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[maybe solved] Failure modes of hot end MOSFET?

Posted by Mazaw 
[maybe solved] Failure modes of hot end MOSFET?
November 09, 2012 11:07PM
I'm using a Sanguinololu 1.3a, but I think this might be an applicable question for any electronics, so that's why I'm posting here.

In which ways can a MOSFET fail?

Started a big print and when I came back, I found that Marlin prevented cold extrusion halfway through the job (thank goodness!) and the hot end was cold. In stopped mode, I set the temperature to 185 and it started getting hot (got to 60 before I decided the test was good enough) so I figured it was a glitch. I started the print job again and it was drawing it's first layer, but before I knew it, it stopped extruding with the same error message again. I check and the hot end is cold again! It heated up and started printing, but it just gave up partway though.

I broke out the multimeter and measured my hot end resistance (about 7 ohms, using a Makergear hot end). That checks out. Checked the other end and it was putting out 12V when the hot end was turned on, so that checked out. Then I switched to current mode, put the probes in the right holes, and noticed it was pushing a piddling 6mA... not sure how accurate that is because I had it in the fused 10A setting but definitely not what I was expecting.

Then I disconnected the hot end heater plug from the Sanguinololu and pulled some wires from my 12V power supply and plugged it directly in to the hot end and, yep, it was getting hot.

Here's the weird part. I then disconnected my hot end heater plug from the 12V power supply and back into the header on the board. When I switched the temperature on, the hot end started to get hot again!

Is this a sign that the MOSFET is failing, or is there something more sinister going on? The above behavior is pretty schizophrenic, since doing the same thing should cause the same results. When I assembled it, I used the Mouser BOM on the wiki which uses some pretty hefty transistors so I'm kind of stumped.

Edited 1 time(s). Last edit at 11/10/2012 05:44PM by Mazaw.
Re: Failure modes of hot end MOSFET?
November 10, 2012 05:18AM
Imo, measure the resistance of the heater wires, one time i had a short on them via the heater block itself, so if thats the case, the heater would draw out a lot of amps, hence pushing the fet into its thermal cutoff. Might be your case too - are the heater resistor wires completelly insulated from the heater block?
Re: Failure modes of hot end MOSFET?
November 10, 2012 03:04PM
Resistance checks out, 7 ohms, same as when new. The Makergear hot end is basically nichrome wire, insulated by kapton tape, over a few loops in the threads of a heater barrel. Then it's entombed in a ceramic goop. I don't think it's broken in there.

But now that you mention thermal cutoff, it might make sense why the heater all of a sudden decides to start working after a period of "off" time. Would the transistor get hot to the touch? I think I can check if it gets too roasty and, if so, look about replacing the hotend I guess. sad smiley
Re: [maybe solved] Failure modes of hot end MOSFET?
November 10, 2012 05:49PM
Following up, I set up two meters in the path of my hot end wiring, one for voltage, the other for amperage. Switched on after a few hours of time for the electronics to rest. Turned on the heat and noticed zero volts and zero amps. Probed the MOSFET with a thermometer, room temperature. Touched it and cool to the touch. Then I rubbed up against the connector and all of a sudden the meters showed activity and the temperature sensor (on the electronics) started reading an increasing number.

Not sure why I would read volts but no amps when I first saw the issue, but maybe it was crappy crimping on my wires? I guess. I knew I should have assembled that board with screw terminals. smiling smiley
Re: Failure modes of hot end MOSFET?
November 10, 2012 06:31PM
The STP55NF06L's aren't the greatest FET's on the block, but for driving the heater element they should be fine. I personally use FDP8870's, which have 1/10th the Rds(on) resistance (their internal resistance when switched on).

FET's usually fail either short circuit or open circuit. Short seems to be the most common (from memory here), which then tends to cause other issues.

