Raspberry Pi Forums

Hello Pi users.
I have two 5v 8 channel relay boards that are active low. They are working fine Pi B+ version connected direcly to the GPIO 0-7, 21-28 and 3.3v. Both relays are being powered by a saperate 5v power supply (connected to Ground and JDVCC).

Pi gets hotter when relay are connected (few degrees) so would some IC curcuit like Darlington Arrays (ULN2804A) release the power load?

I would like to relase the power load as much as possible. Perhaps best way would be NPN transistor but that needs in addition two more resistor and the circuit looks uglier

So, question is, what IC circuit would be the best to release the load on the Pi to simply pwer a relay board?

Since not all 8 channel relay boards are equal, it's difficult to give definitive advice.
However, it sounds like your particular ones are perhaps not buffered - the relay coils are connected directly to the inputs.
If so, be aware that the Pi's GPIOs aren't designed to withstand the 5v that would be applied through the relay coils. Also they can't safely supply the current the relays require.

A ULN2804A wouldn't work because it requires 6v to drive it, but a ULN2803A would work well.

That package is convenient, but nFETs on low side would work fine, with 5V on high side. Use appropriate pull-up/downs. Not sure what you mean by active low if these are controlled directly. Apply voltage, relay contact closes.

Do you have a link to show us the board ?

Some models have opto inputs so shouldn't take much current.

Hello Pi users. Thanks for the replay.

The board I bought is this one http://www.ebay.com/itm/111283816540?_t ... EBIDX%3AIT

Two similar boards are connected to the Pi and on the 3.3v pin for 12 hours. I made Pi outputs low to enable all 16 channel.

Disconnecting the board with all channels still active low, the Pi is not that hot.

The inputs to that board are through opto-isolators driving transistors, so the relay coil current won't be an issue for the Pi.
The only thought I have is that the E-bay listing doesn't specify the drive current or voltage for the optos.
When the input is driven low, what current does it take? And when it isn't driven low, what voltage does it put on the GPIO?
I don't think it should be a problem, but I don't know.

If the Pi gets hotter with the board attached it means more current is flowing. There is a limit beyond which the Pi will be damaged. Without full specs, only by measuring that current can you find out if it's safe.

I connected my multimeter on the 3,3v and the relay VCC input. While all 8 channals were active low (meaning relaysactive), the multimeater was showing 3,7 mA (for the entire board of 8 channels)

The other 8 channel relay will probably consume the same amount. So about 7.5mA both.

The opto-isolator probably does not consume that much power but these relay boards have in LED to show the state and the LED part is connected parallel with opto-isolator. I have noticed that when JDVCC is disconnected, relays will not activate but LEDs remain on. These relay boards could have LEDs powered by the JDVCC.

if you want to offload the switching from the pi you could use a MCP23017 I/O expander to use the I2C bus to give the signals. The MCP23017 can be powered externally from the pi.

Thanks BMS Doug

I was planning to use expanders for the inputs and PI's GPIOs for outputs. The calculated current from the Pi to relay board is not that much but still, heating PI board a bit more. I want to use PI for home automation so I want a reliable solution avoiding anything that could lead to a failure. I don't want all the lights to go off while kids are alone or something.

br dsgashi

Have a look at this Texas instruments page: it lets helps you easily select components based on performance requirements.
http://www.ti.com/lsds/ti/interface/per ... ducts.page