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Power
The challenge here is to get the Mac mini powered from the car's 12v power supply. I refused to use an inverter as they're heavy and inefficient (In this case, it'd go 12v - > 240v -> 18v) and I can't be bothered to wait for the Carnetix P1900 power supply. So, with the help of this post at MP3car.com, I got it working.
This project lets you easily swap your Mac Mini from your car to your home by placing a connector in it's power line.
WARNING: This is dangerous - we're playing with electricity. Test all pins with a voltmeter before you connect it to your Mac mini, just in case you get it wrong. This WILL void your Mac mini's warranty. I can't be held responsible for any damage caused to yourself, your computer or anything else.
What you'll need
- 1 x Mac mini and power supply
- 1 x 12v -> 18v DC-DC convertor (I used a laptop car adapter like this one)
- 1 x Male 4-pin connector
- 2 x Female 4-pin connector (for these, I used a set of 4-pin hard drive power plugs)
- 1 x 6.8k Resistor (blue-grey-black-black)
- A soldering iron and an ability to solder
- Wire strippers
- Small, sharp knife (modelling knife, etc)
- Electrical tape
Ok, let's get started!
First, cut the Mini's power cable (the one that goes from the power brick to the mini, not from the wall to the brick) cleanly in two. I cut it fairly close to the Mini, but it's up to you. Just bear in mind that having too much cable is better than having not enough.
Then, carefully strip the outer plastic sheath from the end of one of the cut bits of cable. I recommend about an inch, but that depends on the connectors you bought. You must be very careful when stripping the wire as there is a very finely woven metal mesh just inside. You should be seeing this:
Using your knife, make a slit in the mesh along the wire from the sheath to the end, pull it round and twist it into a wire:
The white 'fluff' is very annoying filler for the wire. Try and keep it all together and cut it off - if it frays and goes everywhere it's a pain in the arse! You will then finally have 3 wires in addition to the twisted sheath:
It's a good idea to wrap the twisted sheath in heat shrink or electrical tape seeing as it is just bare metal. Above I've done it red/black, but yellow/green would be a lot better as this is an earth.
The wires you see above are as follows:
- Red: +18v DC
- Black: Earth
- Grey: "Control"
- Twisted sheath: Earth
As we're just making a connector to join the cut cable at the moment, we don't need to worry about what they mean just yet. Get your 4-pin connector and wire the 4 wires to the pins. It doesn't really matter which order you put them in, as long as you use this arrangement for your other connectors. Don't completely cover up the wires if you can help it, so you can identify which one you put where later on. I used a hard drive power connector, like this:
... and soldered the wires to the pins ...
Now, repeat the all above with the other side of the cut cable, making sure you wire the pins on your connector up correctly so red meets red, etc. To test, put your Mac mini a long way away, plug your connectors into each other, and plug the brick into the wall. If the brick doesn't explode, you have no shorts :-). Now, go and get your Mac mini and connect it. All being well, it'll boot up as normal. This is what mine looks like:
If nothing happens, check all your connections and wiring. If something explodes, run and get your fire extinguisher. Not a water one, mind - this is an electrical fire and water will make it worse.
Ok, so now we have the Mac mini working exactly as it was before we started cutting things up. Brilliant!
Now to make the car's side of the connector. This is where we need to pay attention to what the wires are. The following three are required for bootup:
- Red: +18v DC
- Black: Ground
- Grey: Control
The sheath is a ground which I assume is connected to the Mini's metal body to ground shorts. It is recommended you wire this to a grounding point on your car, but isn't required.
The grey control wire is needed for the Mac mini to boot up. This is a low-load wire similar to an amplifier's remote wire - it's needed for operation but no serious current goes through it. However, you can't just wire this up to live or ground as it is. This is where our resistor comes in - you need to connect the grey control wire to the black earth wire through the 6.8k resistor.
Get your 12v -> 18v DC-DC convertor and cut off the connector at the end of the 18v side of the wire. You should have two wires - a red +18v wire and a black (or white) ground.
The process is very similar to wiring the other connectors, except you need to wire the grey control pin to the black/white ground pin through the resistor. Remember to wire the pins correctly so they match up with the Mac mini side of the connector! There are many ways to wire the control, such as splicing it with the black wire. I simply bridged the pins with a resistor:
And now, I have this:
And there you have it! Assuming you soldered everything right, you can now power your Mac mini in the car and your home by simply connecting it to the connector in your car or your power brick at home. All without having to get a second brick! :-)
Here's a picture to prove that it works for me (note the little white power light on the front is on!):
At the moment, the 18v laptop car adaptor is connected to the car's cigarette lighter, but it will be connected to a proper power supply soon.
Closing Thoughts
There are some things to think about when doing this:
- I haven't wired up the ground sheath yet, but you really should do. It won't be hard to connect the last pin on your car connector to a grounding point. I don't recommend you wire the sheath to the ground pin - I've no idea why, but I'm assuming if Apple didn't use the ground wire for that, we shouldn't either.
- I'll be connecting the DC-DC convertor to a proper carPC power supply/voltage regulator. The convertor I used seemed to keep the voltage constant during cranking, but I'd rather not trust my computer to a cheap cigarette plug adapter.