D9 Max Current Output

Hi All

Does anyone know what the max current output on Pin D9 is as it powers my 3.5 watt laser fine but not my 8.5 watt laser. I am getting only 2 Amp reading from D9 and I need 5 Amp

My PSU is a 12v 20A.


Well your laser should have it’s own power supply and driver the ttl or PWM should just be a trigger. This is the correct way.

Depends on your board but typically fan and hotend ports share 5A, bed port gets 11A.

Hi Ryan

I can power it externally, was just trying to avoid this as my system has plenty of power available thinking D9 would output 5A. My laser does have a 12v TTL / PWM line.

If I was to power it externally would I just attach the TTL/ PWN Line to either the + or - on D( or is it not that simple?


Oops forgot to say its a Ramps 1.4 board running on a Mega



You should have a power and a PWM port, careful ttl is usually 5V. There is a write up in the how too section on using a laser with a ramps board.

Hi Ryan

I already had a 3.5 watt laser up and running and working with my MPCNC, it run directly of ports D9, basically laser ground to D9 - and both laser positive and TTL line into D9 +. Worked great

I upgraded my laser and wired up exactly the same but noticed that the laser wasnt operating at full power, stuck a meter on it and D9 was only supplying it 2A and not 5A for full power.

Yes most lasers have a 5v TTL line but the ones I have both use a 12v TTL line.


If you bought an imported Ramps board, that is the issue, the reason I no longer carry them. It seems every single component has been substituted for a vastly inferior part. The 5V regs pops when trying to power the lcd lights, the mosfet has apparently now been swapped as well.

As for how to hook it up there has been three threads about it in the last 3 days. These are not the lasers I have so I have no experience with them. From my understanding this is not how they are properly wired but seem to function for all of you, a bit…at half power or with poor grey scale. The power supply should be external and proving the laser with all the current it needs, the trigger should just be a high speed trigger, not trigger and power. I very well could be wrong.

You’re basically correct, Ryan. Since this particular instance is a RAMPS setup, I’ll offer this… 12v power and TTL inputs on laser controller to 4-wire cable (first photo) and, at the RAMPS (second photo), 4-wire cable green wire to pin 44 (remapped D9 fan output) and blue wire to a GND pin for the laser TTL input… and an external 12v to laser Power (could optionally be connected right to 12v power to RAMPS green connector).

[attachment file=62520]

[attachment file=62521]

There’s so much confusion, here’s an attempt to clarify: TTL is Transistor-Transistor-Logic and speaks to voltage levels… 0 to +5 volts. PWM is a pulse-width modulation method… and can be any voltage levels. D9 fan control output is PWM alright but at 0-12 volt levels and not appropriate for the laser’s TTL input… hence the need to remap to pin 44, a TTL-level signal. D9 fan control output applied to laser’s +12 volt Power input is akin to leaving the light switch on and turning the light on and off from the breaker panel…

Hope this helps? – David


Thanks dkj!

Hi David

Thanks for the reply.

When I first got my first 3.5watt laser I followed the laser guide on here but my laser would not work at full power on the mapped pin 44, after contacting the laser supplier they told me that the TTL line needs 12v.

They done this to make the laser compatible with most 3D printers running the laser off an extruder output. They suggested that the laser was to be run of D9 on the ramps board, this worked great for my 3.5 Watt.

I have now upgraded to the 8.5 watt and hooked up exactly the same way but my laser will not run at full strength, they can only assume there isnt enough current being supplied via D9, however after using an external power supply I am getting the same issue, so I think the issue is with the laser driver itself.


What lasers are you using, Douglas? IIRC JTech laser controllers accept the 12v PWM from the D9 fan terminals directly… no need to remap. I’m sure there are others, similar, out there.

Mine are the Banggood 3.5 watt units and specify “TTL” modulation input… 0v off, +5v on. Whether that input is “protected” or not against application of 12v PWM… I don’t know and haven’t been gutsy enough to try it and find out.

The “true” TTL devices that I actively worked with in industry (in the 70’s, 80’s, 90’s) was powered with +5vdc power but the actual signal levels didn’t fully attain the power supply levels: i.e. input signals <0.8v were interpreted as low (0) and >2.4v were interpreted as high (1)… output signals had to guarantee <0.4v low and >2.8v high IIRC.

So, thinking out loud… if the “true” TTL output from pin 44 on the RAMPS is, for example, only 2.8v max (100% duty cycle), is it really telling the laser controller to turn on fully? Is the Banggood “TTL” spec of “low level (0V): OFF, high level (5V): ON” really saying it’s looking for a true +5v for full on… and 2.8v is only half-power? Hmm…mm…

I’ve always wondered why my Banggood 3.5 watt laser really didn’t seem any more powerful than the original 2.8 watt laser I started with. Looks like I need to check some stuff… :wink:

– David


Never mind. Checking pin 44 with a scope shows it is not the “true” TTL I was assuming… it’s genuinely 0-5v levels.

Issuing “M106 S127”…

[attachment file=62680]

and “M106 S200”…

[attachment file=62681]

– David