Heated Bed Control 24V

Hi

I am using a Prusa PCB heated bed. I am setting up my Mendel to run on solar power (24volt batteries). The first step was to switch the heated bed. I am using the Ramps output to drive a 30amp relay and the system works. However the bed draws around 13 amps and is causing voltage drop on the 24v volts which also runs a lot of my IT equipment. The relay duty cycle is about 80% on.

I would like to improve the control in a way that controls the current. It would seem a limit of about 10 amps would provide the required power and reduce the voltage drop. I would appreciate any suggestions of a better way to control the heated bed.

---

Regards

Ian

Running a 12V heated bed on 24V doubles the current going through the bed and quadruples its heating power. If it's still 80% on, something is seriously wrong. Are you sure you measured right?

To tame the situation I think a custom heated bed, designed for 24V, is required. With something like 3 ohms cold resistance.

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

I have now added insulation below the PCB and the duty cycle is now about 30% on when the bed temperature stabilizes. It takes much longer than I realised to stabilize so the earlier readings I gave were incorrect.

I am also measuring the current with an ancient clip on ammeter which may not be accurate.
I don't have an in circuit ammeter that goes above 10 amps.

I am also using an aluminium plate which may be radiating the heat rather efficiently.

The same heated bed worked fine on 12 volts with a heatproof glass bed but I never measured the current.

I would like to use a glass bed but my latest beds have 4 keyhole slots so the bed surface height never alters if I change the bed (As per the Ultimaker).
I found a local water jet company that can cut profiles at a very reasonable cost. However all attempts to cut key hole slots in heat proof glass ended in failure with the material cracking round the slots. The aluminium cut perfectly.

The problem with a custom PCB is my design skills. I guess reducing the track width on the standard design would do the trick. I have the capacity to mill PCBs if the cutter would last long enough for a board the size of the heated bed.

Edited 2 time(s). Last edit at 10/20/2012 10:53AM by emt.

---

Regards

Ian

For me V-bits last for milling some 20 Gen7 boards, so you should manage to mill a single heated bed.

Regarding dimensioning the tracks, the wiki knows a few bits: [reprap.org]

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

Another option would be to use the stick-on 24V heated bed elements that are used on the Mendel Max (in place of a PC. I use that on my 24V setup with no issues, though my bed is slightly longer/wider than your average bed (260mm x 220mm), to accommodate the stick-on heating element.

Many thanks.

I will study the PCB info and investigate the MendelMax heater.

---

Regards

Ian