Proper Resistance of a Heated Bed November 17, 2014 10:32PM |
Registered: 10 years ago Posts: 363 |
Re: Proper Resistance of a Heated Bed November 18, 2014 02:36AM |
Registered: 11 years ago Posts: 1,592 |
Re: Proper Resistance of a Heated Bed November 18, 2014 02:53PM |
Registered: 10 years ago Posts: 14,672 |
Re: Proper Resistance of a Heated Bed November 18, 2014 05:10PM |
Registered: 10 years ago Posts: 869 |
Re: Proper Resistance of a Heated Bed November 18, 2014 09:50PM |
Registered: 10 years ago Posts: 363 |
Re: Proper Resistance of a Heated Bed November 19, 2014 09:29AM |
Registered: 10 years ago Posts: 869 |
Most likely, yes. Polyfuses have a hold current rating, which is the current that they can run at more or less indefinitely without tripping. As the current exceeds that threshold, it will start to heat. The material the fuse is made of is temperature sensitive and increases resistance as it heats. If it runs at current that exceeds it's hold rating long enough, eventually enough heat will be generated to cause the fuse to trip. The more it exceeds, the faster it will trip. As your heated bed cycles on and off, 12.55 amps probably is pushing it, but you'll just have to see. Hopefully your bed has an LED so you should be able to tell if the fuse trips when the LED on your electronics indicates that the bed should be on, but the bed's LED is off.Quote
My PSU puts out 12.55V. With 1.1ohm resistance the bed will draw 11.4A when cold which is just over the 11A polyfuse fuse capacity. With 1ohm resistance the bed will draw 12.55A when cold which is over the polyfuse capacity. Would this be too much? I understand the bed will draw fewer amps when it gets warm but can the fuse handle this surge when it's cold?
Re: Proper Resistance of a Heated Bed November 22, 2014 02:34AM |
Registered: 10 years ago Posts: 363 |
Re: Proper Resistance of a Heated Bed November 24, 2014 09:53AM |
Registered: 10 years ago Posts: 869 |
Yes, 11A is the hold current. There isn't an absolute tripping current, it's all based on how much current for how long causing a temperature rise. The datasheet for the polyfuses speced for RAMPS say at 25 degree C ambient, 55 amps should trip in a max of 7 seconds.Quote
Garry Bartsch
So that 11A is the holding current? What amperage will it trip at?
There is some variance just like with any other electronic components, but yes. I have a fan that blows on my RAMPS board keeping the drivers and mosfet cool, so that may help some. But 12 amps should be allowed to run for some time without too much of an issue. Once the bed reaches temperature and duty cycling kicks in, that will also help.Quote
If your polyfuse didn't trip pulling 15A then mine shouldn't at 12-ish, granted polyfuses are all the same.
Presuming wiring was large enough gauge, I'd be most concerned with the heated bed screw terminals, as well as the traces. The screw terminals are already marginal at best.Quote
But say I did wire in a 15A car fuse. It would not trip till 15A but what would get hot before it did?
Re: Proper Resistance of a Heated Bed November 27, 2014 01:43AM |
Registered: 10 years ago Posts: 363 |
Re: Proper Resistance of a Heated Bed November 27, 2014 04:37AM |
Registered: 10 years ago Posts: 14,672 |
Quote
Garry Bartsch
The PDF shows a trip rating of 18.7A beside the 11A holding rating. So the fuse would trip at 18.7A? But I assume it would not trip instantly at this amperage due to it's construction.
Re: Proper Resistance of a Heated Bed November 27, 2014 05:44PM |
Registered: 10 years ago Posts: 363 |
Re: Proper Resistance of a Heated Bed November 27, 2014 09:29PM |
Registered: 14 years ago Posts: 1,092 |
Re: Proper Resistance of a Heated Bed December 01, 2014 02:10AM |
Registered: 10 years ago Posts: 363 |
Quote
Cefiar
Note: I've successfully used Phoenix removable screw connectors (the two-part ones) in place of the standard screw connectors.
Re: Proper Resistance of a Heated Bed December 01, 2014 11:32AM |
Registered: 10 years ago Posts: 869 |
That datasheet doesn't explain what the ratings mean, but from another one:Quote
Garry Bartsch
The PDF shows a trip rating of 18.7A beside the 11A holding rating. So the fuse would trip at 18.7A? But I assume it would not trip instantly at this amperage due to it's construction.
Re: Proper Resistance of a Heated Bed December 01, 2014 11:41AM |
Registered: 10 years ago Posts: 869 |
If you're making a 1000 boards, saving $.10 on a connector makes you $100 more. They may not be inferior connectors. They may just be produced in larger quantities or they otherwise get better pricing. The dimensions may also be better fit for the existing design without having to recreate masks for etching the circuit boards.Quote
Garry Bartsch
I'm guessing that different RAMPS manufacturers use different terminals? Would the Molex terminals called for in the RAMPS wiki be about the best quality going? Without having many connectors to make a comparison I assume the cheaper poor quality ones would be lighter, weaker and made from inferior plastic for the housing? That would be the standard way to make a cheaper product.
Re: Proper Resistance of a Heated Bed December 01, 2014 03:57PM |
Registered: 10 years ago Posts: 363 |
Quote
cdru
In other words, in a 20°C room, you can expect to get at least Ihold but not more Itrip on a sustained basis.
Re: Proper Resistance of a Heated Bed December 01, 2014 04:02PM |
Registered: 10 years ago Posts: 363 |
Quote
cdru
Also, Molex is just a brand name. Saying that a Molex connector is better suited for a job doesn't mean anything unless you state which Molex connector.
Re: Proper Resistance of a Heated Bed December 01, 2014 04:54PM |
Registered: 10 years ago Posts: 869 |
TE Connectivity puts the derating numbers on their catalog pages, not their data sheet for whatever reason. Bottom of page 4 (172) for RGEF1100 it shows 11 amps rating at 20°C, 9.7 at 25°C, and 8.8 at 40°C. Page 7 also has a nice little graph that fills in the inbetween values. A fan wouldn't hurt...especially since the MOSFET and drivers will like the moving air as well.Quote
Garry Bartsch
Thanks for elaborating. And as the ambient temperature rises both of those numbers would go down? In my case in summer my room with the printer gets hot: 28-30C sometimes. So a fan would definitely be needed I think.
Highest quality? Probably not. I'm sure there are better connectors, but you'll pay more. They are generally good connectors. So are Phoenix. So are FCI...again...in general. The ones that are spec'ed are only good for 15 amps. Some beds push that limit and if their connections are marginal, they may overheat. Stranded wire sometimes doesn't get the tension it needs and can loosen up over time with thermal expansion and contraction. Or as pointed out above, the screws can strip out.Quote
Garry Bartsch
Yes. I guess I'm just wondering if Molex is known as a highest quality brand? Or do other manufacturers make products with the same or higher quality? Molex certainly has name recognition going for it.
Re: Proper Resistance of a Heated Bed December 03, 2014 02:09AM |
Registered: 10 years ago Posts: 363 |