Upgraded Heated Bed heating unevely

[addisonElliott]

Hello everyone,

My Rostock Max has been apart for 5-6 months now because of college getting in the way. During the summer and on any free weekends, I have been working on upgrading the heated bed. It started off I was going to remove the bed completely, buy a MIC-6 plate and lay nichrome wire over that. In the end, it did not give me good results, so I decided to try something else. I bought two Dell 12V 41A power supplies and hooked them in series to give 24V. (I isolated the ground on one of these so that they did not short). This is inline with a SSR to relay the 24V. After much work with that, I finally got it up and working.

However, now my issue is that the bed is heating unevenly and I'm not sure why. I stuck two temperature probes on the bed(one in the center and one on the edge). When I heat it up with 24V, the edge of the bed goes to like 131*C and the center is at like 100*C. I don't have the exact numbers with me, I will report them when I get a chance.

Is this alarming? Why doesn't the temperatures even out even after time? Should I get a new bed?

What are some more tests I can do? I'm going to try and measure the resistance of the bed when hot and cold to see if its within spec. I'm worried I may have shorted it somewhere.

[Tincho85]

Is it safe to connect them in series? I thought that it was quite dangerous, because one PSU can overdrive the other.
But... I'm no expert, just repeating what I read somewhere

[addisonElliott]

Is it safe to connect them in series? I thought that it was quite dangerous, because one PSU can overdrive the other.
But... I'm no expert, just repeating what I read somewhere

I believe you are referring to connecting them in parallel. If you connect them in parallel, then if one power supply is at a higher voltage than the other(even by a small amount), then that potential difference will flow INTO the other power supply. It may be fine in most cases but some power supplies may not be able to handle this, especially the cheaper ones.

By connecting them in series, the voltages add together. The only thing is that each power supply needs to be able to supply the total current in the equation. So if you assume that the bed resistance is 1ohm, each power supply needs to be able to supply 24V / 1ohm = 24A.

[Eaglezsoar]

Does the heat plate help to even out the temperatures?

[teoman]

Switch the location of your temperature probes and see that you get different results.

It could be that they are reading differently.

[addisonElliott]

Whenever I get home this weekend, I will try what you have suggested. I will move the temperature probes around and see what the temperature is at other places. I will also try using a heat spreader and seeing if that does help.

What if it reads a much higher temperature on the outer edges of the bed?

What if it's just that one spot?

I'm not sure what to do with this data. I guess I find it weird that it is not heating up evenly. Did anyone else have this problem when they upgraded to 24V?

[Eaglezsoar]

Whenever I get home this weekend, I will try what you have suggested. I will move the temperature probes around and see what the temperature is at other places. I will also try using a heat spreader and seeing if that does help.

What if it reads a much higher temperature on the outer edges of the bed?

What if it's just that one spot?

I'm not sure what to do with this data. I guess I find it weird that it is not heating up evenly. Did anyone else have this problem when they upgraded to 24V?

I believe once you put in the heat spreader and let the temps stabilize, you will see mostly even temperatures. If that is not true let us know. Are you using Thermocouple
probes attached to a meter to get your temps and not one of the laser type guns?

[addisonElliott]

Yeah I am using temperature probes(type K) and not the laser gun.

Is there a specific heat spreader you would recommend? What is a desirable thickness?

My goal with this project is to have the bed heated up within a few minutes. I don't want too thick of a spreader to make it take forever.

[Xenocrates]

If you have access to machine tools, or a good bandsaw+mill, I suggest making your own out of aluminum or copper plate, (copper is better, but more expensive), and really you only need it to be thick enough to stand up to being moved and removed (Mine are thicker as I don't use glass, but instead bond PEI straight to them, which is an option)

If you don't have shop access, I know Tricklaser sells one purpose designed for the Max. I can't find any commercial copper ones, but some people have made their own. If you want to experiment a bit, you can use 2+ layers of heavy duty aluminum foil. The glass will keep them flat and they should transfer heat well.

[KAS]

This is the one I use http://tricklaser.com/Heat-Spreader-for ... R-ONYX.htm

[Eaglezsoar]

I agree with the Trick Laser heat spreader, it is not to thick and already cut to size.

[addisonElliott]

Alright guys, I bought the heat spreader and it is on its way.

I should have some free time today to measure the resistance across the bed to make sure it is okay there.

I will report back when I have more information.

[teoman]

Watch KAS s thermal video camera videos while waiting.