I'm upgrading a project that controls several motors via a relay board. Previously, I was able to power the relays from an arduino UNO alone. I'm going to be controlling more relays, and although I'm now using a MEGA, I'm afraid it won't be able to source enough current to flip the relays. I know I've read about a product that provides the current needed to flip the relays, but I can't remember what it is or find it. A mosfet per relay makes sense at first, but the relay board already has that so I figured that might not be the solution.
I'm flipping up to 16, 5V relays. Each relay needs 15-20 mA. The MEGA can source 40 mA per pin, and 800 mA overall, though only so much current to a group of pins (http://arduino-info.wikispaces.com/ArduinoPinCurrent). There are other components to this project that will take up power too, and I want to be as gentle on the arduino as possible. Has anyone heard of such a product or circuit? Thanks.
d_vee:
A mosfet per relay makes sense at first, but the relay board already has that so I figured that might not be the solution.
I'm flipping up to 16, 5V relays. Each relay needs 15-20 mA.
Well then your "relay board" has this covered. The "mosfets per relay" require very little current from the Arduino output/s.
The relays draw their current through those very transistors.
But. what is the source of the "5V"?
"Arduino +5"?
I'm flipping up to 16, 5V relays. Each relay needs 15-20 mA. The MEGA can source 40 mA per pin, and 800 mA overall, though only so much current to a group of pins (http://arduino-info.wikispaces.com/ArduinoPinCurrent). There are other components to this project that will take up power too, and I want to be as gentle on the arduino as possible.
Has anyone heard of such a product or circuit? Thanks.
Keep in mind that there are two 'current loads' that the relay board presents. First is the digital inputs of the relay board, most use a opto/led input that require about 20 ma per relay channel to turn on. The second load is the relay's coil consumption, and depends of the specific relays used in the module. As far as being able to power all 16 relay channels at the same time while being powered by the arduino's 5V pin that would probably be more then the arduino can supply. Powering the relay board with an external regulated +5vdc of sufficient current rating is probably called for.
The relay board I'm using is this: http://www.amazon.com/Kootek-Channel-Module-Arduino-Raspberry/dp/B00C8O9KHA.
I figured that you needed the 15-20 mA at the pin (ie. for the mosfet), though I realize now that it may be per relay. I have tried powering all eight at once before and it seemed the arduino couldn't supply the current because only some of the relays flipped. I do not recall, however, if I ran the 5V to power the board from the arduino or from the power supply itself. I'll test tomorrow, hopefully, using the 5V from the power supply and post the results here. Thanks for the responses.
15-20 mA... for the input. ? ?
I can't make out what the semiconductor/s are (optos of FETs), but either way, that's a lot of current.
Get out the DMM (milliammeter) and find out.
The amazon description doesn't mention "MOSFETs" as you did previously.
In the comments section someone mentions that they're EL817 optos.
I hooked a 4N25 up with a biggish resistor on the LED input the other day and measured the current at about 5-6mA iirc; at that current the LED (edit: the opto's led, that is) happily triggered the transistor. (edit: the opto's internal photo-transistor, that is).
I tested the 8 relay board both hooked up to an Arduino UNO alone, as well as enabling the opto-isolation and providing power from an external source. Both worked without a problem. In a project I had a problem where I couldn't power 6 relays at one time; 1 or 2 relays wouldn't flip. I put a delay between switching on the relays, figuring the inrush current was greater than the Arduino could supply, and it fixed the problem. I suppose the problem was with the code and not power, and that's why I had a problem then but not now? Either way, I don't think I should have a problem with the MEGA.
There are opto-isolators (optocoupler?) on the board, but there was also components labeled Q1,2,3..., transistors. I guess I had heard at one time that its standard to use a MOSFET with a relay or maybe I just assumed it was a MOSFET as opposed to another transistor. It's labeled J3Y.
The claim that it needs 15-20 mA "driver current" seems dubious. That is really too much for a mosfet or opto input. And too little to drive the relay actuating coil. It's not clear to me what that is actually refering to.
Try measuring the current that the relay coil actually uses. Make sure you have a separate power supply for that with enough capacity.