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I am trying to build a driver for my M140 laser diode. I made a circuit following a guide. Then I made a test load. When I change the pot value to the minimum I get a maximum current of <100mA on the 1 ohm resistor.

Can someone please explain why the maximum current is only <100mA?

Thank you in advance!

Laser driver

Laser driver

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  • \$\begingroup\$ Assuming it's not a pinout mistake, what was your test load? \$\endgroup\$ – George Herold Jul 23 '14 at 13:08
  • \$\begingroup\$ I used a power supply and it showed aprox. 90mA and also got the same value on the multimeter when the pot resistance is at minumum. \$\endgroup\$ – disoderly Jul 23 '14 at 13:38
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Assuming you are using a TO220 package for your LM317, your pinout is wrong.

1 - Vadj

2 - Vout

3 - Vin

Also, just as a bit of friendly advice, running an amp through most pots is a bad idea. The wipers will not handle it in the long run. The failure mode will be an open circuit at the wiper, which in your circuit will produce as much current as the LM317 can produce. In this case, that is not enormously more than you want, so you may be alright, but it's still not a good idea.

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  • \$\begingroup\$ Thanks for the response! The pinout is correct, it is wrong in the schematic (sorry). I know that the pot would not handle the current for a long time that's why I am just testing :) Any other suggestion on what could be the problem? \$\endgroup\$ – disoderly Jul 23 '14 at 13:33
  • \$\begingroup\$ I suspect that your dummy load is not right. By my calculations, at 1.25 amps the voltage out of the LM317 needs to be 6 volts, and an LM317 will not do that. It needs ~ 2 volts across it at that current. Try increasing your 7 volt supply to 9 volts. \$\endgroup\$ – WhatRoughBeast Jul 23 '14 at 15:51
  • \$\begingroup\$ Tried supplying the driver with 9V and when the pot is at minimum, the current on the test load resistor is 0.5A. Should I just try increasing the power supply voltage till I get my wanted ~1.5A? \$\endgroup\$ – disoderly Jul 24 '14 at 9:45
  • \$\begingroup\$ No. Leave the power at 9 volts. Just get rid of the pot and see what happens. You should get 1.25 amps. And trying for more is slightly iffy. The LM317 is rated for a maximum of 1.5, and trying for absolute max of any device is asking for trouble. And anyways, going from 1.25 to 1.5 amps is only a 20% increase, and it's hard to see how that will benefit you. While doing all this, I do hope you have properly heat sunk the LM317, and are checking its temp. You may be getting near its limit anyways. \$\endgroup\$ – WhatRoughBeast Jul 24 '14 at 12:38
  • \$\begingroup\$ Actually, I just realized your BIG problem. You already have a 1 ohm resistor in your current path - the one associated with the LM317. You don't need the one you've added to your dummy load. Get rid of the resistor in the dummy load, and measure the voltage across the one in the LM317 to determine current. Then try the circuit with 7 volts. Also, if you're not getting the current you should be, use your meter to check that the 7 volts is good. Gad, I should have realized this sooner. \$\endgroup\$ – WhatRoughBeast Jul 24 '14 at 14:57
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Remove the pot and 1 Ohm rsistor and replace both with a 0.8 to 0.9 Ohm resistor, preferably Mil-Spec and at 1% tolerance. Has to dissipate at least a watt. Don't be a Dodo bird and do this with 7 Volts present; you'll kill your laser! Ramp up voltage from your power supply until you achieve approximately 3.9 to 4.1 Vdc; maximum light output shall be achieved here. Things to remember... The Equation:

It=Vref/R1 (Ohms law).....................................eq. 1

Since Vref is always 1.25Vdc, then any resistor that's also 1.25 Ohms will deliver one Amp to the laser:

1A = 1.25Vref/1.25Ohms ...................................eq. 2

So, if you want max brightness, then shoot for about 1.3 Amps, or a resistor that's just shy of One Ohm.

No pots, nothing outside a one ohm resistor.

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