non linear hfe npn transistor?

I recently bought an smd transistor thinking it would work directly driven from the arduino, as I thought the hfe was 100 so 10ma could saturate it at 1a, however after looking at the datasheet I notice that at 1a current the hfe is 30? Does this mean that I can't directly drive this from an arduino pin because then it will take ~33ma , which I can't alot that much, atleast id rather nor

Almost foegot datasheet

Fzt491 its called , sorry I guess my phone doesn't want to upload

Hfe is not constant, this is true. Usually drops off quite a lot as current reaches max, and tends to drop off at low currents a bit too.

33mA is pushing it. Find a better transistor? Modern (small) switching transistors can be quite good up to 1 or 2A. I've used STX724 and ZTX851 before, min Hfe's at 1A are 80 and 100 respectively, and they have ultra-low Vsat.

Or use a logic-level MOSFET and finesse the problem?

I was looking at those, but slightly confused on one part the vgs(max), I know to look for low vgs(th) and low rds(on) but what does the max mean? I got. One where the max was 4, the 5v one had too high rds, it should still work with 5v out from a digital pin right?

If vgs Max is 4V, then driving with a 5V output is not advised. Use a resistor divider, like two 3k resistors perhaps?

the datasheet says min 2, typ 3, max 4 so I should aim for 3v? Or 4 to ensure its fully on?

the datasheet says min 2, typ 3, max 4

But you haven’t told us the FET part number.

IRF7351TRPBFCT-ND srry about that, this is the one I picked

Actually, srry to be slow to realize this, but im thinking now th max means that the threshold value at max can be 4 volts? Mosfets confuse me, now im thinking I may have made a bad choice, maybe I want a low max threshold to guarantee. Its on all the way @ 5 volts?

/Is this abetter choice? FDS9926ACT-ND

IRF7351TRPBFCT-ND The data sheet measures the device with a 10V GS voltage, so clearly 5V is not going to damage it. The data sheet also says: "20V VGS Max. Gate Rating" What the data sheet says about 4V is that you are guaranteed that 4V will turn the device on, and in all cases, at least 2V is required before the device turns on. Driving it with 4.5-5.0V (as you get out of the Arduino/AVR) might be a little too little. The device has nice low on-resistance -- but notice that they measured that with 10V into the gate!

FDS9926ACT-ND This seems to be more efficient at 4.5V. However, it doesn't handle as much voltage (20 V DS only -- for a 12V motor, I imagine this is OK) and has a slightly higher on resistance at the rated voltage. If the original part has an on resistance at 4.5V that is competitive with the latter device, I'd probably stick with the former. (Looking at Fig12 of the original data sheet, you really want to drive this with 6V or so, though) http://www.irf.com/product-info/datasheets/data/irf7351pbf.pdf

Btw: Your part numbers are the Digi-Key specific part numbers. The actual part number is slightly different, as found in the data sheet.

NO!! Vthr is not the voltage at which it turns on, its the voltage at which some specified TINY current starts to flow. Its the threshold voltage that you plug into the FET equations.... Here the datasheet is specifying Vthr for a drain current of 50uA (microamps). For a power MOSFET 50uA is definitely in the "OFF" category, not the "ON" category!!

If a MOSFET datasheet only quotes the Rds(on) at 10V (as here), then you must supply 10V to the gate to get it properly switched on.

Logic level MOSFETs have Rds(on) specified at Vgs = 4.5V or thereabouts. (And Vthr will be more like 0.5 to 1.0V)

IRLB8743 RDS(on) 3.5m? Typ, 4.2m? Max @ VGS = 4.5V, ID = 32A.

$1.41 @ Digi-Key or Mouser.

MarkT: If a MOSFET datasheet only quotes the Rds(on) at 10V (as here), then you must supply 10V to the gate to get it properly switched on.

That data sheet had a nice diagram of resistance as a function of gate voltage, which is where 6V came from. The 10V number is for max efficiency.

Not quite, that graph will be a "typical" performance curve, not guaranteed performance for all devices at all temperatures/loads. There is significant device spread of Vthr in MOSFETs (several volts), note (hence the min 2V, max 4V - so all voltages could be +/- a volt or two out worse case from that curve.)

Basically ignore all specs that aren't "min" or "max" if you don't want to gamble on being lucky.

Thank you for the help, im just starting to get the hang of mosfets im gonna see about that one suggested,