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Is this circuit too good to be true

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cyrusthevirus

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So I am near finishing my build to create a sunrise/sunset high power LED lighting system for my aquarium.

As I started to build out the constant current component, I can across the following circuit. It is much simplier than mine. (Mine required an NPN N-Channel mosfet, a mosfet driver, an lm317 and various other resistors.) This circuit seems very simple and very efficient with limited overhead. Is it too good to be true? I have posted the picture, but the original posting is located here:Super-Simple Power MOSFET Linear Current Regulator - CandlePowerForums

Here is the circuit:
**broken link removed**
 
It looks like it'll work.

I'd probably reduce the value of R1 to 100k to improve the transient response or even lower if the PWM frequency is really high.

R1 pulls Q1's gate up turning it on, when the current through R2 exceeds the turn on voltage of Q2 (about 0.6V), Q2 turns on which diverts the current to 0V, therefore reducing the qate voltage to Q1.

When Q3 is turned on, the gate of Q1 is shorted to 0V causing it to turn completely off.

The drop-out voltage will be equal to 0.6V plus the voltage drop of Q1, assuming the current is 0.6/0.82 = 730mA, the datasheet for Q1 says the resistance is 0.27R so the voltage drop-out will be about 0.8V

If you replaced Q1 with a lower resistance MOSFET such as the IRL540, the dropout voltage can be reduced to 0.656V.
https://www.electro-tech-online.com/custompdfs/2010/04/irl540.pdf
 
The current limit value depends upon the forward-bias base-emitter voltage of Q2. Since this voltage is temperature sensitive, the current limit will vary with temperature (≈-0.3% per degree C). So if that amount of change in the current limit with temperature is not a problem in your application, then the circuit should work ok for you. Assuming you are using the current limit to control the LED current, then that amount of variation will likely not be noticeable in the LED brightness level.
 
Another circuit using only BJT's
CurrentSourceBJT..png

A workaround if those transistors won't match properly is to add resistors to emitters for Q1 and Q2.
 
Great. To make this work for my situation, I changed the Q2 to a MPS8098_D26Z which can handle 60V. Here are the specs: **broken link removed** Any suggestions for Q3 so it can handle 60V? I saw a couple of options, but they operate at different frequencies than the 2N3906.

BTW Hero999 you have been a great help for me on this project.
 
If you need to power the circuit from voltages >20V, you need to add a zener diode across the gate of Q1 so it won't get damaged if a surge across the gate exceeds the maximum rating of 20V, a 10V zener will do.

The zener will also mean the voltage ratings of Q2 and Q3 don't need to change.
 
Not to good to be true, I think. Her is just another circuit doing the same using just BJT transistors:
alternative..png

This should be more reliable than the circuit one I posted. And again, it's R1 together with Vbe on Q2 that decides the current.
 
Hero999 said:
If you need to power the circuit from voltages >20V, you need to add a zener diode across the gate of Q1 so it won't get damaged if a surge across the gate exceeds the maximum rating of 20V, a 10V zener will do.

The zener will also mean the voltage ratings of Q2 and Q3 don't need to change.
Click to expand...

Okay here is an updated picture of the circuit. I have several IRL520's sitting around so I am going to use them instead of the IRL540.

I also added values for all of the components.

constant_current&#46.png
 
No, the zener needs to go between the GATE and 0V.

The purpose of the zener is to protect the gate, which has a maximum voltage rating of 20V, look at the datasheet.

If the power supply is going to remain near 60V then you don't need a zener, you can use a 33k resistor which will form a potential divider.
 
Grossel said:
Another circuit using only BJT's
View attachment 41619

A workaround if those transistors won't match properly is to add resistors to emitters for Q1 and Q2.
Click to expand...

while i do not think he cares about good temperature and accuracy, the best would be a dual transistor instead of his Q2 with the other one just using the BE junction as a diode to offset the tempco of the control transistor.

and of course it is that easy...
 
Hero999 said:
No, the zener needs to go between the GATE and 0V.

The purpose of the zener is to protect the gate, which has a maximum voltage rating of 20V, look at the datasheet.

If the power supply is going to remain near 60V then you don't need a zener, you can use a 33k resistor which will form a potential divider.
Click to expand...

Okay how is the diagram below? Is there an advantage or disadvantage of the zener over the resistor? I am not mass producing these so cost between a resistor and zener is minimal?
Constant_Current&#95.png

Ubergeek63 said:
while i do not think he cares about good temperature and accuracy, the best would be a dual transistor instead of his Q2 with the other one just using the BE junction as a diode to offset the tempco of the control transistor.

and of course it is that easy...
Click to expand...

My two concerns are:

1. Loss due to heat. I have eight PWM strings and I am still trying to determine where to mount these circuits in relation to the PS and the PIC.

2. Efficiency. The circuit only has between 1-2V to spare.

Other than these, I am open to using any circuit.
 
cyrusthevirus said:
Okay how is the diagram below?
Click to expand...
The zener is the wrong way round.


Is there an advantage or disadvantage of the zener over the resistor? I am not mass producing these so cost between a resistor and zener is minimal?
View attachment 41652
Click to expand...

A zener means it can be operated over a wider voltage range than a potential divider.

I suggest researching potential dividers and zener diodes and it should become obvious.
 
Hero999 said:
The zener is the wrong way round.
Click to expand...

Okay, I flipped it.

Hero999 said:
A zener means it can be operated over a wider voltage range than a potential divider.
I suggest researching potential dividers and zener diodes and it should become obvious.
Click to expand...

From what I could tell, the zener diodes switched faster and provided a more constant voltage than the resistor. One thing I did come across was a couple of posts showed a resistor in series with the zener. Is this overkill?
 
Grossel said:
Another circuit using only BJT's
View attachment 41619
A workaround if those transistors won't match properly is to add resistors to emitters for Q1 and Q2.
Click to expand...
The disadvantage of a current-mirror, as shown in your attachment, is that you need to put as much current through the control mirror transistor as goes through the controlled transistor. Thus you double the power consumption (assuming the control mirror gets it's current from the same source as the LEDs).
 
Hero999 said:
I suggest researching potential dividers and zener diodes and it should become obvious.
Click to expand...

Maybe an ECE background would help me better, but let's see if I am reading this correctly.

A resistor would provide a set amount based on it's resistance properties (i.e. ohms value) and only resist a certain amount of voltage. A zener is rated for a specific voltage and will maintain that voltage based on it's specs.

For my circuit because I have a fairly constant Vin and Vout, a resistor would work fine. However a zener would provide better flexibility if I wanted to change the PS or the number of LEDs in the circuit. Did I get that correct?
 
As the supply is fairly constant, just use a resistor and it'll be fine.
 
crutschow said:
The disadvantage of a current-mirror, as shown in your attachment, is that you need to put as much current through the control mirror transistor as goes through the controlled transistor. Thus you double the power consumption (assuming the control mirror gets it's current from the same source as the LEDs).
Click to expand...

What voltage output would be needed on the PWM signal? Can you suggest some part numbers or specs to look for?
 
Last edited:
1) What's the normal turn on voltage for a BJT? Look it up if you don't know.

2) I think his criticism is aimed at the circuit posted by Grossel which contains nothing by BJTs, not the original circuit posted by you.
 
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