Heatbed SSR + Power resistors

Hi Everyone,

Im building a aluminium heatbed for my printer. And i think i have everything i need but to be sure i ask you opinion about my setup.

Im want to use the following components 1xAluminium sheet of 300x250x10mm 1x CARLO GAVAZZI RP1A23D5 SOLID STATE RELAY 3-32 VDC , 5 A 230 V AC 9x 50-watt wirewound resistor, RH050, 12 Ohms 2x Thermal fuse, 10 A, 142 °C 1x Thermal compound, metal oxide powder, 5 g syringe 1x Thermisor 100K 1x Rumba 3d printer controller based on Atmega2560

I put the Resistors in series mounted to the aluminium sheet with 2 screws and some thermal compount thats a total of 9x12 Ohms =108 Ohms The Thermal fuse also in serie with the resistors and mounted just below the headbed. Thermister connected to the Rumba and placed in the center of the heatbed Port 3 of the SSR connected to a logic pin of the Rumba Port 4 of the SSR connected to the ground of the Rumba board

Port 1 and 2 of the SSR between 230AC power and the Resistors Ground wire connected to Heatbed

The Rumba board is running a PID controller to get the temp. to a maximum value of 110 degrees C

230VAC / 108Ohms = 2.13A 2.13A * 230V = 489W

489W/9 Resistors = 54Watt P/resistor

But because the are mounted to a giant heatsink i hope the extra 4Watt would not be a problem.

Thanks

489W/9 Resistors = 54Watt P/resistor

But because the are mounted to a giant heatsink i hope the extra 4Watt would not be a problem.

That could be an issue. If the resistor is designed to be mounted to a heatsink, I believe that's already been taken into account and 50W might be too much without a heatsink.

Normally, you select a resistor to run at less than it's maximum power rating. For regular resistors, you usually want a resistor rated for twice the actual power, but I assume you don't need as much safety margin with power resistors.

And... You don't really have a heatsink "taking away" the heat... You're trying to make a hot plate... What's the temperature rating of the resistor? The power cycling may save you. 110 degrees on the resistor body is probably OK, but the resistor body will run hotter than the overall temperature you're trying to achieve. (There's a rule-of-thumb that a component shouldn't be too hot to touch, but it's just a general rule and it doesn't necessarily apply to "power" components.)

Here is the specs of the power resistor i have the RH050 http://cdn-reichelt.de/documents/datenblatt/B400/RH_NH.pdf

It can run on 250 degrees for 2 hours. and yes 50 watt is including heatsink And i see the max power will go down with the temp. So maybe 1 need to add 2 more to be save. then the will use 36 watt

If you're running them at 100C, which I think is typical for a heated bed, you have to pay attention to that derating curve. Your RH050 drops to 75% of its rating at that temperature.

Heater cores, like the ones in your extruder, are usually the better choice for creating heat. They're designed for the sort of thing. Unfortunately they're much more difficult to mount to the build plate.