Beefcake Relay / Power Supply Question

Community Projects
1

Okay, I’m looking for help with using a relay

to power the Seaflo bilge pump. I’m using

the Beefcake relay board for now.

So, the pump is 12vdc/1.8amps.

I have a 12v, 2A wall wart that I wanted to use for the project.

So, my ignorance is about how to power everything.

Can I use the single 12v 2A power supply for everything?

I was thinking of running a direct line from the 12v source

through the relay, and connecting the pump ground directly to

the source.

And then, also connecting a 7805 to the power supply to power

the micro controller, and ultimately the beefcake brd.

Will this work, or will the 7805 get too hot dissapating the

12vdc?

If heat is a problem, would using a voltage divider before the 7805

solve this? Or would I then have a hot resistor?

A sample circuit diagram would be wonderful, but a few helpful

comments might be enough too.

Thanks.

I want to cycle the pump on for 3 minutes, and then off for 1 minute.

So surges might be an issue too. And I want to be able to change this

interval to, so I want to use a microprocessor to add this feature.

2

Your approach might work, but it is marginal. Need to know more about the relay (what is the coil resistance?) and the wall wart. Many of those are unregulated and one rated for 12V under load might output 20V with no load. You should measure the output voltage with no load and under the full expected load.

The power dissipated by linear regulators depends on the difference between the power supply voltage and the load voltages, as well as the load current.

Finally, if the power supply is not actually capable of supplying the entire load without significant voltage drops (which includes the startup current for the pump - much higher than the running current), you will probably see the microprocessor resetting every time the pump starts.

3

What’s a better way then? Or correct way.

I could use the wall wart only for the pump.

And use a 9v battery for the micro controller,

but I was trying to avoid a battery.

(I don’t have the wallwart specs in front of me,

the relay is the Sparkfun product)

All the examples use the relay to control 120vac,

and I’m trying not to burn the house down,

with a lower horse power project for now.

And I’m trying to do a little bit of design,

before I begin experimenting.

Thx

4

A safe approach would be to use a regulated 12 V supply capable of supplying several amperes, maybe 5 A. A 7805 would be fine for the microcontroller and perhaps also the relay. However, you MUST determine the relay coil current requirements, and don’t forget the flyback diode on the coil.

Sometimes you can find discarded 12V/5V dual (regulated) adapters at thrift stores for just a few $.

Finally, for 120 VAC projects, a solid state relay will work much better than an electromechanical relay, and you don’t need to worry about coil current.

5

Thanks jRem.

I’ve decided to buy a dual power supply. This is my first attempt

at escaping my breadboard into the real world.

I googled, and looked around digikey a little bit. And I found my self

at digikeys power transformer selection matrix, and it was a little

too much for me. So I went and bought a ready made pwr supply,

this time.

I found a schematic,

I found this L01-6310 transformer at digikey.

And I see how you get the 7vac, and in series the 14vac.

Is this schematic a useful power supply, otherwise?

It doesn’t seem like there is too much to it.

I’ve learned a little bit, thanks.

6

This transformer, L01-6310 transformer, is $58 bucks.

I found a dual 12v/2A, 5V/2.5A power supply for $11 usd

I’d like to build the power supply in the schematic above,

but I’d prefer to use a cheaper, old fashioned, non toroidal,

transformer.

Can anyone point me to cheap 2x 7vac transformer???

Please help, a part number would be great. Or a link

to some place?

Thanks.