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Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors

Posted by terramir 
Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 12, 2018 02:54PM
The specs of the "arduino chip" atmel either 644p or 1284p allow for a max sink of 40mA and since the combined supply is 200mA the max current for all pins should be around 200 mA using a 10 ohm resistor allows for a current of 500mA @5v while in version 1.3.1 the resistors signaling the mosfet were 1k ohm resistors which limit the current for signaling to 5 mA was this an error or is there an internal resistor on the pins which already limit the current??
Could use some input for another design project.
terramir
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 12, 2018 11:30PM
I haven't driven power mosfets from an atmega chip for some time, but when I did I used 100 ohm series resistors. In conjunction with the output resistance of the pin (about 30 ohms AFAIR), this value ensures that the peak output current is within the 40mA limit.


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VDX
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 13, 2018 12:23AM
... I'm driving power MOSFETs with 1 kOhm directly or with a "Darlington-like" setup with a smaller MOSFET (BS170) and 10 kOhm resisitor.

Don't count on the max stated values! - I'm avoiding to drain more than 10mA from the "40mA"-ports ...


Viktor
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 13, 2018 03:06AM
Quote
VDX
Don't count on the max stated values! - I'm avoiding to drain more than 10mA from the "40mA"-ports ...

I agree in general terms that it's best to work well within the maximum rated values. However, in this case the current draw from the pin is only significant when the pin changes state; so the average current will be very much less than the 40mA limit.

The value of the series resistor affects the time taken for the mosfet to turn on and off. Higher resistance leads to slow switching times, which increases power dissipation in the mosfet. Lower resistance leads to faster switching times, which can cause the mosfet to avalanche when it turns off due to parasitic inductance in the wiring. Such avalanching is normally well within the rating of the mosfet, however it increases EMI so it is best avoided.

On the Duets we did some measurements and ended up using 180 ohm resistors. This is a good compromise for the mosfets we use when using relatively low (1000Hz or lower) PWM frequencies.


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 [www.duet3d.com].
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 13, 2018 05:04AM
Quote
terramir
The specs of the "arduino chip" atmel either 644p or 1284p allow for a max sink of 40mA and since the combined supply is 200mA the max current for all pins should be around 200 mA using a 10 ohm resistor allows for a current of 500mA @5v while in version 1.3.1 the resistors signaling the mosfet were 1k ohm resistors which limit the current for signaling to 5 mA was this an error or is there an internal resistor on the pins which already limit the current??

I don't think there's a real need for a current limiting resistor. The one on the Gen7 board mostly exists for bridging the high current track underneath it. And a little bit for the general rule of thumb to put a resistor everywhere.

Semiconductors get damaged by overheating. Simplified, switching a MOSFET happens very fast, so there's no time for the switches inside the MCU to develop substantial heat.

If you really want to develop a mathematical model for such a situation, also take switching time of the driving transistor into account, as well as the charge needed to fill the MOSFET's gate. I'm pretty sure the MOSFET's gate is fully charged already before the MCU internal driver is done with switching its self. So yes, there's kind of an internal resistor limiting the current: the resistance of the driving switch during switching its self.

If you plan to switch the MOSFET in the megahertz range, the picture might change.


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Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 13, 2018 06:42AM
Semiconductors get damaged by electromigration at high current densities as well as by heating. In a professional design, a current limiting resistor would always be used. However, I agree that the risk taken by not using one is very small.

The mosfet gate charge time depends on load current and load voltage, but is likely to be of order 100ns with no series resistor and a heavy load, which is a lot greater than the output switching time.


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 [www.duet3d.com].
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 13, 2018 10:38PM
Megahertz range..... LoL more like less than 0.5 hertz max and in the ideal more like 0.1-0.001 Hz range.
With the loads I'll be switching total load 600 W nominal
I.e 4 100 w heating elements and 4 50W element (ceramic in aluminum) each might draw up to 3x that power while cold which is why I will be switching them staggered until they reach like 60c this is an extermination gadget I'm building (heating my bed to a toasty 122F air temp goal is 130F to 150F depending on what I'm heating at the time. but I wanted to reuse the gen7 mosfet and temperature sensor circuits, albeit I'll be using a mini pro clone and an i2c 16x2 display and a rotary encoder. Just was a bit confused about the switch from 1k ohm resistors to 10 ohm ones. I will be programing this thing to check the air output sensor, intake sensor, and some buried in the target object. Also flexible 4 inch dryer hose going in and out of the "tented area" will be using my prusa2 with gen 7 1.31 electronics to print the 4 inch 10 cm collars the hoses will go on. Tent material is thick waterproof nylon fabric big enough for a queen size bed but it will be wrapped around smaller furniture objects as well. So my project is completely bang on bang off.
What would you guys recommend going with 10 ohm or 1k resistors for the bang on bang off approach. BTW I'm going to be using two pc power supplies to power this thing each will power 2x 100 W AND 2X 50W elements since I want to distribute the surge load. During warm-up phase the "arduino" will switch on one element wait for air temperature rise then the others will be added at intervals I will have to experimentally determine.
Will use a shunt and a multimeter to determine the warm-up spikes b4 programming the time. Ordered 15 IRLB8743 from eBay got them from China today :-) .
Wondering if I should double up on the 100W elements but the heated bed of the prusa never even made this mosfet warm .
Well that was my project and why I wanted info on why the resistor was switched. Any input would be appreciated before I go etching a circuit board for this project. Also I'll be using the 104laf thermistor but my working range will be 50C to 75C on the reprap we have a 180C to 235C working range any tweaks I should do on the temp circuit?
terramir
PS: incase anyone is interested 122 degrees F is the temperature where bedbugs and their eggs die within minutes but because heat transfer takes a while in wood and mattresses as well as other materials you should shoot for a little higher temps and give it a few hours.

Edited 2 time(s). Last edit at 02/13/2018 10:49PM by terramir.
VDX
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 14, 2018 01:32AM
... btw - if you're interested in switching high loads with Arduinos digital I/O's and frequencies of up to some hundred Kilohertz (or less), then this "scalable" circuit could be a good basis winking smiley




Viktor
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 14, 2018 04:41AM
Hmmm my German is like hmmm last time in Germany was like 94 lol klemme2pol must be a 2 pin connector
Or terminal. Seems my memory wants to translate it as clamp. I used to speak German like a German thanks to Herr rektor telling my mother to only speak German with me when I came to germany as a 9 yr old.but I am aus Der uebung.
For those that are german challenged einzel under gesamtStrome translated to single and total possible current.
terramir
VDX
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 14, 2018 11:55AM
grinning smiley


Viktor
Re: Beginning with gen7 1.4.1 10 ohm mosfet drivers resistors
February 16, 2018 09:05AM
12V elements like this should transition current around 25A max for the 100w 12v ones half for the 50w ones. regular current should be about 8.34 and 4.16 respectively.
The mosfet I choose is one we used for the heated bed
Without needing a sink.
Hmmm max specs for this one is like 30v 78A :-)
I wonder if I should use the 180 ohm someone mentioned to ensure the mosfets dun get toasty.
terramir
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