12v - 5v USB Power Socket

[Matty80 Avid PP Poster! Find all posts View Profile Close Matty80]

Re: 12v - 5v USB Power Socket

I've had a look into what might constitute a safe 5v power supply for sensitive electronic devices such as iPods & gaming devices that are connected to an automotive system.
Found a few ideas for standard 5v power supplies, but not much really covering the ellimination of electronic noise.

Bear in mind that I have not had a chance to test the circuit, as we do not have breadboarding facilities within the electrical department out here on the rig. So anyone wanting to use this, still needs to verify that the figures listed for the capacitors and inductor are suitible for removing and filtering any transients in the system, and adjust as necessary.

Basically, as we already know, the prados electrical system is 12v, charged by an AC generator (the alternator) and rectified to DC for charging the battery and maintaining voltage throughout the car. However, the filtering on this is crude. Any sensitive electrical system connected directly to the car must perform is own additional filtering to eliminate potential spikes of up to 60vdc, during startup, charging and running. An easy way to eliminate these spikes is to use electronic filters, my circuit uses a capacitor-input filter which is described as a 'Low-pass Pi Filter'.


(Image not entirely clear - can provide a full size PDF version of this if someone wants it)

How the circuit works:
The 7805 voltage regulator should do most of the work so long as at a minimum capacitors C2, C3, C4 & C5 are included.

ZD1 - this zener diode will prevent the output voltage from ever exceeding 5.6v, optional.
R3 & D2 - Power indication LED, optional.
D1 - to disipate any reverse polarity that may occur and damage the regulator, very unlikely - optional.
C1, C2 & L1 - the Low-pass Pi Filter, if not using the filter, C2 should remain in circuit as a decoupling capacitor.
F1 & L1 - marked as 5A rated components, no reason why they can't be 1A rated (smaller & cheaper).

Capacitor values may need to be experimented with, but this system should work ok.

I've included below, some industrial style USB sockets which would be more durable that what can normally be aquired from an electronics store, we have variations of these in our Cyberbase chairs.



Cheers

Matt

[sandman Member Find all posts View Profile Close sandman]

I've just bought one of these to run my SPOT as it needs 3.0V. For 5v on the USB, these units would be great :-)

http://www.goodluckbuy.com/lm2596s-d...er-module.html

Cheers..Pete

[geograzza Member Find all posts View Profile Close geograzza]

Unfortunately most devices won't work with the basic design you have. To get them to charge you need to set voltages at pins 2 & 3 to tell the device how much current it is allowed to draw.
pin2=3v & pin3=3v = 100mA (won't work with iPhone)
pin2=3v & pin3=2v = 1A
pin2=2v & pin3=2v = 500mA
the voltages can be set with simple resistor voltage dividers eg 22k & 33k resistors. If setting both to the same voltage, don't be tempted to tie the pins together - use a separate set of voltage dividing resistors for each pin.

[Chris220220 Senior Member Find all posts View Profile Close Chris220220]

Think I'm going to gut one we have and try mount that in the back. It's a pain with no outlets for the back seats to keep the kids happy on long trips.

[BigDog816 Senior Member Find all posts View Profile Close BigDog816]

I've included below, some industrial style USB sockets which would be more durable that what can normally be aquired from an electronics store, we have variations of these in our Cyberbase chairs.
Cheers

Matt

That chair looks like a pretty nice piece of kit. The USB outlets aren't bad either.

[bworth Junior Member Find all posts View Profile Close bworth]

About a month ago I was replacing the 2-Din 120 Radio, and decided whilst I had the dash out I would also put in a 5V supply for USB Charging, and as a side benefit provide a 5V line (via RCA Plugs) to our TomTom, this eliminating one messy "Cigarette Lighter" cable**.

I know that a tablet requires about 2amps, and you can add another 500ma (x2?) for another USB device including the TomTom, so I figured that to build any long term usage into the system 3amps output would be desirable.

I looked at the various typical solid state regulators, in TO-3 (Metal) packaging, because of the current and power dissipation of the chip (discussed above) would at 3amps, with wattage = 3*(13-5)=24 watts, it was obvious that the heat-sink was going to have to be fairly serious. There was also the need insulate the 12v/5v voltages without the insulation trapping the thermal load in a layer of electrical tape.

The LM317 family is well suited to the task, but I actually settled for a 78H05 series TO-3 device primarily because the outer metal casing of the device is at ground, whereas the LM317k case is at 5v, and would need to be protected. A side benefit is an output of 5amps, (although that also means if used, the heat-sink would be required to dissipate 40 watts), and a slightly lower component count.

Fortunately I had all the bits at home, and used a single capacitor on input (1uF TagTantalum) and output (.1uF Greencap), as well as a protective diode Output > Input. No advantage to extra caps.

Warning: don't use a zener diode as in the above mentioned circuit, they are not designed for fault protection and will be the first thing to vapourise if called to action.

A retrospective: Would I do it again?

Firstly, the cost and effort of the above simple circuit was probably above the simple commercial solution, which I gather is to feed a USB cable to the middle/back of the car??? Jaycar sell a small 2-way USB plug [Search: Jaycar "3.1A Dual USB Car Adaptor"] for $20, that plugs into an existing or new (rear of center console??) "Cigarette Lighter" socket.

One output is a generous 1000ma output, the other is set for 2.1 amps, and with the cost and the neatness, it would be a better solution for many; certainly you can also take it from car to car. Ultimately they all have wires, it s just a case of minimising a few?

For me, the clincher was the TomTom output, as that is one cable that is now 100% tidy! The USB output is potentially handy, but we already have it covered with the above 3.1 amp Jaycar adapter, which can be plugged in to the rear 12v connector as desired.

Does it work? Yep, it works like a charm.

bworth

**The original TomTom Cable length was cut to about 20cm, and plugs in to a short length (10cm) of figure-8 cable coming out high above the dash. The Cigarette lighter end of the factory charger cable was also kitted with a suitable RCA socket, to allow the TomTom to be used in other vehicles.

[gxl_d4d Advanced Member Find all posts View Profile Close gxl_d4d]

The LM7805 will get hot if you try to pull 1A out of it without a heatsink. If the alternator is running, it will put out (13.8-5)*1= 8.8W of heat.