electronics advice

[Anton Piatek]

Hi all, looking for some electronics advice for something pretty basic.

I've got a few strips of 12V leds that I want to hook up on my daughters toy kitchen to light things up. I'm thinking of controlling it all via an arduino for nicer push-button, door open/close configuration so theres two things I'm not sure about.

What should I use for a 12->5v regulator for the arduino? Would a L7805CV (£1 from bang good) be a sensible choice, or is there something else more suitable?

What about powering the leds from the arduino - I obviously need something to switch the 12V, so does this mean a mosfet? What sort? I have no idea what sort of specs I need here (and do I just wire it up like a transistor with an arduino pin high to turn it on?)

Thanks for the advice, Anton

[Bracken Dawson]

7805 will be fine for 5V, there are actually better and cheaper linear regulators but it's barely even worth researching.

To switch the LEDs you want an n-channel FET, got any random ones lying around? I know we do at SoMakeIt.

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:wq

[Anton Piatek]

No, not got any FETs - No idea what sort of thing I'm looking for when choosing one - I don't need very high current for this job. Other than n or p type and current ratings, is there much else to base a choice of FETs on?

[Philip Stubbs]

I am not an Electronics engineer, but I did read The Art of Electronics once! :-)

I think a low value for Rds is important if you need efficiency. There is also two values for Vgs that you may want to consider. Vgs is the voltage between Gate and Source. The first value is the threshold, or where the device starts conducting. The second is the voltage where Rds is specified. If you need the low Rds, then you need to make sure that you can achieve the required Vgs.

So, as you want to drive these from an Arduino, I guess that as long as Vgs is lower than 5, it should work.

For playing with simple circuits, I have found Qucs[1] to be fun.

[1]http://qucs.sourceforge.net/

Philip Stubbs

[Bracken Dawson]

This ought to handle it: http://uk.farnell.com/diodes-inc/zvp2106a/mosfet-p-60v-0-32a-e-line/dp/9525505

:wq

[Anton Piatek]

I thought you said n channel - thats a p...

[Philip Stubbs]

Try this:-

http://uk.farnell.com/vishay-formerly-i-r/irlz14pbf/mosfet-n-60v-10a-to-220/dp/8651370

[Anton Piatek]

Thanks, how does that compare with either of these (which are cheap on bang good)
http://www.banggood.com/20-Pcs-2N7000-N-Channel-Transistor-Fast-Switch-TO-92-MOSFET-p-943285.html

http://www.banggood.com/10Pcs-IRFZ44N-Transistor-N-Channel-Rectifier-Power-Mosfet-p-953277.html

I'm not sure if im understanding the data sheets on those, particularly Vgs values

[Philip Stubbs]

The 2N7000 will work with the arduino. But it will only cope with 200mA max and at that current will drop ~1.2 volts.

The IRFZ44N will handle much more current with minimal voltage drop, but will not be fully 'on' with only 5 volts between Gate and Source. You will need to be closer to 10 volts.

Now you could use both. Have the IRFZ44N in series with the LED's. Connect a pull up resistor to the Gate to turn it on. Then use the 2N7000 to pull the Gate of the IRF device to ground or at least <2V to turn it off. The Logic from the Arduino will have to be inverted, but that is only software! :-)

[Anton Piatek]

thanks!

[Bracken Dawson]

oops, guess I clicked the wrong checkbox

:wq

[Dave Rowntree]

You can get so-called 'logic level' Mosfets that have low Vgson, and also low enough Rds to be efficient.  Can't remember any specific devices of top of me head - but they do exist for exactly this purpose.

Dave.

:wq

thanks!

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Philip Stubbs

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Philip Stubbs

[Anton Piatek]

So I ended up buying a couple of each of these
https://www.fairchildsemi.com/datasheets/bs/bs170.pdf

http://www.irf.com/product-info/datasheets/data/irlr024n.pdf

The LU024N ones work fine (once I realised you need to have a pull down resistor on the gate to keep it off, but the BS170 never quite turn off. Can someone tell me what I got wrong as I'd like to understand why they don't quite turn off. I'm using 12V on the drain, 5v on the gate. I was worried about having a high enough voltage to trigger it on from an arduino, I didn't expect I'd end up with one I couldn't turn off. 

The LU024N will be fine to use, but looking at the size of the package I suspect they are total overkill for switching <150ma