# Constant Current Source Simulation Question

I found this constant current source circuit over at instructables.com. This circuit calls for 2N5088BU for Q1 and FQP50N06L for M1, but I replaced those parts with 2N2222 for Q1 and PHT6NQ10T for M1 because that's what I have on hand for simulation. I calculated R2 = 0.5/0.125 = 4 ohms, but in the simulation I got a current of 58.72 mA across the LED instead of 125 mA. Why? Did I use the wrong simulation type, which was time domain (trainsient)? Is it also possible that these custom made parts for PSpice was done wrong?

I'll probably just use a resistor in the actual circuit, but I wanted to understand this.

Thanks,
Mike

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vk6kro
That mx3s LED has about 9.7 volts across it. I looked it up and it seems to be a 10.7 volt device, perhaps with several LEDs in it.

I would just remove it from circuit and try again. 125 mA is too much current for normal LEDs.

AlephZero
Homework Helper
From the data sheet of the MX3S, its max current is 175mA and the forward voltage as 115mA is 10.7V typical, 12V max. So your 10V supply probably isn't enough to drive it at 125 mA.

I calculated R2 = 0.5/0.125 = 4 ohms,
Your 0.5V across the base junction looks too low to turn Q2 on properly. Operating it with currents of nA and pA doesn't look right. R1 = 100k suggests the emitter current in Q2 should be about 0.1mA.

Try increasing R2 by a lot (e.g. 40 ohms) and see if the circuit behaves properly but at a lower constant current than your requirement of 125 mA.

The LED's are rated at 10.7 volts and 115 mA. Since, I lowered the voltage to 10, I thought I should raise the current slightly to compensate. Shouldn't the LED's be in the circuit for a more accurate simulation? The reason I lowered the voltage to 10 is because that's one of the voltages I found in fixed voltage regulators. I may just use a 12 volt regulator and a resistor in the actual circuit.

Alright, raising R2 to 44 ohms lowered the current across the LED to 12.42mA. When I lowered R2 to something like .00001 ohms, the current goes up to around 95mA.

Thanks,
Mike

AlephZero
Homework Helper
The LED's are rated at 10.7 volts and 115 mA. Since, I lowered the voltage to 10, I thought I should raise the current slightly to compensate.
That doesn't make sense. If you lower the voltage across the LEDs, the current through them will be lower not higher.

For a 10V supply you have less than 10V across the LEDs, because they are in series with R2 and M1

Alright, raising R2 to 44 ohms lowered the current across the LED to 12.42mA.
So you have 0.55V across the base-emitter of Q1, which is probably on the edge of the linear region, and a lot more sensible than the 0.23V you had when R2 was 4 ohms.

When I lowered R2 to something like .00001 ohms, the current goes up to around 95mA.
Again that doesn't make any sense for a practical design. R2 should be controlling the current through the LEDs by setting the base-emitter voltage of Q1. If Q1 is completely turned off because R2 is so small, you might as well just connect the LEDs straight to the 10V supply. In fact it would be worth simulating that to see what current they draw at 10V. (My guess is about 95mA!)

If you want to run the LEDs at 115mA, you need a higher supply voltage, say 15V, so there is some headroom for the constant current regulator to actually regulate. Then, R2 should be something like 4 or 5 ohms. When you have the value of R2 to give the current you want, then try reducing the supply voltage to 14, 13, 12, etc and see how the current changes. That will show you the minimum supply voltage you need.

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I did what you suggested and connected the LED directly to the supply. At 10V I got 70.37mA and at 12V I got 177.4mA. I think I understand, I can't make the LED's 'eat' more current than they want for the given voltage? My goal is to get the maximum lumens out of them, that's why I thought, if I lowered the voltage, I should raise the current to balance it out. The LED's will be going into a flashlight. I've attached a simulation with the power source at 12V and R2 at 5 ohms. The data sheet gives the max voltage with 115mA as 12 volts, so is 12 volts too much?

Thanks a lot,
Mike

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AlephZero
Homework Helper
I think I understand, I can't make the LED's 'eat' more current than they want for the given voltage?
That's right. If you repeat the "LED and battery" simulation for more voltages between 0 and 12V, you can plot a graph of current against voltage. It won't be a straight line, but more voltage will always give more current. Whatever circuit you use, the LED will always be operating at some point along that graph.

I've attached a simulation with the power source at 12V and R2 at 5 ohms. The data sheet gives the max voltage with 115mA as 12 volts, so is 12 volts too much?
The data sheet says 12V maximum, 10.7V typical to get 115mA. Unless you were unlucky and bought a component that was right on the margin of meeting the specification, 12V would give you a higher current (but probably not high enough to damage the LED).

Looking at your results, the voltage at the "bottom" end of the LED is 1.228V. So the voltage across the LED is 12 - 1.228 = 10.772V. The simulation of 110mA at 10.77V is pretty close to the "typical" specification of 115mA at 10.7V.

If you increased the supply from 12V to 13V, the voltage at the bottom of the LED will also increase by about 1V to about 2.2V. The voltage across the LED and the current through it will stay about the same. That's how the circuit is supposed to work.

On the other hand if you reduce the supply voltage too much, the circuit can't create more voltage from nowhere to boost the voltage across the LED back to 10.7V, so the current will be less (as in your first simulation).

Try it with a supply of 10, 11, 12, 13, 14V (leave R2 at 5 ohms) and see what happens to the current.

You could increase the supply voltage a lot higher than 12V without damaging the LED, but that would just waste energy making Q2 hot, so it isn't a very useful thing to do in real life.

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I ran the simulations with the other voltages and with 16V the current went up to 112.8mA. I think I'm going to stay with a 12V source. I'm planning to use the CSD17571Q2 transistor from Texas Instruments because it has a lower threshold voltage than the one in the simulation and I want to use it as a switch also, but the model given by TI requires a license for OrCAD PSpice to use so I couldn't make a custom part with it. I don't have \$10,000 for this software.

I just need to know how to use the thermal characteristics given in the data sheets for the transistor and LED. I don't know how to use that information. I'm attaching the thermal graph for the LED and the thermal characteristics for CSD17571Q2. This transistor has a total power dissipation of 2.5 watts. With 12V and 110mA the total power will be 1.32 watts, which is well within the limits of this transistor. Am I looking at that right? Another thing for me to consider is the space where I'm going put this circular circuit board is fairly airtight so I'm planning to add a few holes in the circuit board for air circulation. Maybe I could also add some notches around the outside edges of the board for additional air circulation.

Thanks,
Mike

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