Powering and controlling a relay from an Arduino Ethernet with PoE

Other Hardware Motors, Mechanics, Power and CNC
1

Hi there, I have a switched power cord (commercial solution with a Finder 40.61 DC sensitive relay, 5 V coil, 50 Ohm, 4-7.5 V working range) that is supposed to be plugged into a USB port. If there's 5 V on the USB line the relay closes.

What I want to do is control (and power) the relay from an Arduino Ethernet with PoE module, taking the power for the relay from the Vin pin on the Arduino. Since I have one lying around, I was thinking about using a DC-DC converter to get the 9 V from the Vin down to 5 V for the relay coil. The converter in question is a Recom R-78E5.0-0.5 (three terminal non-isolated, internal short circuit protection, 7-28 V input range, 5 V output, 5-500 mA output). The idea is to connect the power cord's USB line directly to Vout of the converter and to switch the converter to control the relay (the start-up time of the converter appears to be sufficiently fast), because being able to completely turn off the converter would make me feel better.

Essentially I have two questions. The first one is about the DC-DC converter, with which I have little to no experience. The application note by the manufacturer (mostly about isolated converters though) makes all kinds of suggestions for input and output filtering, limiting inrush current, overload protection, no load overvoltage lock-out, etc. My feeling is that I don't have to worry much about input and output filtering in this case. But what about limiting inrush current and the rest?

The second question is about switching the converter on and off. Playing around with it, I found that disconnecting the converter's ground connection causes Vout to sit at Vin (I'm guessing that's normal behaviour). That rules out using an NPN transistor as a low side switch. Instead I would use a PNP transistor on the high side.

Here's what I have so far:

So far it's working. R3 is probably not necessary. It makes the DC-DC converter turn off faster, but that doesn't seem to affect the relay's switching behaviour.

Have I committed any obvious stupidities? I'm particularly interested about possible failure scenarios.

2

Can't you just use a single PNP transistor to switch 5V from the Arduino regulator?

3

Good question.
I guess I could take the power directly from the 5V pin, although in that case I'd use an NPN to switch the relay. However, I am a little concerned about the amount of power the 5V pin can source. According to the Arduino Ethernet's schematic the 5V regulator is a Motorola MC33269ST-5 (the datasheet for which states output current "in excess of 800 mA"), but that's clearly not the regulator on my board. The regulator on my board just says 117-5, and there seems to be a ton of different 117-5 regulators out there, all with different max. output currents. Probably it would be fine, but then I'd rather know than guess...

4

"5V coil, 50 Ohm" I think that means it draws 100 mA which I hope is plenty low enough both for the Arduino regulator and the PoE supply.

5

Well, depending on how much current the Arduino (and all other connected circuitry) draws, it might be enough. Or not. My position on these things is that if I don't know the numbers (and I don't, since I don't know which regulator is actually used on the Arduino) I try to find a solution for which I know the numbers. I know the PoE module is good for 1 amp, and this definitely is enough for everything.

Of course I'd love to get rid of the DC-DC converter, but until I find out how much current the regulator on the Arduino can source I'll probably stick to the converter.

6

Oh well, I guess I should've just done the obvious and check the schematics of other Arduino variants. Sure enough they give the correct regulator, an ON Semiconductor NCP1117 with an output current of more than 1 A. Problem solved.

Thanks for suggesting this, probably wouldn't have occurred to me.