24v heated bed running directly from the duet2

BlueDust

I am new to the whole greatness of the duet2 and would prefer to run my heated bed directly from the board to make sure the duet has complete control and directly removes power from the heated bed if there is a problem.
I know the duet2 1.04 can handle 18amps on the heated bed and 400w/24v=16.6 amps... But is there any reason I shouldn't don't his? Is it better to use a SSR?
Heated bed I bought.
https://www.amazon.com/approx-KEENOVO-Universal-Flexible-Silicone/dp/B01ERAE0FY

Will adding heatsinks to the duet2 improve its cooling in regards to the 16.6 amps/heated bed that would be going through the duet2? Where would I put it?

Anything else I didnt think to ask above I should be aware of when doing this?

Thanks in advance!

dc42

It's not worth adding heatsinks, however a fan blowing cool air along the underside of the board in the area of the bed heater mosfet will help to keep it cool.

If you have the version 1.04 Duet, bear in mind that the bed heater fuse is 15A (because we couldn't find a 18A fuse), so at 16.6A it will eventually fail.

If you don't need the full heating power, you could turn down the output voltage of your 24V PSU to about 21.5V to reduce the current to 15A.

When using such high currents, it is important to make sure that the bed heater and VIN terminal block screws are tight and remain tight, and you must use the proper ferrules to terminal the wire ends.

If that all sounds too difficult, use a SSR instead. See https://duet3d.dozuki.com/Wiki/Choosing_a_bed_heater#Section_Low_voltage_12V_or_24V_DC_bed_heater for recommendations. The Auber Instruments one is good value.

BlueDust

@dc42
Any negatives running the 24v heater at 21.5 volts? (just slower warm up times?) Or to any of the fans, heater cartridge, or other devices on the printer?
I expected a fuse to blow right away if the amps surpassed its max amps. Your saying you would expect it to last a little while, but but just have a shortened life span?

Is this the SSR you recommend? I searched for the one in the wiki, and it didn't come back with an exact model number but this was the 1st result in the search.

https://www.auberins.com/index.php?main_page=product_info&products_id=288

Thanks for the quick reply!

dc42

Fuses only blow really quickly if you give them a massive overload. They will even take double their rated current for a little while. You could fit a 20A fuse instead if you need to use more than 15A.

Reducing the PSU voltage will reduce the heating power of both the bed and the hot end. The hot end heater is typically over powered anyway if you have a silicone sock over the heater block. The fans will run slightly slower too.

The Auberin SSR you linked to is a good one.

BlueDust

If I use an SSR, I would like to add a thermal fuse to it. I currently have an SSR on my current printer and using the Duet2 on the one I am currently building. Since researching the parts for the new one, I have read people talking about the SSRs failing closed, and the heated bed just keeps getting hotter and hotter etc... until problems arise. I am struggling to find a thermal fuse because I don't know if the power specs listed for them matters, and haven't found one for the same needs as the heated bed I ordered.
Can you tell me if I am making the right assumption with this statement?
The AMP and Volt specs on a thermal fuse are only for MAX use, and the temp is really only the part I car about if I am not going to hit the max amp and volt specs.

Thank you for your help!

dc42

The alternative to using a thermal fuse is to choose the heating power so that at full power the bed doesn't reach dangerous temperatures. For silicone heaters, this typically means ensuring that the temperature doesn't exceed 200C.

The Auberin SSR is no more likely to fail that the bed heater MOSFET on the Duet - probably less likely, because it is rated for a higher current. Any high powered bed heater needs to be controlled with safety in mind.

BlueDust

I would rather keep the system running on 24v. I ordered the SSR mentioned.

Thanks!