Author Topic: T2 laser comes on when going to laser control. SOLVED!!  (Read 8321 times)

a_kent

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #15 on: March 01, 2017, 12:21:52 PM »
@a-kent - I consider it more than an engineering oversight. The entire MOSFET drive circuit is deficient. Each MOSFET has has definitive current and voltage requirements to reliably turn fully ON/OFF in a specified time. The AVR328 I/O pin barely suffices. I have 2 cheap NANO clone boards that generate less than 4.2V for a logic high and they cannot fully turn the MOSFET ON. The result is excessive heat. Eventually (2 minutes) they get stuck in the ON state and melt the PCB trace.

I have posted before and some users say my analysis is wrong. I contacted the MOSFET manufacturer and they agree with me. My boards now contain dedicated MOSFET driver chips. They work reliably and run cool with 3.3V ARM processors.

Perhaps there are power MOSFETs that do work with 5V logic. IMO - This circuit is something that a legitimate engineering department would investigate and make rock solid before releasing onto the consumer market.

@ggallant571, mine (Mana SE) has a single Mosfet to drive the motor's low side on the red connector. I don't see a problem there.

My board does not have a Mosfet driving the Laser. It is a signal directly from the Nano. The same signal also drives the servo port.

Pascalp, that is perfect!
Kent



ggallant571

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #16 on: March 01, 2017, 04:15:20 PM »
The MOSFET driving the "Motor" port is the problem.
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Zax

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #17 on: March 02, 2017, 06:36:45 AM »

mrehmus

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #18 on: March 02, 2017, 08:55:12 AM »
Thanks for the pictures.
Where do the wires go and for what purpose?
Thanks.
mrehmus
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a_kent

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #20 on: March 02, 2017, 11:11:33 AM »
Thanks for the pictures.
Where do the wires go and for what purpose?
Thanks.

The wires on there are to some kind of limit switch as mentioned by Pascalp, and are not a part of this fix.
I have not looked into that mod.
Kent

a_kent

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #21 on: March 02, 2017, 11:31:23 AM »
The MOSFET driving the "Motor" port is the problem.

Ggallant571:
The Mosfet used on this board is rated at 10 amps continuous and 35 amps pulsed.
In this design, it is switching the low side of the motor port (ground side).
The gate voltage threshold is from 1 to 2 volts (Vgs), and is rated for logic levels.

So in this design, it is well implemented.

I have attached a datasheet for the part.

Kent

ggallant571

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #22 on: March 02, 2017, 12:46:12 PM »
Thanks. Will order a tube and replace my current IRLR7843. They are cheaper than the mosfet driver chips.
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Koskee

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #23 on: April 02, 2017, 06:44:12 PM »
So I did this mod with a 1k resistor, but the only effect was that it made my laser unresponsive. Trying to send Gcode wouldn't work, although i could still manually jog the steppers. As soon as I removed the resistor, it was business as usual. This leads me to believe that the pulldown that was installed was too strong for the signal from the arduino to be properly transfered. Are we sure that 1k is the appropriate value for the pulldown? Most any pullup/down I've ever seen are more on the order of 4.7k-100k+ with 10k being a very common value.

ggallant571

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #24 on: April 02, 2017, 07:04:54 PM »
I agree that 1K is quite strong but I would expect the CPU to be capable of driving it high. I use 10K on my boards. I don't use the same CPU or MOSFET circuit so I cannot verify. Do you have a scope?
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Koskee

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #25 on: April 02, 2017, 07:09:31 PM »
I have a really old analog scope, not good for much other than getting an idea what's going on. I'm not sure how to actually read voltages off of it and besides, it hasn't been calibrated since I got it off of eBay so I doubt it would be all that accurate anyway.

If you're using a 10k then maybe I will try that instead of messing about with measuring anything.

ggallant571

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #26 on: April 02, 2017, 07:22:45 PM »
Please try and measure. The MOSFET spec sheet is brutal reading but I recall 3.0V being a good turn on voltage. Which controller do you have?
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Koskee

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #27 on: April 02, 2017, 09:01:29 PM »
Mana SE, without the jumper. I bought it around Xmas 2016.

denmark219

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #28 on: April 04, 2017, 09:58:06 AM »
I'm about to order a 1k resistor.... I have a Mana SE, this is the correct resistor for this mod correct?

a_kent

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Re: T2 laser comes on when going to laser control. SOLVED!!
« Reply #29 on: April 04, 2017, 11:46:05 AM »
1k is a value that will work with the Laser output of the Mana SE, IF the laser itself does not already have a pull down.

Again, the way to tell, is to turn on the Mana SE with the USB unplugged.

If the laser goes full on, then you need this mod. The laser will also go full on for a second or so when you go to Laser control from the T2 software if you need this mod.

Also, the MOSFET has no bearing on the port labelled 'Laser'. The MOSFET is used for non-TTL lasers on the red port labelled 'Motor'. It switches the low (ground) side of the laser power on this port.

The Laser port PWM control signal is a direct digital output from the Arduino. The Arduino has no trouble driving this signal with the mod.

Best,
Kent