Bed heater fuse always blowing

Triet

The 15A fuse for the bed heater in my Duet2 WiFi gets blown aprox. every week, or perhaps every dozen of times used. This always happens when starting to heat the bed (at aprox. 40 deg.C).

I have read that this is because the circuit is dimensioned up to 18A, but since there are no such fuses (at least I can't find one with the proper size), I have to use 15A, which is too small.

Is there a trick to limit the current used to heat the bed? Doing the heating slower for example? How would I do that?

Thanks in advance.

Phaedrux

What are the details of your bed heater? Perhaps a better option for you would be a relay capable of your heater current draw and using the Duet to switch that?

dc42

@Triet here are three possibilities:

Turn down the power supply voltage a little, to reduce the current.
Replace the 15A fuse by a 20A fuse. Keep a watch on the fuse holder because you may be running it a little above its rating. The rest of the Duet is designed to handle up to 18A. As it's a Duet 2, also make sure that the VIN and bed heater terminal block screws remain tight.
Use an external mosfet to switch the bed heater.

Triet

@dc42 Thanks!
I never thought it would be possible to use an external switch because I was asuming that the current is somehow regulated (not just on/off).

What I already tried was to glue an aluminum heat dissipator on the fuse, but surprisingly, it blows even earlier then.

I will go for the mosfet switch but that is an effort.

dc42

@Triet TBH I doubt that you would hit any issues if you just used a 20A fuse.

Have you measured the resistance of the bed heater?

Triet

@dc42 said in Bed heater fuse always blowing:

Have you measured the resistance of the bed heater?

I should have begun with that question.
I measured 1.6 Ohm. Which brings us to 15 A. Nice, no? At times it takes weeks to blow, at times the second day. Chinese fuses are more tolerant

I replaced the fuse with a 20A one and will keep an eye on that. I re-tightened the bed heater terminals to avoid additional heat there (I once already had an issue with bad loose contacts).