Use 2N4401 transistor to drive 59mA stepper motor - Page 1
Forum section: electronics / general

author: circuit12
author_rank: Newbie
timestamp: July 21, 2018, 02:35:37 am
content: A while ago I asked a question about driving a stepper motor using a 2N5550 and was advised that it wasn't really capable/a good idea. I received a good answer but would like a bit more information really so that I can make these decisions for myself in the future.2N5550On the 2N5550 transistors PDF it says it can drive up to 600mA (DC) under the maximum ratings (am I interpreting this correctly?):Ic Max ratingIf this is correct, why does the DC Current gain graph only show up to 100mA, is this just because it can handle 600mA but would be almost pointless to actually use it for that because of the low gain at that level?hFE2N4401So this brings me to the 2N4401. The2N4401 applicationalso says the max collector current can be 600mA (again, am I interpreting this correctly?).Now the current gain graph on this transistor shows it going up to 300mA however it says "pulsed" in the title of the graph and there doesn't seem to be a graph for continuous so I'm a little confused why this might be? I suppose driving a stepper motor would come under the category of pulsed though?So assuming that this is suitable for driving my 259mA stepper motor I would need to supply the base with about (259/150) 1.7mA (259 being the number of mA needed by the stepper motor and 150 being roughly the current gain (derived from the graph)?), is this correct?I understand that from my other question I should really use a MOSFET as they are better suited to the job, I just have a handful of these 2N4401s kicking about that I thought I could test my code and theory with.Thanks in advance and I'd appreciate a response to even the little questions I raised like the "am I interpreting this correctly?" ones, just to reassure me that I'm on the right lines (or not).

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2018, 08:54:28 am
content: Just a few points:The entire operating area of a transistor is seldom given on the data sheet.It's bad practise to run a component, continuously at its maximum ratings.The higher current gains are only specified with a relatively high VCEof 2V or 5V, which will be far too higher voltage drop for your application.1.7mA of base drive will be nowhere near enough for a collector current of 259mA. To saturate a BJT s ratio of 1:10 between the base and collector current is normally recommended, but it can be lower or high, depending on the part and application. Quickly looking at the data sheet for the 2N4401, you could probably get away with a base current of1/20of the collector current, which would be 13mA.http://www.farnell.com/datasheets/661741.pdf

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2018, 12:15:11 pm
content: Quote from: circuit12 on July 21, 2018, 02:35:37 amThe 2N5550 is intended for higher voltage applications.QuoteThat is right.QuoteThat basically sums it up. Datasheets usually only include specifications suitable for the intended applications. For instance the 2N5550 datasheet specifies current gain from 1 to 50 milliamps while the 2N4401 datasheet specifies current gain from 0.1 to 500 milliamps.QuoteYes, and it even has specifications for operation at 500 milliamps.QuoteYou will need to provide a link because my 2N4401 datasheet does not have that.Pulsed usually means that the transistor is operating at high power dissipation so the test has to be short.QuoteMore like 13 milliamps like Hero999 says. Current gain falls at high collector currents.QuoteThere is nothing wrong with using the 2N4401 in this application.