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Old 05-16-2012, 02:56 AM #1
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Default PWM Laser dimmer

Hello,

I have been looking on ways to dim lasers from their full power to make their output adjustable. The best way I've found is using PWM to control the output. I found a circuit that uses the 555 timer to dim LED's here. DPRG: A Simple PWM Circuit Based on the 555 Timer Could it also be used to dim lasers?
Also, the 555 timer can only allow 200mA of current through. Is there something similar to the 555 timer that can allow 1A through to dim a 445nm laser? Also, can I just directly attach this circuit to a laser diode with driver? If anyone has better circuits that uses PWM to dim laser diodes please share

Thanks!



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Old 05-16-2012, 04:06 AM #2
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Default Re: PWM Laser dimmer

You have the 555 timer's output turn on/off a power transistor between the power supply and the laser driver. if the driver has a lot of lag time or capacitance it will distort the signal though.

The main problem is that PWMing a laser gives the illusion of it being low power and thus safer. This is not what happens, our eyes only perceive it as being lower power. Thus you are more apt to be injured by a PWM'd laser than an analog modulated one.
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Old 05-16-2012, 04:59 AM #3
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Default Re: PWM Laser dimmer

You don't want to drive the laser directly with PWM. Use the signal from the PWM to drive the gate of a mosfet , with the mosfst acting as a switch between the battery and the driver. -OR- if the driver has an EN pin, stands for enable, connect the output from 555 to that pin. Hope this makes sense. I am using this (2nd) circuit in a build that I will be writing up soon. The Ben Boost has an EN pin which is meant for PWM dimming, which s the preferred way to dim LEDs. Sig is correct that it does not increase safety. It is just as dangerous as the full load current at 100 duty cycle.
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Old 05-16-2012, 06:57 AM #4
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Default Re: PWM Laser dimmer

So the driver like the Ben Boost has a mosfet built in? And I guess the PWM is used to control whether the mosfet is on or off and that's the pulse. Also, is it possible to smooth out the PWM signal so that it is an flat wave regarding the safety concern?
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Old 05-16-2012, 01:36 PM #5
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Default Re: PWM Laser dimmer

Ben boost doesn't need a mosfet. It has PWM dimming control built in to the IC. All it needs is a square wave.

There is no way to smooth out the pulse to increase safety. The reason is, even though it is happening hundreds of times per second, each pulse from the PWM generates thousands of cycles of the driver because the driver operates on MHZ frequency not KHZ like the PWM signal.

The main benefit is increased control and much better battery life.
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Old 05-16-2012, 06:34 PM #6
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Default Re: PWM Laser dimmer

You can use something like this. It also ensures you won't exceed your maximum current for the current-passing part of the duty cycle.
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Old 05-17-2012, 04:06 AM #7
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Default Re: PWM Laser dimmer

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Originally Posted by Bionic-Badger View Post
You can use something like this. It also ensures you won't exceed your maximum current for the current-passing part of the duty cycle.
Does the PWM signal goes in W4? When I saw this post before in my other thread about dimming I didn't really understand it but I read about PWM and it seems like a good method. Thanks!

Also, do I use linear drivers for new 3.8mm red 650mm-660nm diodes while boost drivers like the benboost for the blue 445nm diodes since the Vf for the blues are higher than the reds?

I also have a green DPSS module that I want to dim with PWM. However, it already came with a driver. I don't know what type of driver it is. How do I attach the PWM to it?

Thanks everyone for your input!
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Old 05-23-2012, 02:44 AM #8
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Default Re: PWM Laser dimmer

Sorry for this noob question but, where do I connect the ground to? Isn't the ground supposed to be attached to the Earth?
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Old 05-23-2012, 04:04 AM #9
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Default Re: PWM Laser dimmer

Yes, it's a noob question, but probably just because you haven't dealt with basic electronics.

You need to read up on some basic electronics before proceeding. You're liable to break things otherwise.
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Old 05-23-2012, 04:13 AM #10
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Default Re: PWM Laser dimmer

Do you have any suggestions on where and what do read? Would Wikipedia be a good place to start? And yes, I'm not an electrical engineer... I'm a chemist. :P And yes, when I read a little bit more carefully I found

"The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in portable electronic devices such as cell phones and media players as well as circuits in vehicles such as ships, aircraft, and spacecraft may be spoken of as having a "ground" connection without any actual connection to the Earth. This is usually a large conductor attached to one side of the power supply (such as the "ground plane" on a printed circuit board) which serves as the common return path for current from many different components in the circuit."

I got confused by this notion because I'm only used to connecting the - and + ends of a power supply/battery to things

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Old 05-23-2012, 04:56 AM #11
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Default Re: PWM Laser dimmer

Here's a site I had bookmarked for basic electronics. I cannot actually say whether it is good as I don't use it (I don't use specific references, but just look up the specific info via google). Regardless, it should be a good start.

The post above was not meant to be condescending, but really, you might end up breaking a lot of stuff before you get good use out of it. I remember wasting so much money on laser diodes before properly heatsinking them, etc.

As for "ground" itself: it's just a voltage reference point. You simply specify the other voltages in the circuit with respect to "ground"--even negative voltages that are "below" ground. It's called "ground" because that's the "zero potential" voltage level, like the ground would be potential energy in a falling object. The concept is somewhat hard to grasp by some people because people want absolutely measurable quantities, not relatively measured quantities, which is what voltages are.

As for return paths, the return path concept is more the domain of transmission lines/conductors, where a signal/power goes out one conductor, and must return via another. The latter conductor could be the "ground" voltage reference of your circuit; however, it doesn't have to be. For example, the "ground" in your circuit could be some voltage between the voltage of your two power terminals, and therefore the negative terminal would have a negative voltage--with respect to ground. If this is still confusing, see if the site above helps somewhat.
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Old 05-23-2012, 04:59 AM #12
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Default Re: PWM Laser dimmer

Quote:
Originally Posted by Bionic-Badger View Post
Here's a site I had bookmarked for basic electronics. I cannot actually say whether it is good as I don't use it (I don't use specific references, but just look up the specific info via google). Regardless, it should be a good start.

The post above was not meant to be condescending, but really, you might end up breaking a lot of stuff before you get good use out of it. I remember wasting so much money on laser diodes before properly heatsinking them, etc.

As for "ground" itself: it's just a voltage reference point. You simply specify the other voltages in the circuit with respect to "ground"--even negative voltages that are "below" ground. It's called "ground" because that's the "zero potential" voltage level, like the ground would be potential energy in a falling object. The concept is somewhat hard to grasp by some people because people want absolutely measurable quantities, not relatively measured quantities, which is what voltages are.

As for return paths, the return path concept is more the domain of transmission lines/conductors, where a signal/power goes out one conductor, and must return via another. The latter conductor could be the "ground" voltage reference of your circuit; however, it doesn't have to be. For example, the "ground" in your circuit could be some voltage between the voltage of your two power terminals, and therefore the negative terminal would have a negative voltage--with respect to ground. If this is still confusing, see if the site above helps somewhat.
I didn't take it to be condescending! Thanks so much for you help I understand what ground means now I'm not the expert at electronics that's why I'm here for help :P

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Old 09-25-2012, 04:42 AM #13
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Default Re: PWM Laser dimmer

Hello all,

I have made a PWM circuit using the 555 timer. However, I have a little problem. The NPN transistor I used, TIP31C seems to only allow small amounts of current through at low voltages even though the PWM is turned up to maximum (95%) duty cycle. The data sheet for the TIP31C can be found here, TIP31C Datasheet

The PWM circuit works at dimming but only for low currents. Did I use the wrong transistor? If so, which transistor should I use if I want to modulate a M-140 445nm laser? I appreciate all help!

Thanks!
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Old 09-25-2012, 12:19 PM #14
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Default Re: PWM Laser dimmer

Quote:
Originally Posted by Apocalypse View Post
Hello all,

I have made a PWM circuit using the 555 timer. However, I have a little problem. The NPN transistor I used, TIP31C seems to only allow small amounts of current through at low voltages even though the PWM is turned up to maximum (95%) duty cycle. The data sheet for the TIP31C can be found here, TIP31C Datasheet

The PWM circuit works at dimming but only for low currents. Did I use the wrong transistor? If so, which transistor should I use if I want to modulate a M-140 445nm laser? I appreciate all help!

Thanks!
Post an exact Schematic/Drawing of the circuit you built.
It will be easier to find a problem that way and everyone
will be using the same reference points.


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Old 09-25-2012, 03:35 PM #15
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Default Re: PWM Laser dimmer

I built it based on PWM tutorial video in High-def
I find that I need like 15V to get 100mA through. The PWM just controls the square wave and the NPN transistor is supposed to be the switch to turn the current on and off right?

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Old 09-25-2012, 04:50 PM #16
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Default Re: PWM Laser dimmer

Quote:
Originally Posted by Apocalypse View Post
I built it based on PWM tutorial video in High-def
I find that I need like 15V to get 100mA through. The PWM just controls the square wave and the NPN transistor is supposed to be the switch to turn the current on and off right?
Yes the TIP31 NPN transistor is the switch but it must be near
saturation to supply full current/voltage.

If you built the Linked circuit EXACTLY as shown and you are
using 12Volts as shown the circuit should work as shown.

If you had made a change or a mistake then your circuit will
not work the same.

You didn't say how you measured the current nor did you
say what the output frequency of the 555 was during the
current test.

How about a few closeup pics of your circuit... We might see
something you could have missed.


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