1. The flyback diode is on the wrong place in that schematic, it doesn't protect the transistor when the motor switches off. For that, you need a flyback diode connected in parallel with the motor.
2. IMO darlingtons should almost never be used, because they have high saturation voltage. If the motor draws a substantial current, this results in a lot of power being dissipated in the darlington, so it will get hot and need a heatsink.
A power mosfet is a much better solution in such a case.
If the current is not so high (say 500mA or less), then a single transistor will do the job more efficiently and with less heat generation than a darlington, especially if the transistor is a high gain low saturation type such as the ZTX851.
I agree. The MOSFET would be my first choice as a switch like this. We were talking about transistors though.
divide it by the minimum hfe(gain) of the transistor
QuoteI agree. The MOSFET would be my first choice as a switch like this. We were talking about transistors though.When did a MOSFET stop being classified as being a transistor? That's what the T in MOSFET is all about! Lefty
I agree. The MOSFET would be my first choice as a switch like this.
Quotedivide it by the minimum hfe(gain) of the transistorhFE is a concept for linear amplification and has very little meaning for switching applications. Typically, you design a circuit assuming the switcher is driven to saturation: Ic / Ib < 10.
IC = 1000mahFEmin=100 (between 10ma and 2A)
Mine too, if I was making a PCB for it and could use a mosfet in an SMD package, for example http://uk.farnell.com/diodes-inc/zxmn2f30fhta/mosfet-n-sot-23/dp/1583664RL. Unfortunately, there do not seem to be any medium-current mosfets available in non-SMD packages. It surely wouldn't be difficult to make a 1A or even 2A mosfet in a TO92 package, but all that seems to be available is the 2N7000, which has a continuous drain current limit of only 200mA. Mosfets in TO220 packages are a lot more expensive and physically too large in some applications.
Your transistor would have exploded at that point.
I have used the IRLU3410 quite a bit. Sure, it's over-kill for 90% of what I use it for, but is a decent MOSFET at a decent price (about $0.75) and is much more compact than a TO220.