For example automotives use the chassis ground as a common rail, and all devices are turned on or off by switching the positive supply. Note - if R2 is on the gate side of R1 it forms a voltage divider that reduces the gate voltage.įor many applications switching the ground rail is not suitable. R2 (typically 10k) ensures that if the control signal becomes open circuit the MOSFET will still turn off. R1 (typically 220 ohm) with the gate capacitance dampens sharp edges that could cause oscillation. Sparkfun (and other suppliers) offer the FQP30N06L (60V 30A TO220 package) at around £0.80 Why have the resistors? The IRLZ44NPBF will take 40A and 55V - price 0.77p Mouser. If driving from a digital circuit such as an arduino you will need to choose a logic level operated MOSFET! In this figure you can see a power MOSFET can be used to turn the motor on and off. You may need to access the registers to choose an appropriate frequency for your application. This has two main advantages firstly that the full torque is available and secondly that there is very little power dissipated in the control transistor, as it is always "fully on" or "fully off".Įach Arduino has pins that can directly generate a PWM signal, but So we can use "pulse width modulation" to control the motor speed. If we speed it up the pulses smooth out, and the motor turns at a speed determined by the "mark to space ratio" of the pulses. If we do this repeatedly, we see the motor pulsing, going, stopping, going. We can use the simple circuit we saw above to turn the motor supply on and off. However, when the motor is running at low speeds lots of voltage is dropped across TR1, wasting battery power and producing lots of heat. The no-load speed is proportional to Vin. Increase Vin, the motor voltage increases.Ĭonnect Vin to the positive 12V supply and you get 12 - 1.5 = 10.5V at the motor. Now if we apply a voltage - say 5V - to the input the emitter voltage will rise to 5 - 2*0.75 =3.5V and the motor will turn. R1=10k makes certain there is no base current. In the absence of a voltage V1 the darlington transistor TR1 (BDX33C again) is off, and no current flows. This is easily done but has the disadvantage that the maximum available torque (driving power) is also reduced. So we can control the speed by reducing the voltage applied to the motor. If the no-load speed is 10,000 rpm with a 10V supply, then it will run at 6000 rpm from a 6V supply. The speed of a dc pm motor depends on the load, and the supply voltage, as shown here. R2 is there to remove base current when TR1 is off - 10K is fine.Ģ: Proportional speed control without feedback We need a base current of 2A / 750 = 3mA so R1 = (5V - 1.4V) / 3mA = 1.2K For this circuit I chose a power darlington type BDX33C (shown RIGHT) which has Icmax=10A, Vcemax=100V, Pt = 70W (on heat sink) and Hfe > 750. Suppliers often have selectors that let you choose the parameters you need. As before Diode D1 prevents a voltage spike that would damage the transistor. When this current ceases the mottor stops running. The transistor is turned on by a positive voltage applied to R1. Here the motor current is being switched by a transistor. The other factor to consider is that the motor may act as a generator if the shaft continues to spin. This is wired so that it is reverse biased when the motor is running, but conducts when the negative spike begins, and "shorts it out". adding a series capacitor C1and resistor R1 across the terminals (interference suppressor) OR.This can be provided as shown here (left) by: If you dont deal with this either the switch will arc (or if its a semiconductor it will fry) or the winding insulation will break down. The same applies to any inductive load such as a relay. When the supply to the inductor is interrupted the energy has nowhere to go, resulting in a negative voltage spike. The windings of the motor are inductors, and store energy. However it introduces two important factors firstly, what happens when you remove the drive current to a motor. The simplest form of control system can often be achieved with a simple switch or relay.
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