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Topic: Quick transistor question (Read 1 time) previous topic - next topic

I need to switch about 14v (before current limiting resistor) at about 490mA (actual calculated number without buffer) at PWM speed for one channel of LEDs. What are my best options, and what am I looking at for heatsinking needs?



I would say most tranhsistors will do the job.
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A logic level FET should do that without the need for a heatsink.

May 22, 2012, 06:43 am Last Edit: May 22, 2012, 06:51 am by stoopkid Reason: 1
I tried finding some common N channel Mosfets, but they all seemed a bit overkill and expensive. So would this work?:

oops, wait... I'm not sure that one is logic level... how about this one?:


Yes that one will turn on quite well with a logic level signal.


If one reads the data sheets for each, the relevant characteristic is Vgth and from the charts for that parameter the devices are similar and both are easily capable of > 7A Drain current with 3.5V Vgs and as 'best part' the IRL520 is capable of about 20% more current for the same level of gate enhancement, further the '3055 needs to be soldered to a heat sink whereas the IRL520 is a standard TO-220 case and thus (if required) is easier to mount. Were it my choice I would use the '3055 part because the design requirements ~.5 A wouldn't require a heatsink to be used and the "D-PAk" part is easier to use (smaller) and thus more devices will fit in the same PCB area. FYI, the 'Older' IR parts the L in the part number means logic level drive.   IMO

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super-beta NPN switching transistor will do nicely too (no heatsink) - go for Vsat <= 0.1V at Ic >= 1A. 

What PWM clock rate were you wanting BTW?
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I don't know, whatever the arduino rate is. I was thinking about using the Arduino Tiny Library with an attiny84, is that the same rate?

I'm not really sure how to find a super-beta NPN, I'm using Newark.com and it has no filter for saturation voltage.

I just learned that I will require the IRL520 to sink about 1.5A at 5ish volts... How do I calculate how much power it will dissipate and whether or not I need a heatsink?



For a MOSFET that is properly switched on the dissipation is I-squared-R, where R is the Ron value.  For the IRL520 Ron = 0.27 ohms and at 1.5A that means the power will be 0.6W.  A small heatsink will be needed probably.

When off the current is tiny so dissipation can be ignored.  When switching the dissipation is much higher though, so you have to look at that if using high-speed PWM.

There are lots of far superior MOSFETs these days, with Rds(on) of 0.01 ohms or less, which wouldn't even get warm:  1.5^2*0.01 = 22mW

Hint - if you don't need a high voltage device the lower voltage devices will have lower Rds(on) for the same price.
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Hmm ok how about this one?

I'm still not sure what spec tells me whether it is logic level or not... Its Rds(on) is less than .01ohms. The threshold is 1.8v, which I am assuming is the voltage required to switch it on so I need it to be less than 5v, correct? Can I switch it directly with an arduino pin?

When you say that the dissipation is higher when switching, do you mean that actually switching off/on, on/off requires more dissipation than just having it switched on, or did you mean more dissipation switched on?

I'm having a pretty hard time completely understanding these things, I appreciate your help.


The threshold is 1.8v, which I am assuming is the voltage required to switch it on so

No that is the voltage when the FET starts to turn on.
You need to look at the on resistance when you apply 5V. In this case this is a logic level FET.


Jun 10, 2012, 10:16 pm Last Edit: Jun 10, 2012, 10:19 pm by dc42 Reason: 1
For switching 490mA I would use a ZTX851 transistor, base driven from the Arduino through a resistor of about 220 ohms. Simpler (1 less resistor) and cheaper than a logic-level mosfet. But for switching 1A, I would use a mosfet.

[EDIT: I might be wrong about it being cheaper, depending on your supplier. However, for switching 490mA, a BC337 is just about adequate - and they really are cheap.]
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There is one thing not yet considered here and that is that 'pulsed' or PWM control heating effects are "Averaged" in the transistor (Mosfet or Bi-Polar) and dependent on duty cycle which is generally much lower than the steady state 'On' current. The Best choice IMO is the Lowest Rdson and least expensive Logic Level Mosfet you can find. Simple? Yes, IMO

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"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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