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Will this work? DIY TTL driver for BDR-209

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For the cnc laser engraver that I have been wanting to build for a while, I wanted to build my own driver rather than buying one just so I could get more experience and understanding of the inner workings of laser drivers. I like BionicBadger's design from this thread http://laserpointerforums.com/f51/simple-linear-mosfet-dimming-circuit-pwm-70812.html
because of its high efficiency, versatility, and TTL compatibility. The only issue is that I am going to be using a 12v power supply, which is needed for the stepper motors on the machine but is too much for the BDR-209 diode. Would it be possible to use a differential amplifier to reduce the 12v down to about 8v and then use the mosfet driver to regulate current? This scenario is different from handheld lasers with 3.7v batteries, so it could raise some new questions. :thinking:
 

Cyparagon

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The circuit will work as-is on 12V.

Would it be possible to use a differential amplifier to reduce the 12v down to about 8v
Who gave you the idea that op amps or diff amps are appropriate for high power voltage regulation? Stop taking advice from them.
 

Benm

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If you are driving in with PWM you don't need any level translation: 8 volts or 12 volts gate-to-source will drive that mosfet completely, no difference. The only problem that arises is when the maximum gate-source voltage for the mosfet is exceeded - which wold be about 20 volts for the IRF520 mentioned.

But even that doesnt matter since the control signal is first inverted by the pnp 3906 transistor. Since there is a 4.7k resistor between control signal and transistor base you will not damage it with 12 volts at all. Practically speaking that circuit would still work just fine if you inputted 100 volts to it, as long as the 4.7k resistor is powerful enough to dissipate the heat generated.
 
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Who gave you the idea that op amps or diff amps are appropriate for high power voltage regulation? Stop taking advice from them.
I have an app called electrodroid for android used to calculate values for op amps but I have misread it. With a differential amplifier, I would need another voltage of 4 volts to get an 8 volt output, so scratch that.
 
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Thank you both for the input. I am going to construct this driver and use it to drive the BDR-209. Never mind what I said about the differential amplifier or regulating the 12 volts before feeding it into the driver.
 

Benm

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Surely using an expendable test load like a power LED or dummy load is a good idea when developing a new driver circuit - anyone can make a simple mistake and there is no need to blow up something expensive everytime you do.

@cyp:

As far as differential amplifiers as in opamps are a good idea for controlling large constant currents: they are great at it. Obviously not to drive them directly but trough a switching element like a mosfet or (darlington) transistor.

The pictured setup is a classic two-transistor current source, with one of the transistors replaced by a mosfet. It is a reliable circuit really, and counts on the base-emitter voltage of the 'small' transistor as the reference.

While super simple it has the advantage that it is not prone to thermal runaway: when the driver gets hot as a whole, the base-emitter voltage will decrease reducing output current... not by much, but noticable negative feedback none the less. This circuit is a classic and should be in all electronics textbooks, and in fact is in many of them that were still printed on paper ;)
 

Cyparagon

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...with one of the transistors replaced by a mosfet.
For the benefit of others that don't know better, a MOSFET IS a transistor. When Ben said transistor in this case, he meant NPN BJT.

@cyp:

As far as differential amplifiers as in opamps are a good idea for controlling large constant currents: they are great at it. Obviously not to drive them directly but trough a switching element like a mosfet or (darlington) transistor.
You're describing a linear regulator, not an op amp. That's like saying resistors make great oscilloscopes... if you add a few other components.

Besides, it is a waste of time to build a linear regulator out of seventeen parts when you can just buy a single IC for 1/10th the price.
 




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