Arduino Forum

Using Arduino => General Electronics => Topic started by: Joy on Sep 21, 2013, 06:17 pm

Title: Which mosfet to use instead of BC548..??
Post by: Joy on Sep 21, 2013, 06:17 pm
I need to drive a 450mA (1watt) led.
I am currently using a BC548 to drive :P (which is bad)

which mosfet can I use (has to be cheap)..

Also please teach me how to find out a mosfet's output load..?
Title: Re: Which mosfet to use instead of BC548..??
Post by: alnath on Sep 21, 2013, 06:48 pm
if you want to drive it from arduino, it has to be a logic level mosfet (which will be fully on at Vgs = 5V )

other important parameters are the Id current it is rated and the minimum RDSon value, the lower, the better.
450mA is a low value , you'll find a lot of mosfet -

you could have a look here : http://www.nteinc.com/Web_pgs/LL_MOSFET.html (http://www.nteinc.com/Web_pgs/LL_MOSFET.html)

maybe others will give you a ref. number ;)

PS: other parameters may be important if you need to drive it at high frequency
Title: Re: Which mosfet to use instead of BC548..??
Post by: Erdin on Sep 21, 2013, 08:06 pm
You can check Ebay or a supplier in your country.
Almost every logic level mosfet will do.
It shouldn't cost more than 1 or 2 dollars.

You can use a 2A logic level mosfet, but also a 40A logic level mosfet. They both can switch 450mA. The 40A type might have a lower RDSon.
Title: Re: Which mosfet to use instead of BC548..??
Post by: MarkT on Sep 22, 2013, 05:52 am
The current rating is not a useful spec for a power MOSFET - so long as you choose an Rds(on)
low enough for your load current and a sensible power dissipation level (I-squared-R),
then you'll be well within the current spec anyway (its just another way of saying the
ultimate power dissipation limit).

So for example if you want to limit device dissipation to 0.5W and the current is 0.45A, then
you need an Rds(on) below 2 ohms.  Not a problem, 0.05 ohm or less is easy to find.

However if the load current was 7A, you'd need a device with Rds(on) below 0.01 ohm
to stick to the 0.5W limit.

But my other question is what is setting the current limit for the LED?
Title: Re: Which mosfet to use instead of BC548..??
Post by: cjdelphi on Sep 22, 2013, 02:52 pm

I am currently using a BC548 to drive :P (which is bad)


bc548 max collector current is 100ma...

you trying to melt it? lol

I'd be tempted to keep using NPN's and use something like a 2n2222's (or better) (since they're cheap)

Title: Re: Which mosfet to use instead of BC548..??
Post by: dc42 on Sep 23, 2013, 10:05 am


I am currently using a BC548 to drive :P (which is bad)


bc548 max collector current is 100ma...

you trying to melt it? lol

I'd be tempted to keep using NPN's and use something like a 2n2222's (or better) (since they're cheap)


I agree. Unless you are making a PCB and so can use SMD mosfets, then a bipolar transistor is better for switching 450mA. Ideally use a ZTX851, however BC337 will work reasonably well (and better than 2N2222 or 2N2222A, i.e.  lower saturation voltage). Use a base resistor of about 180 ohms so as to give it around 20mA base current.

If you really want to use a mosfet then I suggest ZVN4306A.
Title: Re: Which mosfet to use instead of BC548..??
Post by: fungus on Sep 23, 2013, 12:37 pm


I am currently using a BC548 to drive :P (which is bad)


bc548 max collector current is 100ma...


My datasheet says 500.

The simplest/cheapest thing to do would be to put in a 2n2222A which can handle 1A. Get one of the metal-can versions if possible - they dissipate heat slightly better.

Title: Re: Which mosfet to use instead of BC548..??
Post by: alnath on Sep 23, 2013, 01:18 pm



I am currently using a BC548 to drive :P (which is bad)


bc548 max collector current is 100ma...


My datasheet says 500.

The simplest/cheapest thing to do would be to put in a 2n2222A which can handle 1A. Get one of the metal-can versions if possible - they dissipate heat slightly better.





yep, it says 500mA, but it also says "...as switches requiring up to 300mA" 

the 2222A datasheet says 0.6A Ic continuous, and its VceSat @ Ic=500mA may be 2V !
If I had to chose a NPN transistor, I'd follow DC42 advice and go for a BC337 , only 0.7V VceSat @Ic=500mA .

