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Topic: 12V, 11A waterpump. Transistor or realy? (Read 5673 times) previous topic - next topic

oskarq

I want to control a waterpump (12V, 11A) with my arduino Uno and was wondering about transistors and relays. I read somewhere that you should not use transistors for current loads over 5A, is this right?.

This tutorial (http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads) is pretty much exactly what I want to do...except for the analog input (I want to use a digital input) and the small current motor.

So what should I use to connect this all together?, and why?

/thx

pgmartin

Do you want to turn the pump on and off, or also to change its velocity?

If you are looking just for on/off, using a relay is the easiest path. If not, you'll have to explore other solutions.

oskarq

Just on and off. But how do I know what relay to use?

pwillard

#3
Mar 05, 2012, 05:07 pm Last Edit: Mar 05, 2012, 05:12 pm by pwillard Reason: 1
Quote
I read somewhere that you should not use transistors for current loads over 5A, is this right?.


I don't think so.

A very popular power transistor, the 2N3055, is rated at 15Amps (if properly heat-sinked/cooled)

The convenience of using a RELAY though is that you know your device is FULL ON or FULL OFF and completely isolated from you control circuit (for safety reasons).

You do want to OVER ENGINEER your relay choice... Using a relay with 15 to 20 AMP contacts  @ 12V... but these relays can also be costly and usually do not come with a 5V coil... so you will likely need to drive the relay with a transistor as well.

keeper63


Using a relay with 15 to 20 AMP contacts  @ 12V... but these relays can also be costly


http://www.allelectronics.com/make-a-store/item/RLY-351/12V-SPDT-30-AMP-AUTOMOTIVE-RELAY/1.html

NO contacts rated 40 amps, NC contacts rated 30 amps - $2.55 USD each

Not sure where you get the idea that these Bosch relays are "costly" (heck, even brand new at Auto Zone they aren't that much - but if you wanted one really cheap, a pick-ur-part junkyard might be the best bet, provided that you are picking up other parts so that the "entry fee", if any, doesn't cost you more than the relay).

Maybe you're thinking of some other kind of relay (for instance, a large 12VDC contactor could easily cost a bit - but in this app I think it's uncalled for).
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

retrolefty

I would recommend a automotive 12vdc relay as these are rated at high contact amps and are pretty inexpensive for what you get. You will have to use a switching transistor to power the relay coil under arduino control.

http://www.allelectronics.com/make-a-store/item/RLY-351/12V-SPDT-30-AMP-AUTOMOTIVE-RELAY/1.html

Lefty

oskarq

Ok, so will any 12V relay with high rated amp work?. If it were a 12V, 100A relay (if that even exist), would that work too?. But a 12V, 8A would not work?. Sorry for my beginner questions =).

Also,

You will have to use a switching transistor to power the relay coil under arduino control.


What exactly does this mean?

pgmartin

#7
Mar 05, 2012, 06:22 pm Last Edit: Mar 05, 2012, 06:24 pm by pgmartin Reason: 1
Quote
Also,
Quote from: retrolefty on Today at 13:30:00
You will have to use a switching transistor to power the relay coil under arduino control.

What exactly does this mean?


That you can't drive the relay coil directly from an Arduino digital output pin. Yo'll have to see how much current the coil needs, and size the transistor/s appropiately. Like shown here http://arduino.cc/playground/uploads/Main/relays.pdf

For instance, if you are using this relays http://www.allelectronics.com/make-a-store/item/RLY-351/12V-SPDT-30-AMP-AUTOMOTIVE-RELAY/1.html, the coil is 88 ohm, so it will draw 12/88 = 136mA. You'll have to select a transistor that is capable to draw this current, using less than 40ma at the base to avoid burning the Arduino pin. You might use a Darlington configuration with two transistors  to achieve this.

Linc

Let me ask this…
Instead of the Arduino controlling the power side of the relay coil, can it or would it be better if it controlled the ground side of the relay?
I would think it would be easier to control the grounding of the amperage needed to turn the relay on than to ramp up the amperage to do the same?
Just asking.
What type of water pump are you using may I ask?

Linc

oskarq

I dont know how good you guys are in swedish/norweigan/finnish but here are the specs for the waterpump I´m planning to use =) http://www.biltema.se/Documents/manualer/25/25-9753_man.pdf.

So let me se if I´m getting this right - the pump needs 12V, 11A to run. And the relay needs 12V to run. Would it be correct to assume that I can power both these from the same source?.

retrolefty

Quote
So let me se if I´m getting this right - the pump needs 12V, 11A to run. And the relay needs 12V to run. Would it be correct to assume that I can power both these from the same source?.


Yes, of course, that's why a 12vdc relay was suggested. If your motor ran on 120vac we would have suggested a 120vac relay. Common relay coil voltages avalible are DC 5,12,24,48 and AC 12,24,48,120,220 and of course the contact ratings are a seperate specification you use when selecting your relay of choice.

Lefty

Linc

No my sweedish not to good LOL
I have this pump that I am running on my race car, it has a much lower amperage draw. But will it work for you ???
http://www.dencodiesel.com/store/comersus_viewItem.asp?page=store&idProduct=230&idCategory=67
What is your project? water pumping for ???????

oskarq

The thing is that it needs to be able to pump around 30L/min...and it needs to be avaliable in sweden =). I think I kinda know how to build this now. Thx for all the help!

Tumbleweed

Oskarq,
Another really easy way is to turn on a small 5v relay with the Arduino, then use it's contacts to switch on the larger relay coil.
That way the 12v is totally isolated from the 5v side. I used to do this , but now I use darlington opto-isolators.
Anyway, don't forget the diode from coil + to ground, shoot, I'd use one on both relays, you'll probably use the 12v for all power,they will protect all your electronics from inductance surges.
TomJ
Einstein once said you don't really understand anything until you can explain it to your Grandmother

smeezekitty

What about a mosfet? Relatively high current capacity with relatively low gate drive.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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