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Beware of car chargers
Fri, 02/01/2002 - 14:46 — ea
I've seen that there are iPod car chargers available commercially or as an DIY projects.
A word of caution: You should never, ever, DIRECTLY connect an electronic device to the lighter socket (or to any other vehicle 12V terminal for that purpose).
Comments
Re: Beware of car chargers
Don't most chargers have in-line fuses to protect against a large voltage spike? I'm no expert on these things, just wondering...
Re: Beware of car chargers
"In order to safely connect an electronic device to any car voltage"... what? I am wondering what you were about to say right before you got cut off? I used a 500 mA fuse in my car adapter, equal to iPod's stated maximum current draw, so it would blow right away if there were any problem. The ~14 VDC is not a problem; the iPod can take anywhere from 8-30 V. But what about the voltage spikes - diodes? capaciters?
Re: Beware of car chargers
A fuse will not save the iPod. The problem is current overload; It's very high and low voltage spikes.
The method to protect the iPod should involve a Transorb device, which is designed to positive spikes. A simple diode should suffice to clamp the negative spikes. However, this requires a careful design. There's no rule of the thumb here.
Re: Beware of car chargers
OK guys - here's the missing part:
In order to safely connect an electronic device to any car voltage terminal, one must use a Transorb device (a special kind of Zener diode) which can clamp very high voltage spikes for very brief time (just like the voltage output in the vehicle). A simple diode, which is connected between the +12V and earth can clamp negative going spikes.
In addition, an active voltage regulator is required to be absolutly sure that your iPod gets exact voltage, according to the specifications.
It's important to note here that a fuse protect a device from positive or negative going spikes; it can only blow when high current (according to the fuse spec) flows through the fuse for predefined time.
Re: Beware of car chargers
I think what ea meant to say was:
It's important to note here that a fuse cannot protect a device from positive or negative going spikes...
Fuses basically protect accidently short-circuited wires from heating up too much and causing fires. They don't sense excessive voltage, which can kill electronics. Car power is pretty dirty... you've got a bunch of huge spark gap transmitters in the engine and all sorts of electrically noisy relays, too.
TVS - sholud do the trick, no?
something like a P6KE24C (Radio Shack #
900-2771)
Re: Beware of car chargers
Yes, you are right - a fuse CANNOT protect a device from high voltages.
Re: Beware of car chargers
I posted my charger the other day. It was the direct type with no protection other than a fuse. While I did think that the car voltage was well within the 8-30 FW voltage, I do see the need to have better protection.
My soloution was to get a RadioShack lighter power adapter. It puts out 9v up to 1000mA #273-1863.
Inside the plug, there is a voltage regulator and other circuitry. Among the uses mentioned on the pagage...MP3 players. I will post pictures after I Test it.
Re: TVS - sholud do the trick, no?
Almost!
That's a bidirectional TVS; a unidirectional TVS would work better. The difference is that the bidirectional limits the voltage to +/-30 volts, while the unidirectional version limits it to +30/-1 volts... which isn't perfect, but does a better job at filtering negative-going spikes.
The bidirectional TVS's are not polarized since they work equally well in either direction. The unidirectional TVS's are polarized, so you have to make sure you connect them in the right way -- otherwise you'll short out the battery and blow a fuse. The cathode side (the one with the band) should be connected to the positive voltage; the anode side (without the band) should be connected to ground. Put the TVS after the fuse so that the fuse can protect against a TVS that fails (or turns on for too long of a time because of too-high input voltage).
Here's a suggested part: 1N6278A 5% Uni-Directional TVS
Mfr's #1.5KE20A
JEDEC Part #1N6278A
Case Style DO-201AE
Response time: typical less than 1.0ps from 0V to BV min (This is what seperates this from a normal zener diode)
Breakdown Voltage VBR min @ It: 19.00 (Important! well above the expected max car voltage - otherwise it'll short the battery)
Breakdown Voltage VBR max @ It: 21.00
Maximum Clamping Voltage Vc: 27.70 (Under the 1394 max voltage)
Surge capability: 1500W for 1ms
Steady-state output power of 5.0W
I went ahead and spent the extra money ($0.80 vs. $0.37) on the tighter tolerence, higher wattage part. There should be plenty of space for this bigger part, and the tighter tolerance allows the car to put out a few more volts before we start panicing (which is good because the car can provide a lot of current). Just remember to put the TVS after the fuse and there shouldn't be any fires...
Re: TVS - sholud do the trick, no?
That sounds good to me. One last thing to add might be (it's alive!!) a choke to filter out high frequency noise. You can use either a solder-in-type, or a ferrite bead like you see on the end of monitor cables and laptop power cords. It would be most effective as close as possible to the ipod -- this keeps noise picked up by the cable from entering the ipod, and also keeps ipod-generated noise from escaping . A choke would still be somewhat effective in the charger plug at the other end of the cable -- it'll keep noise from the engine out, but the cable can still act as an antenna.
Re: TVS - sholud do the trick, no?
You suggested that the filter be put as close as possible to the iPod to remove enviromental noise that the Firewire cable may pick up. Isn't the firewire cable coaxial, in which case it is not a very good antenae at all? If the firewire cable is not a good antenae then it would be more convenient to put the filter near the lighter plug.