2012 DSP Laser Firing on Reset/Power Down

[hilo90mhz]

It turns out the input impedance of the laser power supply is low enough that the small current blip I was seeing with my logic analyzer from TTL pin on AWC608 is not large enough to actually fire the power supply. All is good if you modify it as Marco says.

To fix the PWM value being too high for these K40 power supplies:

The output of the AWC608 PWM pin is pulled high with a 1K resistor which means if you use a 1K variable resistor you will be limited to 50% power even with the variable resistor turned all the way up. with my K40 machine I have found that if I put an additional 1K in series with the variable resistor the scale on the variable one is perfect! IE all the way to the right gives me 15MA with the AWC at 100% duty cycle. so you can still use the variable resistor to manually lower the current for quick adjustments at run time.

** Update **
The 1K resistor in series with 1K POT only works right with 10,000 PWM frequency - if I set the PWM frequency to 30,000 the laser power output is much lower with only 5mA at 100% power.. so make sure you have your PWM value at the final value you want to use when choosing the correct series resistor. Other power supplies may have different impedance and require a different resistance for proper calibration.

[SScnc]

hilo90mhz, thanks for your researching this, very interesting, this is the kind of info I was looking for. So if I remove the components you specify, the TH line will work properly ?

Also, where do you have your DSP panel mounted ? (as I see in your pic your original controls are still in place)

Last, so is it advisable to keep the OEM POT in circuit as you show ? It looks like everyone else has removed it.

Thanks again,

Steve

[Gadroc]

Thanks for the info Chester! I'll be adding back in my series resistor and doing the component removal this weekend. Also got my air assist nozzle so another fun weekend of tweaking and aligning

[hilo90mhz]

Yes the laser firing on power off problem will be fixed if you follow Marcos directions on this page: viewtopic.php?f=60&t=902

My DSP is stilll laying on the floor next to the machine I just got everything hooked up to test yesterday.. Haven't decided yet where to mount the control panel, I will probably put it above the existing panel with the POT and current meter.

I have updated my previous post with new findings on the current level in regards to the PWM frequency setting.