LM7809 vs KM317T for a 12v to 9v regulated supply

Sorry, thats LM317T (or even a LM350).

Thanks Roger. I checked the specs of the MP3 player in question and it says: # DC Power: Internal Power: Li-ion rechargeable battery 3.6V x 2(720mA) # External Power: 12V adapter (1500mA) # Supply Voltage: 9~18V # Supply Current: working:12V(360mA) / standby :175mA

Also, I am currently running the player on 8 x 1.2v 2300mah NiMH batteries since the internal Li-Ion one is almost dead. So the player can run on 9.6v DC input.

- Does this mean that the player can tolerate an external DC supply source from 9v to 18v and I can plug the player directly to the cigarette adapter with a fuse in between?

- Would it be a good idea to keep the almost dead battery in the player to act as some sort of protection against a spike?

Thanks,

- Siddhartha

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Could you please suggest a good/simple surge protection circuit?

- Siddhartha

The 317T, is a variable voltage regulator, while the '78' family are fixed voltage. With the former, two resistors will also be present to set the working voltage, while the 78 does not need these. In both cases, the 'car' is a fairly nasty enviroment, and some significant attempts will be needed to reduce spikes on the incoming supply rail, if damage to the regulator, and player are to be avoided. Also the control loop round the regulator has a very high gain, and care has to be taken to decouple close to the output pin to avoid instability. There are a lot of different 'variants' of 7809, and the original standard versions were only rated for 1A. If (for example), you look at the version from Fairchild (MC7809), this is still only a 1A device, while the 'On semiconductor' version (MCT7809), is rated at 1.5A. The LM317T, is a 1.5A device, and will probably be easier to source than the 9v 1.5A version of the 78 chip.

Best Wishes

Given that your mp3 player can tolerate a wide input voltage range it might be best just to power it direct from the car supply via a 'surge protector' to stop the nasty automotive electrical transients getting into it.

Graham

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Not really my area. I just know you need one. I expect someone will offer assistance. I gather you need to protect against possible polarity reversal ( easy - a diode ) and high voltage transients. I guess a small series R and a TVS.

Graham

If you value your MP3 player at all, you're probably best off with a regulated supply. There's really only one sure-fire way to know if your surge suppression circuitry is up to the task or not...

While a 7809 or LM317 will work, they are not usually the first choice for an automotive application. I'd recommend an automotive grade regulator - they are designed to accept the harsh transients routinely encountered. Infineon's TLE 4274 regulator is a three-terminal type that is similar to the 78xx, and is available in 8.5V and 10V fixed output voltages. The TLE

4274 incorporates reverse voltage protection internally (78xx and LM317 need an external diode), has a low dropout voltage, and is able to accept up to 40V at the input continuously.

TJL

You could see if you could get something like one of these:

Good Luck! Rich

In article , snipped-for-privacy@example.net opined thusly:

Neither one will make a difference to your HDD if you use it right. But they come in different current ratings, the "T" version wildo an amp but you're on the limit of its spec. The nore heat sinking you give it the more reliable it will be.

If you look at the power supply circuit inside the player you'll probably find it is all regulated anyway, so you could just use a bunch of diodes in series to drop a few volts off the car's supply, which isn't 12V at all, it's closer to 14V.