What could happen (though unlikely) is that a bad FET will fail with one of the pins coming off inside the FET (usually on the silicon die in the FET, but could be off the pin end that connects to the PCcool smiley. This works fine until it gets to a certain temp, then the pin lifts and it stops working. It's a very uncommon mode of failure, however there are rumours of some cheap knock-offs of the STP55NF06L's out there, and it's quite possible that this could be an engineering fault with a knock-off like this.

That said, the best idea would be to systematically test around things, rather than just blindly replacing stuff because it seems suspicious. I personally don't like replacing things like FET's because there is a small possibility you might damage the board. Specifically, you might remove any hole-plating connecting one side of the board to another when taking the FET out, leaving you with something that may or may not work when you put it back together. A person who has good soldering skills can usually avoid this (especially if they have a vacuum desoldering station), but not everyone has good soldering skills and/or the appropriate tools on hand, and even a pro can make a mistake now and then.

I would test the hot-end (the "apply power directly" method, without the FET involved, maybe with a fuse to protect your PSU) and watch the temp in something like pronterface and possibly watch the current with an ammeter. This way you can drive it up to temp and switch on/off to see if it faults without the FET. If the hot-end is 7 ohms, you should be drawing less than 2 amps at 12V, and most ammeters have a 10A range, so you should be quite safe.

Also check the screw connectors (perhaps using an interior 12V car light bulb - something that doesn't draw more than say 2A of current - in place of your heater element, or even in parallel with it if you're not going to use the "apply power directly" method). See if it stops working, etc. Internally broken screw connectors will seemingly work fine then fail when they get warm or if there is too much current. If you suspect this, an easy test is to solder the wires direct to the board and see if it continues to happen.

Remember also that firmware will usually shut down the heater if it gets an erroneous reading from the thermistor (ie: too HIGH a temp, which is almost the same as open circuit), so make sure you don't have an intermittent connection there (could be in the hot end), and your thermistor is correctly specced in the firmware (eg: you have a 200k thermistor, and it's specified as 100k in the firmware, or even the wrong type of 100k thermistor). This may only happen with a highish temp, so you should catch it with the "apply power directly" technique above.

An advantage of using the "light bulb" and "apply power directly" method together is that when you reach temp, the light bulb should go out (which you should probably be able to notice even if you're looking at a GUI/graph interface on your PC), and you know to disconnect the power to the extruder. If the hot end isn't getting hot enough, but the board has cut out, you know the issue isn't with the heating element in the hot end, and you can start to "narrow down" the possibilities.


- If the through-hole copper on the board is damaged, either from the manufacturer or by desoldering the FET yourself, you can have a FET that will work intermittently (touches the copper - heats up and bends just enough away to break the contact on one of the pins). In this case, I'd heat the pin from the bottom of the board with a soldering iron and apply solder on the other side to bridge the pin and the pad with a small amount of solder (not too much!) - not always an easy job though. Also a bad solder joint (frosted/crystalline surface finish) can have intermittent properties, so if you suspect them, remove some of the solder (use desoldering braid or untinned copper wire to soak up some of the solder, or a solder sucker) and re-do them.

- Using the "Apply power directly" method, DO NOT drive the temp over 220 deg C if you can help it. If you remove the power at 220 deg C, you will see the temp shoot past that (extra power applied from the time you've noticed it go above 220 deg C and when you've removed the power from the hot end - basically your reaction time). This hopefully should avoid it going over 240 deg C, which if your hot end has PEEK or PTFE in it, could lead to those plastics breaking down (let alone burning ABS or PLA). You could probably stop at 190-200 (since you say it happens at 185 in your post). I mention this here though as I'm sure someone else might see this and "give it a try" - I'd like people not to kill their machines or set fire to stuff!

Hope this helps.
Re: [maybe solved] Failure modes of hot end MOSFET?
November 11, 2012 09:30AM

Then I rubbed up against the connector and all of a sudden the meters showed activity

Cleaning the header and putting some battery terminal grease into the connector should help. Especially the latter solves pretty much all my connector problems in the car as well as on the RepRap.

Generation 7 Electronics Teacup Firmware RepRap DIY
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