i3v 12" - Dumping the Bed Heater Relay

[ex-egll]

NOTE
I copied the wrong .jpg file when I first posted this. The file has been corrected.


When I switched from PLA to ABS higher temperatures were required from the heating bed. These higher temperatures meant longer heat-up times for the bed, which in my case led to uncomfortably hot connections and relays. This heat was due to resistance in the relay contacts and other connections which in turn led to a loss in voltage and a corresponding increase in the heat up time. In fact I was losing close to 2v between the power supply and the bed. I reduced the length of (12 gauge) wire that I was using and tightened all the connections, and in a couple of cases replaced crimped connections with soldered connections. Even so the relay was still getting very warm to the touch, and I was worried that the continual on/off cycling of the relay would pit the internal contacts, thereby increasing the resistance over time.


I decided to try replacing the the relay with a solid state device known as a MOSFET. The one I used was a Texas Instruments CSD19535KTT which is capable of switching up to 100 volts at 100 amps, we are switching 12v at 30 amps so there is plenty of margin. I used this device as I had some laying around, but they only cost around $2.50 on line. The only other component needed is a 10K resistor. Any N-Channel MOSFET capable of switching 30volts or greater at 40Amps or greater should work.


Below are two diagrams, one of the current wiring and one of the modified wiring. To get minimal resistance from this particular MOSFET when it is triggered, around 10 volts is required at the gate (G) (this value may vary depending on the MOSFET in use, but most require a minimum of 4.5v to switch them on).
MOSFET.jpg


The easiest way I found to get this switched voltage is to use the original heat bed relay to trigger the MOSFET, the fact that we are switching with 12v rather than 10v is not an issue as the MOSFET can accept up to 20v on the gate. This method reduces the additional wiring and also retains the satisfying (to me at least) click when the bed is powered on and off! Although the idea is to replace the relay to reduce the current drop, using it in this manner is not a problem as in the original setup the relay was switching 30 amps, in this case it is switching around 1/1000 of an amp.


With the bed powered on, there is less than 1/10 volt drop across the MOSFET. In my case this has reduced heat up times and also taken away the worry of overheating the relay.

If you need further details, let me know.

DISCLAIMER!
The above works fine here, but I accept no responsibility if smoke flames or earthquakes occur as a result of your using this circuit

[sniffle]

how hot does the fet get with that much current running through it? Most of us just moved to a 40A automotive relay, quieter click but still there...

[lakester]

..., and is there an explanation for someone who doesn't grok mosfet design..., why retaining the relay was helpful? (apart from the click?)

Thx!

[ex-egll]

how hot does the fet get with that much current running through it? Most of us just moved to a 40A automotive relay, quieter click but still there...

The MOSFET gets to around 39C while heating the bed up to 110C. I went with the 40A relay, but found that there was still some resistance in the connectors and in the relay contacts them selves that led to localised heating in the relay, I could have soldered the wires direct to the relay, but I wanted to try a solid state solution....... just because I could!

..., and is there an explanation for someone who doesn't grok mosfet design..., why retaining the relay was helpful? (apart from the click?)
Thx!

A new word for me "GROK" thanks.

The relay was retained as an easy way to obtain a 12v supply that switched on and off depending on the heated bed requirements. It may have been possible to get a 5v line that switched on and off accordingly, but as the figures below show 12v is more effective.

The MOSFET needs a voltage, applied to the gate, to essentially switch it on and off. The minimum voltage for the device one I used was 4.5V, the maximum voltage that could be applied was without letting the smoke escape is 20V. The voltage applied to the gate has an effect on the efficiency of the MOSFET to a small extent.

5v will result in a resistance of approx 6 mΩ
12v will result in a resistance of approx 2.8mΩ

(A 40A automotive relay can be expected to have a contact resistance of 100mΩ). Plugging some real world numbers in there gives the following:

For a 30A current flow in the MOSFET with:

5v on the gate gives a voltage drop of 180mv which equates to a power loss of 5.40w which is dissipated as heat
12v on the gate gives a voltage drop of 84mv which equates to a power loss of 2.52w which is dissipated as heat

The relay gives a voltage drop of 300mv which equates to a power loss of 9w which is dissipated as heat

I think I've got the figures right unless someone has changed ohms law in the last 50 years.

[lakester]

Sounds good. Heh..., but another question: I thought that the relay WAS being switched w/ 12V.

I.e., The signal side of the relay is being driven by the smaller mosfet on the RUMBA that would otherwise be driving a smaller heating pad directly. The RUMBA's HB driver is simply being re-purposed to drive the relay. It should be 12V already.

???

[ex-egll]

Sounds good. Heh..., but another question: I thought that the relay WAS being switched w/ 12V.

I.e., The signal side of the relay is being driven by the smaller mosfet on the RUMBA that would otherwise be driving a smaller heating pad directly. The RUMBA's HB driver is simply being re-purposed to drive the relay. It should be 12V already.

???

Unfortunately the smaller MOSFET on the RUMBA is switching the wrong way. One side of the relay coil is being held at 12v and when it is switched on, then the MOSFET applies 0v to the other side of the coil.

[lakester]

dang it!!!

Unfortunately the smaller MOSFET on the RUMBA is switching the wrong way. One side of the relay coil is being held at 12v and when it is switched it on, then the MOSFET applies 0v to the other side of the coil.

[ex-egll]

dang it!!!

Was the "dang it!!" comment made because you want to control the MOSFET without the use of a relay? If so it should be possible to a p-Channel MOSFET (this circuit was based on an n-Channel MOSFET). Details can be found towards the bottom of this page under the heading P-channel MOSFET Switch.

[lakester]

Yup! And thank you!

Heck..., I could just mod Marlin to make a click sound on the piezo when the heater is powered on/off...

Was the "dang it!!" comment made because you want to control the MOSFET without the use of a relay? If so it should be possible to a p-Channel MOSFET (this circuit was based on an n-Channel MOSFET). Details can be found towards the bottom of this page under the heading P-channel MOSFET Switch.