Understanding Mosfet Switch Functions
Under previous posts, we have already seen how a bipolar transistor, NPN or PNP, can be used as a single pole, single throw switch. The transistor operating in saturation region with voltage across the switch being about 0.2 volt represents the ON-condition of the switch. The transistor operating in the cut-off region, where the only current that can flow through the switch is the collector-to-emitter leakage current represents the OFF-condition of the switch. We also demonstrated how an SCR can be used as a single pole single throw switch of the latching type. Another very important solid state device that can be usd as a switch in a way similar to the operation of a bipolar transistor is the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) briefly mentioned in the last post.
In the present project activity, we shall experience the functioning of a MOSFET as a switch. Before we do that and go ahead with the description of the project circuit. It would be worthwhile mentioning a few important points about this device. First of all, we should remember that a MOSFET is a voltage-controlled device unlike the bipolar transistor, which is a current controlled device. Secondly, there are two categories of MOSFET namely the ENHANCEMENT type and the DEPLETION type. In case of the former, that is, the ENHANCEMENT type, the device conducts only when the gate-to-source voltage exceeds a certain minimum voltage called the threshold voltage (Vth). The device remains in cut-off region if the voltage is below that. On the other hand, a DEPLETION type device is usually conducting for a zero gate-to-source voltage and it can be switched to the conducting state by applying a gate-to-source voltage of a suitable magnitude. Thirdly, in each of the two categories, we have the N-channel and P-channel devices and ENHANCEMENT type MOSFETs are preferred for switching applications. Lastly, an N-channel ENHANCEMENT MOSFET can be switched ON by making gate terminal more positive than the source terminal by a voltage greater than the threshold voltage and a P-channel device can be switched ON by making gate terminal less positive than the source terminal by a voltage greater than the threshold voltage.
CIRCUIT DESCRIPTION
The first part of the circuit starting from left and comprising of R2. R4, R3, LED-2. MOSFET Ql and switch SW-2 illustrates the use of an N-channel ENHANCEMENT type MOSFET as a switch. It can be demonstrated that this device behaves like a single pole single throw switch between drain and source terminals. The second part of the circuit comprising of R5. R6, R7, LED-3. MOSFET Q2 and switch SW-4 can be used to demonstrate how a P-channel MOSFET can be used as a switch.
The third part of the circuit consists of an astable multivibrator circuit built around timer IC 555. The On-time of the output waveform appearing at pin-3 of the IC is given by 0.69R8C2 and the OFF-time of the output waveform is given by 0.69R9C2. The component values here have been so chosen as to produce ON and OFF times of 1 second each. The output waveform is applied to the gate terminals of the two MOSFETs through switches SW-3 and SW-5. SW-3 and SW-5 allow us to selectively apply the waveform to the two gates if desired.
The circuit operates on a 9-volt battery, which can be connected to the circuit through switch SW-1 .A glowing LED-1 indicates the connection. Also, glowing LEDs LED-2 and LED-3 respectively indicate conduction of N-channel and P-channel MOSFETs.
The switching action of the two MOSFETs can be demonstrated through the use of switches SW-2 and SW-4. The repetitive switching action of the MOSFET can be demonstrated through the use of switches SW-3 and SW-5.
CONSTRUCTION GUIDELINES
The PCB layout and the components layout are respectively shown in Figs 3.2 and 3.3 respectively. The circuit can however be wired on a general purpose PCB without any problem.
| Parts List | Specification |
| Resistors | All Resistors are carbon film or carbon composition type |
| R1, R2, R5 | 1K,1/4W |
| R3, R6 | 150 ohm, 1/4W |
| R4, R7 | 2.2K, 1/4W |
| R8, R9 | 680K, 1/4W |
| Capacitors | |
| C1 | 0.01 μF (ceramic disc) |
| C2 | 2.2μF (Tantalum) |
| C3 | 0.1 μF (ceramic disc) |
| Semiconductors | |
| D1 | 1N4001 |
| LED-1 to LED-3 | Red LEDs (Other colour LEDs like green or yellow can also be used) |
| Q 1 | MOSFET type IRF510 or equivalent |
| Q 2 | MOSFET type IRF9520 or equivalent |
| IC-1 | Timer 555 |
| Miscellaneous | |
| SW-1 | Miniature toggle switch |
| SW-2 to SW-5 | Parts of a 4-pole DIP switch (Fig.3.4) |
| Battery | 9V DC battery (Fig. 3.5) |
| Switches SW2 to SW5 | ON/OFF switches |
| Solder metal, multi-strand wires,General purpose PCB (If needed) etc | |
TESTING GUIDELINES
There are two different experiments that can be performed with this simple circuit. These includes:
- Use of N-channel MOSFET as a switch
- Use of P-channel MOSFFT as a switch
Use of N-channel MOSFFT as a switch
Initially keep all switches as open. Close SW-1 to connect the battery to the circuit. Glowing LED-1 indicates battery connection. Closing SW-2 now lights LED-2 indicating that MOSFET Q1 is conducting. LED-2 goes OFF if the switch SW-2 is opened. Opening SW-2 and closing SW-3 can test repetitive ON and OFF condition of the MOSFET. The timer output, which is HIGH and LOW for approximately one second each makes the LED-2 glow and extinguish alternately for one second.
Use of P-channel MOSFET as a switch
Initially keep all switches as open. Close SW-1 to connect the battery to the circuit. Glowing LED-1 indicates battery connection. Closing SW-4 now lights LED-3 indicating that MOSFET*32 is conducting. LED-3 goes OFF if the switch SW-4 is opened. Opening SW-4 and closing, SW-5 can test repetitive ON and OFF condition of the MOSFET. The timer output, which is HIGH and LOW for approximately one second each, make the LED-3 glow and extinguish alternately for one second.
If SW-1. SW-3 and SW5 are only closed. LED-2 and LED-3 are observed to glow alternately for obvious reasons.
NOTE: Remember that in case of N-channel MOSFET. the device conducts when the gate terminal is made HIGH and the source terminal is LOW. In case of P-channel MOSFET, the device conducts when the gate terminal is LOW and the source terminal is HIGH.
Written by arjun on December 8th, 2009 with
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