Heat bed won't heat above 70°C

Question

I am currently heating a silicone heat bed through the 110v/220v AC mains and for some reason it only seems to heat up to 70°C, but it is heating the bed evenly. The power supply I am using is 24v power supply with the power turned up. The SSR has not been heating up at all during use, so could it be that it's not getting enough power even through the mains?

Answer 1

Preface: Working with mains voltage is dangerous. I'm writing this answer based off limited and incomplete information about your situation. Mains voltage can kill. Be careful, and get help from somebody else if you're not 100% certain of what you're doing. Make sure to connect to protective earth any exposed metal surfaces of your printer.

I think that the heated bed you have is a dual voltage bed, meaning it can be used with either a 110V supply or with a 220V supply. It has three wires, and internally, these are connected as follows (resistances are a guess, this would correspond to a 1000W bed):

Because this is a dual-voltage bed, there are three wires. You need to hook up the correct combination of wires to get the correct power rating - and this depends on the mains voltage in your country.

The formula for power P dissipated by a resistance R across a voltage V is P = V²/R. We can see, that if we apply 220V across wire 1 and wire 3 we get a power dissipation of 220² / 48 = 1000 Watts (because the resistors are in series, we get a resistance of 24+24=48 Ohms).

So, in a 220V country, the wiring is easy: just connect wires 1 and 3 to live and neutral. Note: in a 220V country, don't make the mistake of connecting any other pair of wires. If you instead connect wire 1 and 2 to the mains, your heated bed will draw 4000W. This will (should) trip the circuit breaker/fuse, and possibly damage the bed or start a fire.

So, what does a person in a 110V country do? What happens if you connect 110V across wires 1 and 3? You get a power dissipation of 110² / 48 = 250 Watts. That's only a quarter of the bed's power, so it's not going to get very hot (not even up to the 70 degrees you observed, I would think).

So what if, instead, we hook 110V across wires 1 and 2? In that case, we have 110V across a 24 Ohm resistor, so we get a power dissipation of 110² / 24 = 500 Watts. Better, but still not enough to get a bed this large up to temperature.

So, how are you supposed to get the full 1000W rating of this bed in a 110V country? You need to connect the two resistors in parallel!

If you connect wires 1 and 3 together, and connect wire 2 to one side of the mains supply and wires 1 and 3 to the other, you get two 24 Ohm resistors in parallel, each dissipating 500W, giving you the total output of 1000W (or equivalently: two 24 Ohm resistors in parallel make a 12 Ohm resistor, which will dissipate 110² / 12 = 1000W).

You should use a multimeter to measure the resistance between the wires. Out of the three possible combinations, two should give roughly the same reading (in the 20-40 Ohms range), and one combination should give roughly double that value. This latter combination identifies wires 1 and 3. These wires should be connected together in a 110V country.