If you re-designed it to use a PNP and an NPN as output transistors, and changed it to run completely off 5V, you might get 4.6 volts across the motor at low current.
These diodes will prevent the motor voltage from rising above or below ground when the transistors switch.Ĭonnecting the top of the 1.5K resistor to +5V instead of +3.3V will help, it should buy you more output swing.
Repeat with two more diode for TK4 and TK5. Connect another diode with the anode on the emitter of TK2 and the cathode on the collector of TK2. Connect one diode with the anode on the emitter of TK3 and the cathode on the collector of TK3. You also need to add protection diodes to protect the transistors from the inductive spikes form the motor. If your driver can output 3.3V at 8mA, RB1 = (3.3 - 1.2) / 0.008 = 263 ohms or 270 for the nearest 5% value. Those two points will not rise much above 1.2 volts (Vbe1 + Vbe3). Not much.Īlso, you need to add current limiting resistors between your driver circuit and the bases of TK1 and TK6. Using 100 as the current gain again, TK3 will pull out of saturation around 1.1mA * 100 or about 110 mA.Īt 100mA for a motor load again, the high side would be about 1.2 volts and the low side would probably rise to 0.6 volts, giving you about 0.6 volts across the motor. You are dropping some voltage across the CE junction of TK1 and the 1K resistor to ground gets some of the current, so use Ib3max = ((3.3 - (0.2 + 0.6)) / 1500) - (0.6/1000) = 1.7mA - 0.6mA = 1.1mA. If TK1 turns on, the voltage at the base of Tk3 will be about 0.6V.
The lower transistors will saturate at low current. If you were trying to pull 100mA out of them (not much for a motor), it would drop down to around 1.2 Volts from the base current dragging down the voltage through the 1.5K resistor (assuming a current gain of 100 which is worst case for those transistors as I remember). The highest that the emitter of the top two transistors could be in an unloaded condition is 3.3-0.6V or 2.7 Volts.
The top two transistors won't saturate, they are running in emitter follower mode.
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