Splitting a single power supply usefully?

Using Arduino Motors, Mechanics, Power and CNC
1

I have had this issue before and am yet to find the exact solution.

I want to power my arduino (in this case a 5v-mini-pro) via the same adapter as a set of 12v solenoids.

I'll be using a darlington array to step up the voltage from the arduino signal to the solenoids but want it all from just one adapter.
(two plugs looks a tad rough when you're trying to make a convincing finished prototype.)

I have been told to use a Voltage Regulator, which is fine. However my question is: if the arduino is 5v and the solenoids (we'll say theirs just one solenoid for now) is 12v should the supply be 17v before it is split up? Will the arduino draw its 5v of power and thus effect the solenoid?

Very much obliged if anyone has the solution,

Rob

2

In reply to your specific question :

You consider the arduino regulator supplyy and the solenoid supply as two separate loads wired in parallel on the 12 volt system - not in series as you suggested.

Be aware that most cheap wall-warts provide pretty rough rectification and somewhat more that their specified output voltage at low loads, so it would be advisable to fit a large smoothing capacitor across the 12 volt output, say 1000microfarad rated at about 35 volts.

You feed 12 volts to your regulator circuit, using suitable local capacitors as specified in the regulator data sheet and from it you feed the regulated DC to the arduino.

You also feed 12 volts to your transistor + solenoid circuits taking care to ensure you fit flywheel diodes across the solenoid coils.

The 12 volt negative line is considered as a common ground and connects to all systems : regulator, arduino and either transistors or solenoids (depending on whether you are doing high end or low end switching)

3

Thanks! Great answers.

Thanks KE7GKP, on re-checking the mini-pro's specification it can take up to 12V input.
(I made the assumption it was only up to 5V)
So I don't need to step down the adapter for the arduino.

Which could mean I don't need to add a VR circuit?

Thanks to jackrae for the in-depth parallel circuit description.
Especially the component suggestions! I hadn't even considered diodes over the coils.
I'll go with this model but will perhaps leave out the Voltage regulator.

4

You seem to have ignored my cautionary note re the wall-wart regulation of its 12 volt output. Before connecting your micro use a digital volt-meter to confirm the specified output of the wall-wart really is 12 volts (with no external load applied). I have several of these cheap power supplies and some output as much as 19 volts with no load !!