Constant Current Source Circuit Using Mosfet Series With It

Constant Current Source Circuit Using Mosfet Series With It

Once the voltage has been smoothed by the inductor and capacitor, the Load (an LED in my circuit) will have a voltage across it, and, therefore a voltage across the small resistor (1) it is in series with it will also rise.

To operate the circuit the user will plug the circuit into a power supply; this will keep the load at a constant predetermined current.

Spec (dont worry about these bits yet, i will explain what these numbers mean later) Power supply voltage: 15V,0V,-15V Astable Frequency: 60KHz Astable Space Period: 16s Astable Mark Period: 0.7s Monostable Period: 8s (half the period of the astable) Reference Voltage: 2mV Output current: 2mA Explanation 1.

Block Diagram: Here is a diagram where i have split the project down into manageable chunks, i will explain each chunk individually to help with the build.

Add Tip Ask Question Comment Download Step 2: Stage 1: Astable Chip(s): NE555 Inputs: none, this is turned on when the circuit is connected to the power rails Outputs: Pin 3 (output) goes to the trigger of the Monostable Circuit Diagram (see image) Build and Test Once built test the output pin (pin3) with an oscilloscope and alter the variable resistor to get the pulse width to around 16 micro seconds.

If you do not have an oscilloscope, simply set the potentiometer to the middle of its resistance using a multimeter which should work almost as well Explanation When the system is on, pin 3 of the astable will oscillate between 0V and 5V.

The mark and space of this astable has been set to a small space but a large mark.

Having the output low for a short time enables the circuit to trigger the monostable accurately, and, if the monostables period is set to be very short it wont get triggered twice in one period of the astable.

The output of this circuit goes to the trigger of the Ne555 Monostable.

Every time the astable gives a falling edge the monostable will trigger for a set period of time dependant on its pulse width modulation.

I have set the frequency of the astable to 60 KHz, this should be a fast enough sampling rate for the current to stay constant at all times, and however, it is not so fast that the monostable is triggered incorrectly, or the Mosfet being unable to switch on quick enough.

R12R2)C F 60 KHz C 1nF R1 50K Potentiometer R2 1K R1 1.44FC-2R2 R1 1.44((60x10 3 )(1x10 (-9)))-2000 R1 22K, this value is within the reach of the potentiometer I have, this means it will be easy to trim the circuit to get an accurate result.

The Space (T2) will be set very short, this will make sure that, if the monostable has a short period, it is not triggered twice in one oscillation of the astable.

T20.7(R2)C The Mark (T1) will be longer to accommodate the rest of the period.

T10.7(R1R2)C This means, to get a short T1 (short low pulse) I will need a small R2.

This works out with my equations for the resistors based on the whole period, as, I have set R2 very small.

Outputs: Outputs pulses to mosfet to turn it on and off and allow current to flow for varying amounts of time.

If you do not have an oscilloscope, simply set the potentiometer to approx 15th of its resistance using a multimeter which should work almost as well Your test should look something like image 3, test shows the astable output in blue, the red shows the monostable.

Explanation The monostable will be triggered when pin 2 has a falling edge; this means that every time the astable has a low pulse the monostable will trigger.

When the monostable is triggered the output will stay high for a set period of time.

The period of the monostable will be set to half the period of the astable.

This means that the pin 5 will be able to vary the pulse width either way by the same amount.

I will be using this pin to achieve pulse width modulation which will keep the output current of my circuit at a steady value.

The voltage at Pin 5 is allowed to vary between 0.45 Vsupply and 0.9 Vsupply, the higher the voltage, the longer the pulse width.

To get the correct value for the pulse width, without modulation, I will use the following components: p1.1 x R x C Rp(1.1 x C) C 1nF P 8s R(8x10 (-6))(1.1 x 1x10 (-9) ) 7272 7.2K Again I will be using a potentiometer as a variable resistor (R) in order to get the correct period.

Add Tip Ask Question Comment Download Step 4: Stage3: Mosfet Chip(s): 2N7000 Inputs: Monostable Outputs: Current Set circuit Explanation When the monostable output is high the monostable will conduct between V supply and 0V.

The longer the period of the monostable the longer the Mosfet will conduct for.

Add Tip Ask Question Comment Download Step 5: Stage 4: Current Set Circuit Chip(s): no chips are used in this subsystem Inputs: Mosfet Outputs: Load, Difference Amplifier Build and Test Build but dont test yet, you will blow the LED (or whatever load device your using) Explanation The current set circuit will take the pulses of voltage from the monostable and smooth them out into a smooth voltage that can be used by the LED.