I am using 4 24V pumps and 1 12V pump. I am thinking to use a 5V relay and a 24VDC power source because I only want single power source. Can I just use a buck converter for both 12V pump and Arduino to use them with 24V, connect other pumps directly to 24V and connect relays to arduino(Thinking of using 8 channel 5v relay driver). And is it a big problem to power relays from Arduino, if it is what should I do for this case? Thanks for any advice.
Hi, @ysfblkn
Welcome to the forum.
What model Arduino are you going to use?
If you want to use a 24V supply, then DC-DC convert to 12V then DC-DC convert to 5V, then it is quite plausible.
If you use a 8 channel 5V relay unit, then it will take care of the drive current for the relays from the low control current of the controller.
You will need to do a basic diagram and add up all the 24V, 12V and 5V load currents to find the size of the 24V supply and the DC-DC converters in your system.
Can you please tell us what your project is?
Tom... 
Hi Tom,
Thanks so much for your reply. It is just a simple mixing device, also I do not think to run any of the pumps together so I will just choose the power according to one of the pumps maximum use. I am using an Arduino Leonardo. And do you mean that I do not have to use a separate power source for relays and I can power them directly from Arduino?
You don't power relays from any controller pin. They are designed for such currents. Power should come from the buck converters.
Sorry I could not understand what you mean. There are only 2 buck converters, 1. between power source and arduino, 2. before 12V pump.
What is it You don't understand? Which reply?
I am planning to use Arduinos 5V output to control relays, but I did not understand what you mean by
Yes it is a big problem. If you use 5V relays then you will need another buck to convert the 24V to 5V for the relays
Why don't you use 12V relays?
1 Like
Power the 5 volt relays and the controller from a 24 to 5 volt buck converter.
Power the 12 volt motor from a 24 to 12 volt buck converter.
But I can still use arduino to control relays while powering relays from another source right? And what is the difference between using 5v or 12v relays in this situation? I thought using 5v because I wantes to connect relay module directly to arduino
It depends on the relay module you buy.
With some, the relay coils can be powered by 12V but the control signals can be 5V
Agreed! Those little blue (SPST) relays have a coil power of about 360mW, 72mA @ 5V, only 30mA @ 12V. 72mA is a big load for the Arduino's 5V power supply, but with 12V relay modules the Arduino only has to supply 2~3mA for each relay's opto coupler or switching transistor.
Thanks for your reply. I power relay module from a different power source. Arduino only will control the relays. Is this still apply in this situation as well(Arduino having too much load)? Also can Arduino control 12v relays?
If you have a separate power supply for the relay coils, you should be OK. For future reference, modules like yours are available with 12V relays that are still triggered by a 5V optocoupler from Arduino. Of course, you would need a 12V supply for the relay module.
12V relay module
If I use 12v relays, would I still need to power relays from a separate power source, or would it solve the risk for Arduino when relays are directly powered from Arduino
Well, you already have a 12V supply for pumps, you also have 24V available. 2 12V relays would only require about 60mA, 2 24V, about 30mA.
24V relay module
Yea verily. 
No, you can use the same power supply
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