Adding a heated bed to a cheapo printer

Hi folks.

I'm losing my mind a little bit with frustration, but here goes.

Over a year ago, I bought a real cheap printer with no heated bed.

I recently decided to venture into printing more materials, and have wanted to upgrade it with a heated bed,
so I bought a silicone heated pad, and an external mosfet board for this endeavour.

The bed is a 120x120 (printable area), and I ended up buying a 100x100 silicone pad to account for screws on the edges. The heated pad is a 50w silicone heater, ran at 12v.

Since the printer is a cheapo, it has a tiny 24v psu, so I decided to make use of a secondary PSU that I had laying around (12v 29A LED PSU), which should be more than enough to power the 6A for the bed.

The mosfet expansion board I got is a 12-24v board, so the voltage should be fine (also rated at 30A, but I won't ever be pulling that much current through it).

Now. Connecting the mosfet board to the second psu, connecting the heater pad to the mosfet board, and connecting the signal from the mainboard to the mosfet board went fine. I power everything up, and the power light on the mosfet board lights up. I send a command to heat to 50C as a test. Temperature doesn't change. After a while, marlin responds with a heating failed for the heated bed, needing me to restart the machine. When the commanbd is sent, the blue light on the board also lights up (mosfet board) showing that the command for heading is recognizabily send from the mainboard to the mosfet board. I get 0v on the output for the mosfet.

By this time i was already pulling my hair out trying to figure out whats wrong, and ended up thinking I broke the mosfet, and ordered a new one from a different supplier. Received it, installed it, and exactly the same happens.
I am at a loss as to what is wrong currently, because the printer itself works, its running marlin 2, the heater pad works, as connected to 12v it heats up fine (i did this for like 5 seconds before disconnecting, as it heated from 22 to 30 quite easily). The thermistor works fine, as the temperature reading is close to that of my hotend when starting up, and it showed an increase when the pad was connected to 12v.

I have no idea whats wrong with the setup, and why its not working. If anyone has any ideas I would appreciate it.

What external mosfet did you use? as some of them have > 5v gates (ie will not turn on with 5v), designed to be plugged into an onboard low power mosfet

no mention of updating your machines firmware with bed support?

Edited 2 time(s). Last edit at 10/11/2019 08:07PM by Dust.

[www.diyelectronics.co.za]

This is the mosfet board. The control led on the board does turn on when the signal gets sent from the mainboard, which is indeed a 5v signal I added configuration for bed PID in the firmware, as well as set the correct thermistor value according to the one attached to the silicone sheet

Edited 1 time(s). Last edit at 10/12/2019 01:07AM by niekz.

Please read the 4th technical specification on the web page "Control Voltage (Signal) – 12 to 24V DC"

5v is not enough for this mosfet

See if you can get one of this type [www.aliexpress.com] designed for 5v signals

Edited 1 time(s). Last edit at 10/12/2019 02:55AM by Dust.

Now I feel like an idiot. Thank you for the help though, at least I'm not going crazy.

Unfortunately ordering from aliexpress my packages rarely arrive in my country. Would using a board like this :
[www.netram.co.za]
Enable be to use the mosfet boards I have?

Yes that should work between the controller and the external bed mosfet.

Awesome, that's great news. Thank you very much for the help