Is this current drop expected? I don't see anything in the specification sheet of the TIP120 that lists an internal resistance that would be limiting the maximum current. The lower the value, the slower it will spin. The higher the PWM value, the faster the motor will spin. When PWMing a transistor, it's similar to pulsing an LED. This example uses a TIP120, which can switch up to 60V at 5A. Read item description or contact seller for postage options. Description: TIP120 Darlington transistor, a component that can easily provide you with much higher current gain values. The transistor in this example completes the motor's circuit to ground. I'm pretty sure the transistor is getting switched on all the way as I've experimented with everything from 220 ohm to 22 kilohm resistors on the base to the controlling voltage and the resistance to the base of the transistor isn't having any significant effect on the amount of current flowing through the magnet (collector to emitter). 20Pcs TIP120 TO-220 Darlington Transistor Npn li Condition: New Quantity: 5 available Price: £2.63 Buy it now Add to basket Best Offer: Make offer Watch this item Returns accepted Postage: May not post to United States. When I use the TIP120 to switch the current on/off the most current I can get through the electromagnet is around 240 mA, a 50 mA drop. If I hook the electromagnet directly up to my bench supply I can get 290 mA of current (it has a 39 ohm internal resistance). The circuit is working, but it appears the TIP120 is limiting the amount of current that is able to flow through the electromagnet. ![]() I'm using the TIP120 to switch on/off a 12 V electromagnet. The TIP120 Darlington transistor is actually a dual-transistor configuration made by two bipolar transistors connected in such a way that the current amplified. I'm having an issue with a TIP120 I'm using limiting current flowing through it.
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