DDL driver switch

Hello all,
I'm building a DDL driver which will be turned on and off by a microcontroller. I understand that the power switch must be between the battery and the driver. This is so the lm317 is never operative with no load, correct? For this reason an N-channel Enhancement MOSFET in this configuration WOULD NOT WORK as this would only disconnect the load from the driver.http://www.electronics-tutorials.ws/transistor/tran21.gif(replacing the lamp with the DDL circuit)

Using a P-channel MOSFET will work, however this would be negative logic:http://www.electronics-tutorials.ws/transistor/tran59.gif(laser on when Vg is LOW)

Is there any way to control a DDL driver using a MOSFET with positive logic?

If the LAMP is replaced by the DDL circuit, and the DDL circuit is then powering the LD itself, then I don't see the flaw in your first circuit (conceptually). I'm not well versed in the practical use of MOSFETs, but based on my understanding of how they work and what they do, the first approach seems conceptually sound to me.

I think its easier to just eliminate the LM317 and use just use a transistor based constant current source and hook it up to your PWM micro-controller.

Here's a link that might help you: The new stuff!! Constant Current Source #1

rhd said:

If the LAMP is replaced by the DDL circuit, and the DDL circuit is then powering the LD itself, then I don't see the flaw in your first circuit (conceptually). I'm not well versed in the practical use of MOSFETs, but based on my understanding of how they work and what they do, the first approach seems conceptually sound to me.

Click to expand...

My understanding was that in a DDL driver the LM317 is set up in a constant current mode, adjusting the voltage in order to keep a constant current flowing between OUT and ADJ. Disconnecting ground would turn off the LD, and thus removing the necessary current draw on a lm317. I guess I'm not very familiar with how a lm317 functions though.

Jufran88 said:

I think its easier to just eliminate the LM317 and use just use a transistor based constant current source and hook it up to your PWM micro-controller.

Here's a link that might help you: The new stuff!! Constant Current Source #1

Click to expand...

That sounds promising actually. Thank you. That instructable isn't exactly what you're proposing right? Just a starter point? I adjusting the duty cycle controls the current while the amplitude adjusts the voltage?

sprocketme2 said:

My understanding was that in a DDL driver the LM317 is set up in a constant current mode, adjusting the voltage in order to keep a constant current flowing between OUT and ADJ. Disconnecting ground would turn off the LD, and thus removing the necessary current draw on a lm317. I guess I'm not very familiar with how a lm317 functions though.

Click to expand...

It should be perfectly safe to kill the GND into the LM317. I think I understand your concern. But by disconnecting GND, you're also opening the the DDL driver's input circuit, so the IC stops doing its work. What you don't want to do is open the DDL's output circuit (to the diode) while it's working.

So you should be fine to disconnect either the Driver+In or the GND.

You'll need to calculate for the maximum set current with a sense resistor, R3.

Jufran88 said:

You'll need to calculate for the maximum set current with a sense resistor, R3.

Click to expand...

this looks like the best option for me. I'm guessing this will also be pretty efficient, and allow me to control the power of the laser within software. thank you

No problem, it's all in the link I posted. :beer: