Add-on Dimming Circuit

[Cyparagon]

^Depends on the pulse frequency and how big the filter cap is.

 

[Trevor]

In this case we're just pulsing a regulator, and do not even have the option of analog control...

Trevor

 

[Benm]

At what frequency does this pwm controller work?

This is especially important if people consider it a safety feature - little safety is added at lower PWM frequencies since the laser still runs at full power during the 'on' part of the cycle, and enough energy can still be delivered in a single pulse to do damage.

 

[Sigurthr]

You'd have to do calculations to determine the total energy deposition per pulse based on pulse time, and then consider that it is still likely more than one pulse could reach the eye in a handheld quasi-stationary scenario. Basically it would boil down to it only looking like it is less dangerous, which in fact it is not any less dangerous.

Still an awesome circuit, and great electrical engineering.

I'd have gone the analog modulation route as descussed in the other thread, but that is just my personal way. Digital electronics are still foregin to me, I'm stuck in about 1978 or so.

 

[foulmist]

The frequency is around 150Hz. I could try a higher frequencies.

This is false. You are using PWM rather than true analog control, which means your laser is running always at 500mA.

However, it is being cycled on and off at a given duty cycle. So, your maximum duty cycle is 99% on / 1% off, and will give you what SEEMS like 99% current.

For example, 50% PWM duty looks like '''_'''_'''_'''_'''_'''_'''_'''_'''_''', rather than ---------------------. The latter would be a true 250mA analog output.

ah thanks for correcting me there :beer:

I am kinda new to this digital circuits also

 

[Trevor]

...but, like Cyparagon said, if you take that digital output and feed it into a filter capacitor, you can get fairly solid analog output. This you could feed into a FlexMod P3 or something, and have true analog control.

Trevor

 

[foulmist]

...but, like Cyparagon said, if you take that digital output and feed it into a filter capacitor, you can get fairly solid analog output. This you could feed into a FlexMod P3 or something, and have true analog control.

Trevor

I could try converting it to analog through a RC filter. :beer:

 

[Bionic-Badger]

I could try converting it to analog through a RC filter. :beer:

No, enable pins aren't meant to work that way. They rely on digital threshold voltages. You might be able to affect the circuit with a very fast PWM though, which could get filtered at the output if it doesn't react fast enough. Not sure how this would affect the DC-DC converter.

 

[foulmist]

No, enable pins aren't meant to work that way. They rely on digital threshold voltages. You might be able to affect the circuit with a very fast PWM though, which could get filtered at the output if it doesn't react fast enough. Not sure how this would affect the DC-DC converter.

ahh, I thought something like that would happen.

I've read somewhere that PWM frequencies between 100 and 400Hz are acceptable for those TPS ICs.

on LM3410 it can reach to 200kHz.

 

[Benm]

I would recommend increasing the modulation frequency: 150 Hz or so is -really- slow and on the border of visible. If you move the beam around you'll also see clear on-off-on stripes on the wall even at slow speed.

Something around 100 kHz would probably be preferable, provided the enable pins on the drivers can handle it. You'll still get some dot/stripe pattern when moving it around really quickly, but its less pronounced. Also, from a safety standpoint, modulating at this rate actually makes a difference, altough i wouldn't rely on it.

 

[foulmist]

I would recommend increasing the modulation frequency: 150 Hz or so is -really- slow and on the border of visible. If you move the beam around you'll also see clear on-off-on stripes on the wall even at slow speed.

Something around 100 kHz would probably be preferable, provided the enable pins on the drivers can handle it. You'll still get some dot/stripe pattern when moving it around really quickly, but its less pronounced. Also, from a safety standpoint, modulating at this rate actually makes a difference, altough i wouldn't rely on it.

Do you think instead of the En the PS/SYNC pin is used for TPS63020 and the frequency be in the kHz/ MHz range? or that would just affect the switching frequency of the driver.. hmm

 

[Benm]

Perhaps, i'm not sure the ps/sync can be used for modulation. The EN response is much too slow though - seems to have a rise time of 150 uS, and i reckon a similar fall off time, so little chance to get much beyond a kilohertz using that.

 

[foulmist]

Perhaps, i'm not sure the ps/sync can be used for modulation. The EN response is much too slow though - seems to have a rise time of 150 uS, and i reckon a similar fall off time, so little chance to get much beyond a kilohertz using that.

hmm it seems that I might end up making too versions of these one at 20kHz(LM3410 and similiar that support higher frequencies) and one at 500Hz (for the TPS)

I am thinking of a 3rd option of PWM to Analogue output but this isn't very clear to me .... What frequency should I use, and if I plan to use it with my driver I have to redesign it to support that analogue control at the feedback pin, correct? :thinking:

 

[scopeguy20]

I really want to try 528 Hz for Red LED and Laser for therapy. Let me know if it can be done? -Glenn

 

[Sigurthr]

I really want to try 528 Hz for Red LED and Laser for therapy. Let me know if it can be done? -Glenn

528Hz what? pulse frequency? Got to the "How to Pulse a Laser" thread, I developed a circuit that gives you 1 to 1000Hz pulse rate for any laser.