Flyback diode, relay, and fuse placement

Other Hardware General Electronics
1

I've done my best with the diagram below, but I know it's pretty bad.
2 simple questions...

Question 1: FUSE PLACEMENT
The positive from the battery to the relay runs through a fuse, should the line to the buck converter go through the fuse as well?

Question 2: FLYBACK DIODE
Is the 1N4007 suitable as a flyback diode in this situation, and have I placed it correctly?

909×1134 151 KB

This is a system for pumping water from a large barrel to a small barrel without me having to watch over it. The relay will cut off when the water level in the small barrel reaches the top, preventing overfilling and/or running the pump dry.

This is very similar to a question I've asked here previously, but I'm still not confident to power it up without someone having a look at it. Thanks :slight_smile:

2

Shouldn't the 5V output from the buck converter go to the 5V input on the Nano, not Vin?

3

groundFungus:
Shouldn't the 5V output from the buck converter go to the 5V input on the Nano, not Vin?

Yes, thanks. I've updated the diagram.

4

To answer question, the fuse should stop everything when it opens up.

In addition to you original circuit diagram, I would add a current monitor to the pump circuit so when the relay contacts weld themselves closed, your program can raise an alarm so you can come shut the whole thing off and not burn the pump motor up by running continuously.

Conversely, if the pump operation is important, the current sensor can also cause alarm if the program called for the pump motor to run and it does not, then you will also be alerted.

Paul

5

Paul_KD7HB:
I would add a current monitor to the pump circuit

Thanks. I didn't know such a thing existed. I've just ordered some.

Paul_KD7HB:
when the relay contacts weld themselves closed

Obvious question... how can I stop the contacts welding together in the first place?
The relay will only have 1.5 amps going through it.

6

The relay will have lots more than 1.5A through it, the 1.5A is the running current, the stall current
(which is the inrush current when the relay closes) is likely to be 5 to 20 times that.

7

So what is the solution?
Use a bigger relay?

8

The relay is rated for 10A at 30Vdc and you have a flyback diode and a 5A fuse. I think that the chance of the contact welding is low. Add a 0.1uf cap in parallel with the diode will help, too. Adding the current monitor is not really that hard and you have them on order so the added safety might be worth the effort. Probably never need it, but will be glad to have it if you do need it.

9

groundFungus:
Add a 0.1uf cap in parallel with the diode will help, too.

So the flyback diode is in the correct place?

groundFungus:
I think that the chance of the contact welding is low.

It's a $2 relay, and a $16 pump. It won't be a disaster if it does happen.

10

So the flyback diode is in the correct place?

Yes.

It's a $2 relay, and a $16 pump. It won't be a disaster if it does happen.

That's part of engineering. If the pump catches fire (very unlikely) will it burn down your house with you in it or just stink up the place. The former would call for all safety measures available, the second a fan.