |
=== ===
GND GND

This setup will reliably drive 5V relay coils of 50 ohms or more. You can do
the above with any 5V logic output pin that can source 1mA. Another thing you
might want to look at is solid state relays. They give you electrical
isolation from the load with a built in optocoupler, and depending on the type
chosen, can switch AC loads (triac/inverse-parallel SCR drive) or DC
(transistor-drive) loads. Make sure you specify one which is made for 5V logic
levels, and watch the optocoupler drive current. Most SSRs require several mA
of drive current, and some require up to 20 mA. Many logic outputs can't drive
that much of a load directly. In that case, just use the transistor setup
above to drive the optocoupler (you won't need the diode).

Good luck
Chris

I think the catch-diode in that schematic should be across 'R1.'

7. ### CFoley1064Guest

Small error -- sorry. The anode end of the diode is connected to the other
side of the relay coil/transistor collector rather than the transistor base.

Chris