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Anderzander

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First off, fit a large heatsink. Assuming you are feeding in 12v, if you are pulling 2A the devices will need to dissipate 14W. That's quite a big heatsink.

I'd be inclined to couple them up, but only apply power to them when there is something connected. This should stop them fighting.

Or more a more complex solution would be to use one of them as a voltage reference, and use that to drive a bigger transistor.

Or, find something that will push 2A.

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hi all, sorry for the thread jack, but I'm trying to fix my tomtom rider charger. It's never worked properly and requires 5v and 2amps.

I bought a couple of 7805's and thought I could wire them in parallel to cover the draw, I guess Ideally I need 3. But on the maplin site it said you can't do this.

I tried anyway and while it does work, they get very hot even when the tomtom isn't charging. I assume there's something else going on here.

Do I need to add a diode to the 5v out of the regulator circuit to make this work properly?

hope that makes sense, thanks Ruu

No, it will never work satisfactory for a whole host of reasons.

Use one of these ........... its just a 3 amp 7805 !

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2987252

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With any linear regulator in this package type, you'll always find it's running hot at 2A out, even with a higher power regulator.

13.8V in -> 5V out at 2A means (13.8-5)*2 W of heat = 17.6W

The package type means that the internal electronics is running about 50C hotter than the case when it's on a good heatsink. So if it's hot to the touch, the insides are more than a bit sweaty.

If you use the 3A device that BBC suggests, the numbers are the same, except that at 3A it's more like 75C hotter.

If you can get away with 1.5A, a good solution is RS 672-7155. Just as simple to use as a 7805, but about 90% efficient, so less than 1W of heat at full load and no heatsink required.

They do cost a bit more than a 7805 though :(

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Have you ever noticed how the TO3 version always seems to go into self oscillation just before shutdown :angry:

Try a different manufacturer - they are certainly not all the same. Oddly enough, it's often caused by using too large or too 'good' (low ESR) smoothing caps.

My personal fav. is the surface mount switching FETs that can happily melt the solder joints without failing. They either fall off the board and disconnect the power, or slide sideways and short out the power rails :lol:

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cheers guys, I don't think the 7805's are going to cut it with that much heat generation. The tomtom cradle is fully enclosed and quite thick abs plastic so I think I'd be looking at some serious melting. I'll have a look at the other options, doing some basic testing the charging seems to only be 0.7amps max, but I think I need to do some more testing to be sure.

Cheers again

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