LM1117 Letting supply voltage through when switched off. - Page 1
Forum section: electronics / beginners

author: abdullahseba
author_rank: Frequent Contributor
timestamp: October 28, 2017, 08:42:31 am
content: HiI have a 2N5551 transistor switching an LM1117 3.3v regulator to turn on a laser diode.The transistor is controlled by an Arduino. When I set the transistor to HIGH I get 3.3 volts out. But when set to LOW I get the full 9 volts out of the regulator instead of been truned off.What am I doing wrong?Thanks In advance.

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2017, 08:46:39 am
content: You need to go and study the LM1117 datasheet. A regulator ground pin is never an enable pin.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: October 28, 2017, 09:26:45 am
content: It is turned off, ie not regulating which is what that circuit will do.Think about what happens. How can anything on that regulator not be at 9 volts if the ground connection is removed?

author: abdullahseba
author_rank: Frequent Contributor
timestamp: October 28, 2017, 09:29:08 am
content: Quote from: Mjolinor on October 28, 2017, 09:26:45 amGuess I never thought of it like that just assumed it was not passive. Thanks.

author: janoc
author_rank: Super Contributor
timestamp: October 28, 2017, 09:47:15 am
content: Quote from: abdullahseba on October 28, 2017, 09:29:08 amWell, it is not passive. By lifting the GND pin from the ground you are affecting the feedback circuit of the regulator. In fact, there are some circuits around which put a pot in the GND pin in order to adjust the voltage.The only difference between the adjustable version of this regulator and the fixed voltage version is that the fixed version has an internal resistor in the GND pin. The adjustable doesn't have (and it calls the pin ADJUST instead).Either get a regulator with the enable function or put the transistor in series with the input voltage instead.Apropos, you have mentioned a laser diode - unless the diode has its own regulation circuit already (i.e. it is an integrated laser module and not a bare diode), this circuit will kill your diode in no time.Laser diodes are similar to LEDs, they absolutely need constant current supplies and power feedback. The current must not exceed the maximum current even for a microsecond or the diode blows (or, more exactly - it will still dimly glow but won't lase anymore because the optical surfaces got damaged. Your expensive laser is now a crappy LED). Constant voltage supply will not work here because as the diode heats up, the current will change and you may exceed the limits.Someone at work has managed to blow through several expensive IR laser modules for a touch screen because they were trying to power them from a constant voltage supply. The modules worked for a while - until they suddenly didn't anymore.You may want to look here, especially if your laser is expensive:https://www.repairfaq.org/sam/laserdps.htm#dpstoc

author: abdullahseba
author_rank: Frequent Contributor
timestamp: October 28, 2017, 11:42:40 am
content: Quote from: janoc on October 28, 2017, 09:47:15 amYeah it has a built in controller thanks for the heads up.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2017, 01:01:17 pm
content: If you use the adjustable version and connect the transistor to the adjust pin, the regulator's voltage will drop to 1.25V when the transistor is turned on. That will probably be low enough to turn the laser off and reduce the current draw to a minimum.The LM1117 seems overkill for use with a 9V supply. I'd use the cheaper LM317, as a low drop-out regulator is not necessary.