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Duet 0.8.5 driving a MOSFET

Posted by pkm 
pkm
Duet 0.8.5 driving a MOSFET
December 14, 2015 09:37AM
Hi,

I'm trying to control a case light via output pin on Duet 0.8.5.
First I found that 0.8.5 has different pin numbering as compared to 0.6.
So [reprap.org] is partially wrong.
I.e. M42 P16 still controls TXD1, but TXD1 is actually pin #11 on 0.8.5 and so on.

The problem is 3.3V outputs can not open the MOSFET enough.
The module is similar to [www.ebay.com] , the schematics is [eeshop.unl.edu]
Is there any way to get it working?

Thanks
Re: Duet 0.8.5 driving a MOSFET
December 14, 2015 10:04AM
AFAIK the Duet 0.8.5 expansion connector has the same pinout as 0.6 except for the following:

1. AD12 is no longer present on the expansion connector because that pin is used for VSSA. AD12 is available on the Z probe connector.

2. The 0.8.5 has the 2nd extruder/heater/thermistor on board, so the expansion connector pins intended for driving additional heaters etc. on the DueX4 are moved along to the next channel.

That mosfet switch design is cr*p. The IRF520 mosfet on it isn't a good choice to drive from 5V, let alone 3.3V. The 1K resistor from input to ground should be much greater, like 10K or 100K, to keep the load on the microcontroller low and avoid reducing the mosfet gate drive voltage. There should be a resistor in series with the mosfet gate to limit the surge current drawn from the microcontroller output when the pin changes state.

How much current will the case lamp draw?

Edited 1 time(s). Last edit at 12/14/2015 10:04AM by dc42.


Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet.
pkm
Re: Duet 0.8.5 driving a MOSFET
December 14, 2015 01:02PM
Thanks for your reply, David.

The case lights should draw ~1.5A according to the LED specs.

I bought this MOSFET board a while ago, it's the most sold kind from ebay... and 5V opens the gate enough.
I'm also surprised with that 1K resistor... what I can try is installing it in series with the gate instead of connecting to ground.

I found another MOSFET board [www.ebay.com] it looks better. At least they promise 2.5V driving voltage.
Also this [www.ebay.com] is very simple and has 2V gate threshold.

About the pins: I actually verified that M42 P16 switches pin 11 on 0.8.5. So probably it needs to be changed in the description and the firmware.

Edited 1 time(s). Last edit at 12/15/2015 04:52PM by pkm.
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