I have the "8 Relay Module" with 8 SRD-05VDC-SL-C. It seems they pull up to 85mA while the (Nano) Arduino pins only allow 40 or 50mA if I remember correctly. These relays will stay on and off for hours at a time (they control low-voltage lights). Also the box with all the electronics will be stored in a cabinet of the outside kitchen which gets some serious sun exposure/heat. I am considering my options on how to amplify the signal while staying compact, have little heat dissipation and be reliable for many years. I have some IRL540Ns laying around but they seem overkill and are somewhat large. Maybe some IC with multiple op amps or even just some NOR logic? I really only need to double the mAs.
Are you talking signal or coil power?
mulu:
I have the "8 Relay Module" with 8 SRD-05VDC-SL-C. It seems they pull up to 85mA while the (Nano) Arduino pins only allow 40 or 50mA if I remember correctly.
The relay coils pull about 80mA each from the power supply when active, but the Arduino pin signal current is only 2mA.
Both relay module and the Nano could be powered with a 5volt/1Amp cellphone charger.
Don't power the relay module via the USB connector of the Nano.
Leo..
How about an ULN2803?
e.g. Darlington Driver 8-Channel ULN2803 DIP - COM-00312 - SparkFun Electronics
INTP:
Are you talking signal or coil power?
Coil power. I assume the SRD-05VDC-SL-C are non-latching so I don't want to go above the rated output since the relay will be 'active' for multiple hours.
Wawa:
The relay coils pull about 80mA each from the power supply when active, but the Arduino pin signal current is only 2mA.
Both relay module and the Nano could be powered with a 5volt/1Amp cellphone charger.
Don't power the relay module via the USB connector of the Nano.
Leo..
Wowa, yes it's about 85mA so almost 0.7A for the 8 relays. I also have an RS232 and the Arduino itself. And whatever I use for the amps will have some loss. So I plan to use a 5V/1A power supply wired to pin 27 (5V) of the Arduino Nano. This still gives me 10% headroom. Btw, isn't there some limitation with not using the USB supply? I thought the RX/TX pins aren't active if any power source is used except USB (or maybe I don't recall correctly).
As for the original question, what would you suggest to get to the ~85mA?
Edit: Btw, you said there is only 2mA signal from the Arduino. Is this really true? I thought it was MUCH higher but not quite enough for the relays.
lemming:
How about an ULN2803?e.g. Darlington Driver 8-Channel ULN2803 DIP - COM-00312 - SparkFun Electronics
That seems to be perfect! 8 outputs which is exactly what I need, very low driving current (1.35mA max) and as low as 5V input. Unless a better solution comes up that is probably what I will do.
Connect your relay module like this, use 5V instead of 12V for separate supply.
Graphic by @dlloyd.
Your module probably has switching transistors (Q1 on schematic) built in so you don't need "amplifiers".
When connected correctly, the Arduino output pins are only powering a tiny LED in an optocoupler (U1 on the schematic) that switches the external power to the relay coil, so only about 2 mA.
Hi,
Can you post a link to your relay module please?
Can you post a picture of your relay module?
As @edgemoron has said, your module will probably have the transistor driver on board, so the current from your arduino output will only be about 2mA.
You will need to supply 5V to the board to provide current for the transistors to switch through the relay coils.
Tom... 
You know about bistable latching relays, only take current for a fraction of a second when switching,
no need to power them continuously. They have mechanical state though, sometimes that's a problem.
MOSFETs overkill? In what way? If MOSFETs do the job, relays are overkill! Are the
low voltage lights DC?
mulu:
Btw, isn't there some limitation with not using the USB supply?As for the original question, what would you suggest to get to the ~85mA?
Btw, you said there is only 2mA signal from the Arduino. Is this really true? I thought it was MUCH higher but not quite enough for the relays.
There is tiny backflow protection diode between USB and the 5volt rail of a Nano (drops ~0.4volt), that will fry if you draw more than a few hundred mA from USB to 5volt rail. So don't power the Nano AND 8 relays via the USB socket.
Just power the relay board directly on the relay board with a 5volt/1Amp cellphone supply/charger.
Use the diagram in post#7 (separate supply for the relay board) if you want/need opto isolation (mains power switching).
Join grounds, leave the jumper on, and use one supply if you don't need opto isolation (low voltage switching).
Yes, 2mA. Only the opto LEDs are powered by the Nano's output pins. Not the relay coils.
The Nano has no problem controlling 8 optos.
Leo..
Thank you all for the help. edgemoron you are right, the 8 relay modules actually has an 817c (opto coupler) for each relay so I can hook it up directly to the Arduino.
MarkT, the MOSFETs I have would be overkill for driving the relays (because the Arduino cannot drive the relays directly but it turns out there is already an optocoupler). I just need DC (up to 15V) for less than 10A. I might get away with a MOSFET instead of relays but since the on/off cycles are several hours I think relays are better.
Also I plan to just power both the Arduino Nano and the relay from a 5V/1A power supply so I don't need to bring another (USB) cable into the box. If I understand it correctly, the 5V from the PSU will go to pin27 labeled 5V on the Nano, VCC on the relay module and VCC on the serial board. The 'ground' from the PSU will go to pin 29 labeled GND on the Nano, GND on the relay module and GND on the serial board. For the serial module I will just connect RX from Arduino to RX on the serial board and the same for TX. Are those TX/RX headers on the Nano active when not powered through USB? I think I read that at least the on-board LED will not work properly when there is no power through the USB but are the TX/RX affected, too?
No, I meant MOSFETs to drive the lighting direct... That's why I ask if the lighting is DC.