Heatbed temperature problem... Again

Hi!

I'm having a similar, though not identical, problem. I've already gone through almost all the posts, but I can't decide what to do.
In my situation, it takes my printer 20 minutes to reach 70 ºC and then it simply stops there.

I have a 12 V - 33 A - 400 W dedicated PSU, which seems to work well. I also have a layer of glass fiber and one of poly foil (similar to aluminum foil) between the heatbed and the support.

These are my measurements (voltage measurement apply when the heatbed is turned on):

- PSU voltage: 12.05 V
- RAMPS voltage: 11.7 V
- Heatbed voltage: 11.4 V
- Heatbed resistence: 1.7 - 1.8 Ohm

With these values, I'm having roughly 75W when I should be having around 100W.

Doing a little calculations:

- The loss in power due to the drop in voltage is around 10% of the nominal power. (assuming the nominal voltage should be 12 V)
- The loss in power due to the error in the resistence is around 20% of the nominal power. (assuming the nominal resistence should be around 1.4 Ohm. 30% for 1.2 Ohm)

When I plugged the heatbed directly to the PSU, the temperature reached 80ºC. There, I measured 11.75 V on the heatbed.

So I have three problems:

- The output voltage at D10 on the RAMPS 1.4 is 0.35 V lower than it sould be. How do I fix this? (I'd get a 5% bonus in power)
- There is a drop of 0.3 V on the wires. I should changes them right? (I'd get another 5% bonus in power)
- The resistence of my heatbed is to high. Do you think this is the main issue? (I'd get a 20-30% bonus in power)

I really need some kind of advice.

The basic problem is that your heated bed should measure 1 ohm, not 1.7 ohms. Higher resistance means less power. Less power means lower maximum temperature. You need a new (properly built) heated bed. The "real" heated bed pulls 144W when it turns on. The resistance numbers for the bed (and an explanation of why it's wrong if you cheap out on the construction) are all in Prusa's design notes on the heated bed pc board.

Your wires are a little skimpy, and the connection screws on the board might need tightening. That's not your main problem. You aren't going to fix this and get to a *controlled* 110 or 120 C with the bed you have. To maintain temperate you need enough power to overcome drafts and room temperature variations. Yell at the guy who sold you the pc board, try to make him replace it with a proper one.

There are a *lot* of these bum boards out there. They are a lot cheaper to make than the real thing. The correct board is only about $20 (US) from several places. I'd bet that you paid nearly that for the board you have. The 2/3's less it cost to make the bad board didn't get passed on to you. It got pocketed by somebody else. That somebody might be the guy who sold it to you, it could easily be a supplier who supplies his supplier - who knows ....

@jvega

I have the same problem.

It looks like there are a lot of these dodgy pcb heat beds out there.
Where I'd you get yours from, was it part of a kit

If you look on eBay, you can actually find listings for 2 and 3 ohm heated bed pc boards. I'd bet somebody did a big run of them and then later found out they had run the wrong process making them.....

@uncle_bob

Hi. Thank you for your feedback. I think I kinda expected your answer but didn't want to read it..
I'll have to buy another one.

@rogerclark
I bought it from [www.reprap-france.com], which is a "recommended" place to buy a heatbed mount in this reprap web site...
I'm gonna call them tomorrow and see.

Do you think that in the mean time i could increase the PSU voltage to 13.5 V without damaging everything, to get the 100+ W I need?

Thanks again.

Hi

You probably need the full 144W to get the bed to perform properly.

The risk as you bump up the voltage is that you over voltage the regulator on the Mega board. It's rated to 20V, but it's got the power dissipation to handle a lot of current at that voltage.