Welcome! Log In Create A New Profile


Grounding the hot end

Posted by Buback 
Grounding the hot end
February 24, 2011 12:57PM
I read a blog entry the other day (can't remember who it was, but i think he was talking about a BfB hot end) that said that the hot end acts/can act like a Van de Graaff generator, and that putting a ground wire to the metal fixed ESD problems he was having that caused resets.

Now i haven't had any problems with my hot end or electronics like he described, but i was wondering if it is good practice to ground the hot end?
Re: Grounding the hot end
February 24, 2011 04:38PM
That is interesting. Read about the Van de Graaff generator.

It's easy to accept google's spelling which turns up a rock band instead.
Re: Grounding the hot end
February 24, 2011 05:03PM
The issue with grounding the hot end is the electronics, and any possible short you might get on your hot end.

Most electronics drive the heater element with an N channel FET. This leaves the +Volts connected to the heater element, and switches the -Volts. The main reason for this is that N channel FETs are significantly cheaper than P channel FETs (and you don't require the level shift circuitry needed to drive the P channel FET), which would be needed to switch the +Volts.

The issue here is what happens if the switched -Volts line shorts out to the hot end body? You'll end up with your heater on all the time, and the only way to turn it off will be to cut the power.

Alternatively, if the +Volts shorts out to the hot end, you'll have a direct short, and a fuse on your power supply (you have one, right?) will protect your setup.

If you switch the +Volts line instead (using a P channel FET setup, or a Solid State Relay driven from the standard N channel FET - both of which increase the expense of the electronics), and you then ground the hot end, you avoid the possibility of the heater being jammed on in the case of a short. If there is a short on the switched end, you may blow your fuse when the heater switches on though. To me, this is a preferable set of circumstances anyway.

Note: In many types of electronics, one side of the Thermistor (which is part of the hot end) is grounded. This leads to this exact possibility occurring in many hot ends due to shorts between one leg of the thermistor and the hot end. If there is another short between the heater element +Volts and hot end, you will end up with the heater being stuck in the 'full on' position. The thermistor leads tend to be thinner, and would hopefully melt at high current, but that depends entirely on the hot end heater design and the choice of thermistor.
Re: Grounding the hot end
February 25, 2011 12:09AM
If i followed that correctly, then it's probably not a good idea to ground the hot end. probably not a good idea to even touch the hot end when the PSU is on.

I assume that the static charge will bleed off naturally, or if it got high enough it would jump to the grounded lead on the thermistor (it's hard to get those leads fully wrapped down by the bead). what would the extra charge do? just throw off the readings intermittently? that would be preferable to a stuck-open heater.
Re: Grounding the hot end
February 25, 2011 04:00AM
If you are worried about a heater short then ground it with a 1M resistor. That will bleed away any static but will limit any current flow to only 12uA.

Edited 1 time(s). Last edit at 02/25/2011 05:27AM by nophead.

Sorry, only registered users may post in this forum.

Click here to login