# Choosing the right transistor for a switch

Based on a very closely related question: Choosing the right transistor for a switching circuit I have managed to create a working circuit, but I need the correct transistor.

Some context: I am trying to power and switch on/off an 12V, 4.8W LED strip from an arduino.

I have done the same calculations as in the question above, and arrived at

$$h_{FE(minimum)} = 5 \times \frac{0.4}{0.04} = 50$$

The only transistor I have is a BC413B, which has a maximum collector current of 0.1A. Since my LED strip is 0.4A, based on my limited knowledge, I gather this is not sufficient. Currently, with a 1KOhm resistor at the base, I get very little light out of the LEDs.

My question is:

Which transistor and resistor should I be using instead of the BC413B and the 1KOhm resistor to achieve the full effect of the LED and without destroying the components over time?

• Don't downvone, unless you have a name and reason. UNDERSTAND? Nov 12, 2014 at 21:13
• "...The only resistor I have is a BC413B..." resistor? my god!
– Roh
Nov 13, 2014 at 5:16
• @Roh: Brainfart on my part, that is fixed. Nov 13, 2014 at 12:48

You'd be better to use a MOSFET for this kind of application.

R1 keeps the Arduino output current low during switching (the MOSFET gate looks a bit like a short during the brief switching interval), and R2 makes sure that if the drive gets disconnected the MOSFET switches 'off' and does not hang around half-off and half-on, burning itself up.

simulate this circuit – Schematic created using CircuitLab

MOSFETs have almost infinite current gain, so the $h_{FE}$ equation does not apply, but you do have be sure that there is enough drive voltage to turn the MOSFET well on and off.

In this case, the maximum on resistance of the MOSFET with 4.5V drive is 11.9m$\Omega$, so the power dissipation will be only 2mW, and it will run dead cold.

• Thank you very much for this response. I will try to find an appropriate MOSFET instead - I'm just reading up on those to try to learn which values I need to look for. The only place here that sells that particular model you're referencing is Digikey and they charge a lot for shipping. My local dealer only has MOSFETs rated much higher, like this one: vishay.com/docs/91078/91078.pdf. Nov 13, 2014 at 8:46
• Those are the wrong 'polarity' and need 10V to turn fully on, and have a very high Rds(on). Look for N-channel, logic level gate, and very low Rds(on) with the drive voltage you have (or lower). Nov 13, 2014 at 9:13
• I think it may be beginning to dawn on me slightly. I have found a logic level MOSFET with rDS(on) of 0.2 Ohm and V-SD of 1.4V. I don't know what a 'very low' rDS is though. Once again, this has really been enlightening for me, so thanks! Nov 13, 2014 at 10:05
• I will find another supplier with a greater selection, the important part is that I now (think I) understand what specs I'm looking for. Thank you very much for your help, I will mark this as the answer. Nov 13, 2014 at 10:35
• Got my MOSFETs today, and it works like a charm. Once again, thanks! Nov 19, 2014 at 19:49

I'll just mention that an old favorite transistor among hobbyists is the 2N2222, an NPN which is good to 800mA, and has an hFE of better than 100. Also, if you run into a situation where you need a higher current AND high gain, consider a darlington transistor. I'll agree with Spehro, a MOSFET has a lot of advantages as a switch. But you asked about a transistor, and it is good to understand the advantages of both technologies. For example, if cost is a factor, those 2n2222's are dirt cheap. I've bought them for 5¢ and less in bulk at surplus suppliers.

• Thanks for your reponse. I'm just learning about electronics (apart from a short introduction waaay back in middle-school), so while I asked for a transistor, it might not be the correct choice for my application. Cost is not really that important, since I will only be needing to power 3x12V LED strips in my project. Nov 13, 2014 at 8:48
• As Randy says, it's not a bad idea to get familiar with a few popular 'jellybean' parts that are inexpensive and can be used in many applications- a general-purpose transistor such as the 2N3904/2N2222/2N4401/BC547 can be used from low current up into the 100's of mA range and are very cheap (a penny in volume). The MOSFET I suggested is gross overkill current-wise (tens of A rating), but quite cheap (50 cents in singles, 15 cents in volume) and can be used in many low-voltage applications. For example, you could use a 2N4401 or 2N2222 with another transistor to give you maybe 40mA base drive. Nov 13, 2014 at 10:21
• Thanks to the both of you. I have been looking for a list of basic components to just buy off up front. I find myself spending a lot of time waiting for deliveries of individual components that I need. I can buy a single MOSFET for $2.5 from my local supplier, or I can get 50 of them from digikey for$5, but I have to pay \$25 in shipping. If I would just buy a 'bucket' of commonly used components, that would save both time and money. Nov 13, 2014 at 12:46
• @TroelsLarsen - we all feel your pain on the waiting for deliveries. These days, the "Good-Fast-Cheap" triangle is the rule. You can have any two, but seldom all three. For example, Ali-Express is a great source of cheap parts, and direct China PCB companies often have prices that make PCBs a lot lest painful. BUT... you may wait weeks to more than a month for inexpensive delivery from overseas! :-) Nov 14, 2014 at 3:34