A 700 volt transistor that can pass 8 amps in a 12 volt circuit that needs 1/3 of an amp? You need to start your own Thread for that question.How would that work with a BU208? - ISTR: the gain spread is something like 2 to 8.
A 700 volt transistor that can pass 8 amps in a 12 volt circuit that needs 1/3 of an amp? You need to start your own Thread for that question.How would that work with a BU208? - ISTR: the gain spread is something like 2 to 8.
You generally need to include a safety margin - for 0.3A loads a transistor rated at that is obviously working on its limit.The valves each need 300mA; so that would make the base current need to be 30mA, which is high. So if I were to get MOSFETs for the job, is there a certain kind that would work best in this scenario? I would get NMOS type but I mean chip number
2N2222 is rated for 50 mA Ib and 500 mA Ic in saturation. OP is expected 30 mA Ib and 300 mA Ic in saturation. Well within the 2N2222 spec.You generally need to include a safety margin - for 0.3A loads a transistor rated at that is obviously working on its limit.
A transistor rated at 1A would do the job without breaking a sweat.
You also need to make sure your micro is giving enough base current, take the lowest figure in the gain spread quoted and divide your load current by it - your micro must drive *AT LEAST* the figure arrived at. Once again a safety margin is worth having - if its under driven, it won't fully saturate - it'll drop voltage and dissipate wasted energy as heat.
A good choice for MOSFETs would be from the IRF5xx family - you'll most likely need the logic level type, which I believe have an IRL prefix.
The regulator MOSFETs on old PC motherboards are usually equivalent to logic level - but most only have Vds of 30V, some are as low as 20V. So you'd have to protect them from the solenoid back emf.
No, it isn't. With a 2.2K base resistor and a 300 mA load current, the transistor is *not* saturated. Operating a transistor as a saturated switch does not happen automatically. It takes datasheet values and calculation to assure that the circuit performs as intended. If the Arduino output can make 5 V, then decreasing the base resistors to 330 or 470 ohms should confirm that the transistors were not in saturation before. If the voltage across the valve increases, that was the problem.I am looking at the 2N2222 datasheet.
When transistor is the switch, it either Off or On. When it is On, it is in saturation.
You are right, I forgot Vbe(on). I assumed that transistor was working in saturation and acting as a switch based on OP saying that he did hear the valve activating.No, it isn't. With a 2.2K base resistor and a 300 mA load current, the transistor is *not* saturated. Operating a transistor as a saturated switch does not happen automatically. It takes datasheet values and calculation to assure that the circuit performs as intended. If the Arduino output can make 5 V, then decreasing the base resistors to 330 or 470 ohms should confirm that the transistors were not in saturation before. If the voltage across the valve increases, that was the problem.
Separate from that, another output driver option is to replace the 7 output transistors with a ULN2003 transistor array. It has 7 darlington transistors rated for 1/2 amp each, can be driven directly from the Arduino without overloading it, and has the base resistors and output transient protection diodes built in. It was designed to do exactly what you are doing.
If you want to switch to MOSFETs, again it pays to read the datasheets. You will need what is called a logic level MOSFET, one designed to fully enhance ("saturate") with a 5 V gate signal instead of the usual 10 to 20 V. Also, not all MOSFETs have a super-low on resistance. You know your load current. Decide how much voltage you can stand to drop across the drive transistor and use that to calculate the maximum Rdson (MOSFET "on" resistance) you can tolerate. Then shop for parts that are available to you, meet your budget, and meet your requirements.
ak
EDIT: late to the thread, didn't see the 2nd page of posts. Wally already covered on resistance calcs.
That device has a maximum saturation voltage of 1.6V@350mA so that's not acceptable either.................
Separate from that, another output driver option is to replace the 7 output transistors with a ULN2003 transistor array. ..................
Besides what crutschow has said....Separate from that, another output driver option is to replace the 7 output transistors with a ULN2003 transistor array. It has 7 darlington transistors rated for 1/2 amp each, can be driven directly from the Arduino without overloading it, and has the base resistors and output transient protection diodes built in. It was designed to do exactly what you are doing.
I agree that that is on the datasheet, but my experience with the part is that it is way better than that. And I usually shy away from FETs as drivers for something like this. Personal preference and some early bad experiences.That device has a maximum saturation voltage of 1.6V@350mA so that's not acceptable either.
I think a logic-level N-MOSFET is likely the best solution.
Note that if an Arduino is anything like a PIC, the output cam make the rated current but not at a voltage of Vcc. At 30 mA I'd expect to lose a volt or two of output compliance.You are right, I forgot Vbe(on). I assumed that transistor was working in saturation and acting as a switch based on OP saying that he did hear the valve activating.
I am setting Vbe(on) to 0.7 V. Arduino Uno is 5 V system. So.
5-.7=4.3 V
4.3 V/ 30 mA=143.3 Ohm
They would need 143.3 Ohm or a little less to make sure that 2N2222 gets 0.7 volts to turn On.
Too true. Probably would have to do some practical investigation, take the measurements to see what the actual device does. It might just stay at its rated 20 mA and not get anywhere close to the 30 mA.Note that if an Arduino is anything like a PIC, the output cam make the rated current but not at a voltage of Vcc. At 30 mA I'd expect to lose a volt or two of output compliance.
ak
Not really.; two things -2N2222 is rated for 50 mA Ib and 500 mA Ic in saturation. OP is expected 30 mA Ib and 300 mA Ic in saturation. Well within the 2N2222 spec.
Can you actually control the 5 V pin on Uno? I thought it was always On.Thanks for all the comments; I have saved this info for when I come across more problems. I would like some opinions on this circuit I tried. I had a 5V/89mA relay and decided to try that as the switch instead. On the left, I plugged in the Arduino 5V across the relay coils protected by a diode. On the right, I used the Normally Closed pin connected to 12V and then the COM pin connected to my valve and free wheeling diode. I thought Normally Open was the correct configuration for wanting the applied 5V to activate the relay and connect the 12V to my valve, but that worked backwards, so I'm using NO. I didn't experience any voltage drop on the valve in this circuit. The Arduino can suuply the 89mA in this case since I'm using the 5V and GND pins (200mA rating) as opposed to the I/O pins (40mA). I would assume I can find a relay with higher coil resistance (lower current) so I can safely drive it with I/O pins. I would just like some feedback on the idea of doing it this way as opposed to with a transistor. Thanks
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You are correct, I just placed a switch between it and the board to manually control it for testing purposes. When the final project is finished, I will have to use I/O pins to automatically control outputsCan you actually control the 5 V pin on Uno? I thought it was always On.
Is Figure 3 what we should be looking at?I don't know where you live, but you can get this FET at Mouser.
http://www.mouser.com/ds/2/149/FQU13N10L-244077.pdf
Right you are. They were nice to give those curves. The thing to look for as a starting point is that it is a logic level FET. That means it is made to turn on fully at 5 volts.Is Figure 3 what we should be looking at?
The Vgs=5 volt graph?
It looks like the graph starts at 1 A and at that point the Rds(on) is something like 0.17-0.18 Ohm. The graph also looks "flat".
And the table says Rds(on) for Vgs=5 V, Id= 5 A is 0.158 to 0.2 Ohm. Worst case scenario then 300 mA*0.2 Ohm=0.06 V, 12-0.06=11.94 V across the valve.
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