Guidance for multi-voltage power supply

Using Arduino General Electronics
1

I have a working sous-vide cooking controller project that I want to refine and put into a permanent enclosure using an arduino pro mini.

My initial prototype required 3 a/c plugs:

  • A wall wart for the 5v arduino
  • AC plug for the solid-state relay controlled heating element
  • AC plug for a 120v pump

I want to clean this up to only need one AC plug in the new version.

I also may upgrade the current pump to a 12v DC model with higher temperature tolerances. Since this may not happen until my current pump dies, I want my power circuit to support both a 120v AC or 12v DC pump.

I'm including a first draft of my design for the power supply below. Please note that AC connection is to the left (I couldn't find the symbol in eagle).

1028×526 5.16 KB

In this design I am using an LM7805 to regulate 12v off the rectifier down to 5v for the arduino.

Here are my questions:

Should I plan on wiring all the AC components with large (12-14 guage) wiring ? I'm assuming a circuit board is not appropriate for such voltages and currents.

Where I need help the most is getting from 120v AC down to 12v DC. I've looked around at transformers and all the 120v to 12v transformers I have found are very expensive. Is there a better way to go about doing this? I've thought of finding a used 12v wall wart, opening it's case and wiring it into my project. Is this a reasonable option safety wise?

If I do use a power transformer, how wide should the traces connecting it to the incoming AC be? I'm assuming they will need to be wider than what is typically used in low voltage DC circuits.

Also, do I need C3 in the schematic? If so, should it be used by both the pump and the regulator to smooth the output from the rectifier or will the other capacitors in the standard regulator section of the circuit be sufficient?

Finally, this is something that will run unattended for hours at a time, so I would greatly appreciate suggestions for increasing safety.

Note: the DC pump I am looking at is listed as being rated for 0.46a. I don't expect the combined pump and DC circuitry to exceed about 650ma although I imagine that I will need to expect possible higher spikes when the pump starts.

2

Also, do I need C3 in the schematic?

No, in fact it will not work as it will block the DC.

Where I need help the most is getting from 120v AC down to 12v DC.

Why can't you just use a 12v wall wart then regulate for your 5v. Job done.

I'm assuming a circuit board is not appropriate for such voltages and currents.

Technically it's no problem having 240v on a PCB, but it's asking for trouble and I avoid it at all costs if possible. If you really want to us a power transformer you may ba able to have the hot side run flying leads directly to the input socket, that way there is no HV on the board.

120v to 12v transformers I have found are very expensive.

I haven't looked at them for years, but if you paid more than $5 for a small one I'd be surprised. But as I said above, just use a wall wart and all the above issues go away.

Rob

3

Should I plan on wiring all the AC components with large (12-14 guage) wiring ? I'm assuming a circuit board is not appropriate for such voltages and currents.

You haven't mentioned the currents involved here, specifically for the 120v pump, 12v pump you plan to use (Now I see you mention this at the end), and the heater element. Currents will determine how practical a circuit board might and what gauge wiring is required.

I've thought of finding a used 12v wall wart, opening it's case and wiring it into my project. Is this a reasonable option safety wise?

I do this quite frequently if the current required from the 12v source is less than an amp since I can usually find a 12v 2 amp supply on ebay for less than $5 whereas a comparable transformer will cost 2-3 times that. But if you aren't comfortable with 120v, you may consider a more commercial product.

If I do use a power transformer, how wide should the traces connecting it to the incoming AC be

I would suggest to just go open one up and take a look at the insides of and older adapter that uses a power transformer. Most of the 12v switchers I have opened up have surprising thin wires on the 120v side. I guess it all comes to cost and how comfortable you are with your design.

Also, do I need C3 in the schematic

most certainly not, C3 is not even connected properly, and the pump really wont care about a noisy supply. a flyback diode would be a great idea though as well as extra decoupling near arduino.

I would greatly appreciate suggestions for increasing safety

adequate clearance, seperation of voltages, and when in doubt wrap it with electrical tape.

4

Thanks for the answers. I realized the mistake with C3 after I posted. Looks like I go with repurposing a wall wart for the 12v supply. The pump's specs say it's rated for just under half an amp, so I think a 1-2 amp supply should be sufficient. The only thing I'm not sure of is what would happen if the pump somehow gets clogged and starts pulling more current. I guess I'll just put in a fuse to take care of such an instance.

5