12 volt to 10 volt DC/DC

[chris_gray21]

can anyone help i am wanting to install a wavefinder into my car but it runs of 10v and comes with a 240v to 10v adaptor and im afraid that 12v will blow it and they are'nt very cheap..... i have tried looking for an adaptor but cant find one has anyone any ideas

[Adrian]

How much current does it take? An adjustable three-terminal regulator such as the LM317 is your ideal solution if it takes less than 1 Amp. You'll also have to be handy with a soldering iron and wire it up with a couple if resistors to set the output volts to 10.

<a href="http://www.fairchildsemi.com/ds/LM/LM317.pdf" target="_blank">http://www.fairchildsemi.com/ds/LM/LM317.pdf</a>

R1=180 Ohms, R2=1K2 Ohms in fig. 5 (page 4) gives 9.6 volts if my sums are right.

[Stupink]

I'd probably just run it off an invertor with the 240v adaptor to be on the safe side if its not that cheap [img]/smile.gif[/img] so long as the adaptor is double insulated(suply cable doesnt have earth) so can run on an inverter and u buy a suitable wattage inverter it will be fine.. trouble with any form of 12>10v adaptor is that it assumes 12v, which means you'll only be able to run it with the car off, when running it will produce 12-15v and have spikes and all sorts of tricks [img]/smile.gif[/img]

[Adrian]

<!--QuoteBegin-Stupink+--><div class='quotetop'>(Stupink)</div><div class='quotemain'><!--QuoteEBegin-->trouble with any form of 12>10v adaptor is that it assumes 12v, which means you'll only be able to run it with the car off, when running it will produce 12-15v and have spikes and all sorts of tricks [img]/smile.gif[/img]<!--QuoteEnd--></div><!--QuoteEEnd-->

[img]/wink.gif[/img] Not a problem with a voltage regulator like the LM317. It can handle up to 37 Volts at the input, the typical maximum will be around 14.4 Volts.

I know alot of people solve this sort of problem by using an "mains invertor" - but its a terribly inneficient and expensive way to do it. First you use a big beefy chunk of electronics to transform 12V to 240V, then have to use another transformer to drop it back to the particular low voltage required.

OK, so the obvious attraction is you that you can easily buy an invertor and use the transformer that came with the gadget - but a little bit of ingenuity can provide a much neater and cheaper solution - the LM317T is 49p from Maplin...

<a href="http://www.maplin.co.uk/Search.aspx?doy=10m1&source=15&criteria=lm317" target="_blank">http://www.maplin.co.uk/Search.aspx?doy=10...&criteria=lm317</a>

[Adrian]

OK, here's how you'd hook it up:



Becasue the metal tab of the device is internally connected to the output terminal, resist the temptation to screw it to any metalwork! I would bolt it to a peice of aluminium about 4 cm square (to dissipate a little bit of heat it will generate) and fix that to a plastic panel somewhere.

The resistors and connecting wires could be soldered on as shown in the drawing and (taking care that nothing shorts beforehand) can be insulated by covering the whole lot with hot-melt glue (or silicone sealer).

Before sealing up though, I'd test it by connecting a small 12V lamp (5 Watt sidelight or similar) as a load to the output, and measure the volts across it... 9.6 or thereabouts. This is close enough to 10, and gives you a little safety margin.

(oh, I changed both resistor values up a bit to meet the minimum values suggested by the datasheet. Same ratio though.)

[chris_gray21]

holly cow matey thats fantastic help thank you so much maybe you could help me with another little problem i am installing a carputer which runs on 18volt at 3.2A (it does say max of 24v) it has a transforer like a laptop but i want to run it in my car again like you said with out an inverter i would also like it to hibernate when i turn the of the ignition of or on a separate power supply so it continues to run im not really sure what and how to do it any chance you could help me out mate since you are such a wiz............. thanks so much

[trev150]

Amazing what a simple bit of electronics can do.

Trev

[chris_gray21]

Need more help

i am wanting to install my kodak digital camera dock into my calibra ( not a permenant fix) but it runs on 5 volt how would i use the same set up above to get 5v i have the jack plug i may run it on a switch so its not drawing power all the time and get real hot as the camera will only get put in on certain occations also i am having some trouble finding some switches / buttons i am after 1 momentary and 4 -5 latching but funds are running tight they dont need to be light up just nice looking any help would be great ...... thanks

[Adrian]

The trouble with 5 Volts instead of 10, is that you are going to nearly double the heat disspation. Depending on the current drawn, it may be asking too much of this type of regulator. Of course a big enough heat sink might be able to overcome the problem...

Say you draw 1 Amp at 5 Volts, and the altenator is charging the car battery at 15 Volts (a sort of worst-case figure) then the regulator is dropping 10 Volts across itself at 1 Amp. This makes 10 Watts which is alot of Watts in a tiny object. Do you know how much current your docking station requires?

As for switches - scrap heaps might be your friend!

[trev150]

demon tweeks do some good switches.

You could try a smal fan to help dissipate the heat?

As said above 10 watts is a hell of a lot for a pcb mount component.

Trev

[chris_gray21]

i will have a look thanksfor all your help.....

[chris_gray21]

another equiery

i am installing 3 sets of 12v cold cathodes and also 2 80mm led fans in the calibra they are both 12v but if fitted to a computer they would get 12v dc constant but withg being in the car the voltage would flux and im a bit scared of blowing them up would it be a good idea to install the above regulator without the resisters what would the diagram be for a straight 12v regulator

cheers

[chris_gray21]

i have been advised to connect my cathodes and fans to the lighting circuit in the calibra as this circuit is 12v regulated but is there a good place to take a feed from to do this or should i regulate myself

cheers

[Adrian]

Lighitng circuit 12V regulated??? The only regulator I can think of is for the instruments and that's something like 9V. Regulating to 12Volts is difficult because there isn't any 'headroom'. It's a bit hard to explain but regulators have what's called a dropout voltage - oh, sod it, just take my word for it!

Anyway, cathode transformers take alot of power - usually around 0.75Amp each so you'd be pushing it anyway. To be honest, the only thing that will really do any damage to them is reverse polarity. Use a low value fuse like 2Amps and run off the car battery 12~14.4 Volt supply. It'll give you all the protection you need. Fans' the same story.

Anyway, what's the fascination with cathodes - 'orrible things ~ lot's of nasty spiky current to run them, ridiculously delicate glass tubes, wishy-washy colours You should look at <a href="http://www.litewave.co.uk/iceled_tron.htm" target="_blank">ICELED TRON</a> [img]/wink.gif[/img]

[chris_gray21]

ICELED are bloody fantastic but i cnt really afford it at the mo this pc is costing alot also im after lambo doors to thanks for all your help mate is there any end to you knowledge ( kiss *** LOL )

cheers