200x200 Customized heat bed with aluminium clad resistors
June 19, 2018 02:25AM
Hi guys!!!
I would like to build a customized heat bed (200x200) with aluminium clad resistors...Bed material ALUMINIUM

I am planning to give a separate power supply to bed

(power supply - AC 110-220V input voltage, DC 12V 20A output voltage, 240W power)

How to calculate the type of resistor(say 10w, 25w or 50w) and no.of.resistor according to this power supply what I had???

what type of connection should I give to resistors ( series or parallel) ?

need help reg this...
Thanx guys.
Re: 200x200 Customized heat bed with aluminium clad resistors
June 19, 2018 10:20AM
Working with resistors is the most basic operation of the entire electrical (digital and analog) world. If you're asking how to generate heat, I would NOT recommend you doing this. All you need is one of the most basic formulas of circuits.

I'm not trying to be a jerk, but just buy a heating pad.

Edited 1 time(s). Last edit at 06/19/2018 10:22AM by boredom.is.me.
Re: 200x200 Customized heat bed with aluminium clad resistors
June 19, 2018 11:09AM
A heatbed with power resistors is not a good (or even cheap) idea. It'll have hotspots all over the place and is a fire risk. A 200x200 500 watt kennovo headpad is $40 USD, 500 watts worth of power resistors is going to be at least that much if not more.
Re: 200x200 Customized heat bed with aluminium clad resistors
June 19, 2018 01:05PM
If the bed plate is thick aluminum, and you have a source for many cheap resistors, it might work, but heaters don't care if you give them AC or DC, so why spend money on a regulated power supply to drive a heater? The money you save not buying a power supply will probably pay for the resistors if you operate the bed from line power. The more resistors you use, the less likely you are to develop hot spots. The problem is that the more resistors you use, the cost goes up and the bed becomes very heavy. If you're moving the bed in the Z axis the weight doesn't matter, but if you are going to be throwing the bed back and forth at print speed, it matters a lot. If you use 12V to deliver 240W, you will need wires and connectors that can handle 20A. If you use line voltage, say 240V, depending on where you are in the world, you will only need 1A (so much lighter wiring) to deliver that power.

The formulas needed to calculate the resistance are simple:
power=volts x amps
resistance = volts/amps
power = volts^2 / resistance

For powering from the AC line, just use the rms voltage.

Solving the formulas will give you the total resistance needed. Then you have to split that up into multiple resistors. If you connect them all in series, just divide the total resistance by the number of resistors. The power divides the same way.

You may find that just buying an off the shelf silicone heater is less costly and a lot less labor intensive.

Before you decide on how much power you need, go to this page and run the calculations for the required heat-up time. A reasonable rule of thumb for a bed heater is to use about 0.4W per cm^2 if you plan on printing ABS. You may be able to use lower power to achieve the required bed temperature of about 100C, but you will have to wait a long time for it to heat up. As a point of reference, my printer's 300x300x8mm aluminum bed has a 750W line powered heater and it takes about 4.5 minutes to reach 100C.

Edited 1 time(s). Last edit at 06/19/2018 04:43PM by the_digital_dentist.

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