Anyway, OP asked for a mosfet, I think he wants to give it a try, which I think is a good idea : always good to start with low values and simple designs  ;)
Title: Re: Which mosfet to use instead of BC548..??
Post by: elac on Sep 23, 2013, 01:41 pm
My Vishay and Motorola BC548 datasheets state 100ma.
What type of BC548 are you using that has 500ma rating?
Title: Re: Which mosfet to use instead of BC548..??
Post by: alnath on Sep 23, 2013, 01:55 pm
I think he is talking about this one https://www.google.com/url?q=http://www.philohome.com/sensors/gp2d12/gp2d12-datasheets/bc548.pdf&sa=U&ei=pixAUt-vBKbI0QWMzIG4Bg&ved=0CAkQFjAB&client=internal-uds-cse&usg=AFQjCNGVNAQDPefaQ-qWjsmPygfZKO6crg (https://www.google.com/url?q=http://www.philohome.com/sensors/gp2d12/gp2d12-datasheets/bc548.pdf&sa=U&ei=pixAUt-vBKbI0QWMzIG4Bg&ved=0CAkQFjAB&client=internal-uds-cse&usg=AFQjCNGVNAQDPefaQ-qWjsmPygfZKO6crg)

but I agree, this is the only one  :smiley-mr-green:
Title: Re: Which mosfet to use instead of BC548..??
Post by: elac on Sep 23, 2013, 02:06 pm
That's good to know.
Thanks.
Title: Re: Which mosfet to use instead of BC548..??
Post by: Erdin on Sep 23, 2013, 03:45 pm
I could not believe it: pushing 500mA through a 100mA transistor.
I think that fairchild datasheet is wrong. It also doesn't show any graphs to verify the 500mA.

This is the page for the BC548 at fairchild:
http://www.fairchildsemi.com/pf/BC/BC548.html

The button "Download Datasheet" is for the newest datasheet.
Guess what ? It is 500mW and 100mA. And that is what it should be.

The BC548 is for 100mA.

Title: Re: Which mosfet to use instead of BC548..??
Post by: alnath on Sep 23, 2013, 04:13 pm

I could not believe it: pushing 500mA through a 100mA transistor.
I think that fairchild datasheet is wrong. It also doesn't show any graphs to verify the 500mA.

This is the page for the BC548 at fairchild:
http://www.fairchildsemi.com/pf/BC/BC548.html

The button "Download Datasheet" is for the newest datasheet.
Guess what ? It is 500mW and 100mA. And that is what it should be.

The BC548 is for 100mA.




agreed, this datasheet is weird ! and why do they say it is to be used as a switch up to Ic=300mA ? weird again !

maybe we should send it to Fairchild ?
Title: Re: Which mosfet to use instead of BC548..??
Post by: elac on Sep 23, 2013, 07:01 pm
Ok, so all BC548s are 100ma.
Back to the way it was.


Title: Re: Which mosfet to use instead of BC548..??
Post by: fungus on Sep 23, 2013, 07:03 pm
My copy of the datasheet looks like this:

Title: Re: Which mosfet to use instead of BC548..??
Post by: fungus on Sep 23, 2013, 07:10 pm

Ok, so all BC548s are 100ma.
Back to the way it was.


There's definitely something weird going on... but if it's only 100mA rated then why hasn't it caught fire?
Title: Re: Which mosfet to use instead of BC548..??
Post by: alnath on Sep 23, 2013, 08:18 pm

My copy of the datasheet looks like this:




yes, that's the one of my previous link
Title: Re: Which mosfet to use instead of BC548..??
Post by: alnath on Sep 23, 2013, 08:20 pm


Ok, so all BC548s are 100ma.
Back to the way it was.


There's definitely something weird going on... but if it's only 100mA rated then why hasn't it caught fire?



I got light from a 1N4148 once..... it still works  XD
Title: Re: Which mosfet to use instead of BC548..??
Post by: cjdelphi on Sep 24, 2013, 03:59 am
Grab a multimeter measure the max current (set to 10amps) see what you read, don't do it for long though quick enough...