I would like to use an Arduino controller to measure the consumption of some wireless router and to turn them on/off remotely. I already implemented all the communication logic for the Ethernet shield and I'm using the ACS712 current sensor to measure the current consumed by my device.
The only problem i have is how to choose a relay for my project.
The wireless routers are powered using a POE injector (48V max, 25W) and for some logistic reasons I need to put the relay on the DC line. Moreover I need the relay to be closed by default so if the Arduino breaks the router remains powered.
So far the best choice I've found is the Drydom MPDCD3-B SSR. However, I've also found this Panasonic relay:
While the crydom SSR costs about 20USD, the panasonic costs about 3 USD. Moreover the panasonic is in a DIP package while the crydom uses some proprietary format and I'm not sure if it is easy to solder it on a proto shield for arduino.
its not a proprietary format, its just a different mounting style for different systems
how much current does your router consume? (I would be surprised if its more than an amp) whats its voltage? are you using systems that use more voltage on the POE?
its got to be one hell of a router sucking down 48 volts and needing more than 2.5 amps
hamvil:
The wireless routers are powered using a POE injector (48V max, 25W) and for some logistic reasons I need to put the relay on the DC line. Moreover I need the relay to be closed by default so if the Arduino breaks the router remains powered.
So far the best choice I've found is the Drydom MPDCD3-B SSR. However, I've also found this Panasonic relay:
Why do you need a SSR? How about a normally-closed regular relay? Those are cheap, have reasonable reliability (easily 50,000 operations on the contacts) and most of all, have no appreciable resistance when closed, compared to SSRs which always have some voltage drop.
In fact, some SSRs are built for AC and won't even work right when switching DC (for example, zero crossing detection ones.)
This one is $2.25 for 200V / 500 mA: DigiKey - Electronic Components Distributor
There are many, many, (many,) others. You probably want a "NC" (normally closed) version.
I was going for an SSR because it should be easier to use, meaning I can control it directly from the arduino without using a transistor. The relay suggested by jwatte is not suitable for my project because the wireless router uses about 0.8A.
Moreover I having some difficulties in finding non SSR relays supporting at least 60vdc and more than 1A.
If I go for a mechanical relay what should I use to drive it? Is a regular NPN transistor with a 1k resistor on the base enough? I would then put the emitter to ground and the collector to the control pin of the relay. Right?
Should I use a diode on the control pins with this relay?
Very strange selection. Why must you use a relay with a 60vdc relay coil?
Relays have two independent sets of specification. First the working voltage and current draw for the coil and a second set comprized of the maximum working voltage and current that the contacts can control. Because you are using an arduino to activate or deactivate the relay coil it makes a lot of sense to use a relay coil rated to work with +5vdc as that voltage is avalible from the arduino 5V shield pin. Then you only have to make sure your relay contacts are rated for more then the maximum voltage and current for the circuit they are controlling. If the relay coil requires more then around 30ma (most do) then you will use a npn switching transistor between the arduino output pin and the relay coil.
hamvil:
Moreover I having some difficulties in finding non SSR relays supporting at least 60vdc and more than 1A.
You cannot be looking in the right place. Where are you looking?
Try this:
Click the "power relays" section.
Search for in-stock, Turn On voltage less than or equal to 3.5V.
Sort by price, quantity 1, and look down the coil voltage column for 5V, and you end up with this:
$1.67.
You need a small signal transistor to turn it on. Anything will work. I'd use a BS170.