Prop -> SSR -> mechanical relay

kolyur

I need to drive a 30A mechanical relay from the Prop. Relay coil is 12Vdc and pulls 133mA. My thought was to drive a small solid state relay from the Prop pin. The big relay will be mounted elsewhere and I like the idea of having isolation to off-board components. Usually I'd just buy the parts and try them out on my breadboard, but I'm into holiday crunch time here and need to get PCBs ordered this weekend, so no time to test. I'm hoping the experts here can confirm my thinking.

See attached schematic. SSR is a Toshiba TLP222A. LED forward voltage is 1.15V with a nominal current of 7.5mA. So, value of resistor should be (3.3 - 1.15) / 0.0075 = 287 ohms. All the rest of the Prop pins are handling <1mA so I think my current usage is well within limits. Load side of SSR can handle 60V/500mA so I should be fine there. 1N4001 diode across relay coil to handle back EMF (4148 would probably be adequate but I'll err on the safe side).

Any opinions as to whether schematic A or B is more correct/appropriate? The difference is just the sinking/sourcing configuration of the relay relative to the SSR. Arrangement B makes more sense to me but it should work both ways.

Have I thought all this out correctly?

Mike Green

Either schematic should work. The decision of whether to ground the SSR or the relay depends on wiring and mounting. Sometimes it's more convenient to run a ground wire or a +12V supply wire to the relay or the SSR.

Duane C. Johnson

Be a bit careful about how much current the relay coil draws.

As an estimate, from the spec, it looks like the maximum continuous ON current is about 350mA to 400mA, not the maximum of 500mA with the junction at 25C.

Duane

Rayman

The ULN2803 is also good for driving relays...

kolyur

After some more research I think I'll just use a 2N2222 transistor to drive the relay. This seems like the most common approach and I have the parts on hand to test it.

Duane C. Johnson

How much current does your relay draw?

Duane

kolyur

The coil draws 133mA at 12v.

Duane C. Johnson

Great.

I would suggest the use of a n N channel enhancement mode MOSFET such as the 2SK3065 as the driver.
No added resister, just the transistor.

If you use an NPN bipolar transistor such as the 2N2222A you will need a base resister of about 270 or 330 ohms.

Note, the 2N3904 would also work nicely.

Duane

LoopyByteloose

The UNL2803 actually handles less milliamps than the 2N2222 by a huge amount. Why so? The heat buildup of having 8 darlingtons in one package restricts the UNL2803. The 2N2222 is the real goodie as it might handle up to 500 milliamps of steady on without problems. MOSFETs can be problematic when directly driven at 3.3volts, especially if you want to toggle a 5V item.

As I have gotten into wanting to use heavier and heavier relays, I generally rely on the 2N2222. These work nicely with 30amp 12VDC automotive relays that are made for exposure to engine heat and bad weather.

Duane C. Johnson

Hi Loopy;

In the big picture MOSFETs beat Bipolar transistors in almost every way. Even when driven directly by a Prop running on 3.3V.

The 2SK3065 is very nice MOSFET for medium current applications and is guaranteed to switch 1A with a gate voltage of 2.5V.

I often use 2N7000 and 2N7002 MOSFET for low current applications.

For high current I use IRF3708 and is guaranteed to switch 7.5A with a gate voltage of 2.8V.

Duane