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I am replacing my radio with a 7' LCD and a small android device and I am wondering what the best way to wire the screen to the wiring harness? It's rated at 12v 2a being the best operating voltage. Also for the Android device it runs off 5v from a mini usb so I got this from Amazon.


Just wondering if that will work fine.

Thanks!

 
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Wire the screen to the radio harness ignition 12V so that it turns on with your key.

That buck converter will work fine. I would have found one with a lower output current since your device only needs 2 amps. The device that your charging may have a current limiting circuit in it that will maintain the 2 amps needed to charge its battery.

 
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I tried to wire the screen control board to the 12v + switched wire from harness with a barrel connector cut from a different AC power supply. I had the black wire with the white stripe going to the red switched wire and the black to the black ground wire and the board popped and a small cloud came off so it seems that I shorted it and I'm almost certain I had the wires the way I stated and not reversed. Now the board is fried and a replacement is on its way. So is there no chance really that it got too much voltage or amps? Or did I most likely wire it backwards and I just shorted it? I'm very wary to wire up the new control board and I just want to be sure. Thank you!

 
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I'm also wondering if I'm able to wire the screen and 2-3 buck convertors for running the Android device (MK808B), an HDMI to HDMI + RCA Audio and a USB hub possibly to the 12v from the wiring harness? Would I add up all the amperage being possibly drawn from them together and add a fuse of that rating into the radio/acc slot of the fuse block? Just trying to figure out how to make sure everything is protected.

 
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My guess is you wired it backwards. Voltage shouldn't be a problem, the device is listed for 8v to 22v and the normal voltage in a car varies from 12 to 14 (rounded numbers) volts. If your alternator had generated a spike that took out the adapter it would most likely have taken out other electronics as well.
Amps are not an issue either. In some respects there is no such thing as "too much" amps. As an example if you install an audio amp that needs 15 amps most people will install a circuit capable of at least 20 amps but more likely 30 or 40 amps. The amplifier will only pull the 15 amps it needs no matter what current the circuit can deliver.

A better example may be your car battery. It can deliver hundreds of amps but if you connect a device that only needs one amp, only one amp of current is created.

 
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Each of the converters needs its own fuse and you do not want to change the rating of the fuse for the radio/acc. You should tap into the + circuit on the car side of the radio/acc fuse not the radio side. Yes you add the ratings together to get the total draw but each device should have its own fuse and the + voltage feeding them should be fused with a sized fuse of the devices added up. I believe fuse rating is 10% higher then maximum current draw, but I'd have to check.

 
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I must have just been too excited to wire the monitor up that I did it backwards then, good thing the replacement should be here soon. So when I go to wire it up again can I use a multimeter to make sure the center of the barrel connector is positive by putting the positive lead from multimeter into it and touching the negative to ground, turn the key and it should read somewhere around 12v showing all is wired up right? Also if the buck convertors I use to drop the volts to 5v for the various other devices, say they have surge protection built in I should still put fuses on them? Sorry for so many questions just trying to make sure I'm not frying anything or starting fires haha. Thanks for the help!

 
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Yes using a multimeter and turning the key would be best to make sure the wires are correct. I would disconnect the negative battery terminal also.

If the converters have a built in protector You don't need to but a fuse never hurts. If it had a fuse the first time it probably would have saved it from frying. Fuses are cheap.

 
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Thanks again for all the help! I should have the new controller board soon and I'll be starting a build log thread.

 
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You definately need to fuse the barrel connector. The purpose of the fuse is to limit current in case of a short. The barrel connectors are designed so it is hard for the center to make contact accidentally but it is possible. If the center contacts ground you need the fuse to protect the wires from starting a fire. As far as protecting the device connected, sort of. If the device is rated for 2 amps it will only pull 2 amps. If it is pulling more, enough to blow the fuse, there is a problem with the device. Fuses protect from too much current such as a short, they do not protect from voltage. If your original converter blew because the polarity was reversed, a fuse would not have helped nor would it protect from a spike.

If it was me, I would fuse both circuits just to be safe. I would use a multimeter to verify the voltage and polarity at the barrel connector by touching the probes to the two parts of the connector, just like the device will. Just be careful you do not short the probes while testing.

If the car is not running you should expect 12.4 volts, if it is running the voltage will be higher depending on the alternator.

 
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What size fuse do you recommend for 2 amps?

 
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I think GolNat's suggestion of 10% over the rated draw would be a safe place to start. You could probably get away with a 2 amp fuse if you want to try that first or use a 3 amp. Guessing that your wire is 18 to 20 gauge you would not want a fuse greater than 10 amps.

Different loads behave differently. A light bulb is always going to produce the same load while a motor will draw more current when it starts or if it stalls. My guess is your LCD will be a pretty stable load so I doubt it will vary from the 2 amps it is rated.

 
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Thank you for all the help. I should have the new circuit board for the lcd around Tuesday and I will finish working on my write up about the build after that!