If it can't supply the running current, then it's a new ball game. If it can't supply the starting current, then a big enough BFC from the LM317's output to ground to store the charge to start the pump before the TIP120 kicks in will be required. It's tomorrow, and it looks like Spehro pretty much covered what I was going to, so the only other thing I'd advise you to watch out for is whether the LM317 can supply the starting/running current for the pump motor. I have some other business to take care of so I can't finish this now, but I will tomorrow. More often than not, when you design, you have to work backwards, and since we know the motor's the load and we have to - somehow - drive it, this is one of those cases. Well, first off, the LM137 is a negative voltage regulator and it's a 3 terminal device, so I don't know why that ground - or a positive power supply for that matter - is connected to it.Īssuming you made a trypo and it's supposed to be an LM317, it can't source more than about 1.5A, so if we assume your motor draws about an amp once it's running, we can start to think about the TIP120 as a switch. 1.7 typically)- using that would be more conservative again, but it gets you down into the 330 ohm range and is probably unnecessary for an ordinary design. We ranked and reviewed the top 10 Best Darlington Transistors in 2023. The worst case value is 2.5V at 3A current (vs. High DC Current Gain - hFE 2500 (typ) IC 4.0 Adc. Note that the 1.5V is the typical value from the graph in Fig 2 above, I didn't calculate it. The microcontroller output won't be quite 5V when supplying 4mA, so if you use (4V-1.5V)/0.004 = 625 ohms (pick the next lowest E24 value 620 ohms) and you should be fine. Diodes Inc FZT600BTA NPN Darlington Transistor, 2 A 140 V HFE:10000, 3 + Tab-Pin SOT-223 Item number: 2794E-1222366 Manufacturer: Diodes Manufacturer no. So, at 1A out you need about 4mA base current. That's probably close enough unless you need to operate at low temperatures where hFE will drop. You will get almost no improvement in Vce(sat) numbers by going for more than 1/250 of the collector current, and you may get significantly worse if you go much less, so it's safest to go with 1/250 in the 1A-3A 'wheelhouse' of this part (here is the only place the hFE curve Fig 1 comes in handy), perhaps a bit more at higher or lower currents. The TIP120 incorporates two base-emitter resistors (120 ohms and 8K), which minimize leakage, but reduce the effective hFE (and, more importantly here, give it a kind of threshold that is relatively high compared to an ordinary darlington).įigure 2 in the datasheet shows typical Vce with Ic/Ib = 250, as does the guaranteed Vce(sat) numbers in the datasheet - (both guaranteed numbers are Ic/Ib = 250) so I would suggest that as a useful guideline.
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