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