Proper Resistance of a Heated Bed

I have two heat beads. Both MK2a. One etched copper from a reputable company and the other a cheap copper plated one (copper plating in the holes and copper pads).

They both measure ~2ohms resistance when I apply the multimeter probes to the solder pads. Aren't these heat beds supposed to measure about 1.2ohms?

My multimeter measures other things at expected values so I don't think its defective. Hmmm?

Yeah, 1.2 would be about right.

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Waitaki 3D Printer

What does your multimeter read when you touch the probes together firmly? It will probably be some fraction of an ohm. Subtract that reading from the 2 ohms you measured to get a more accurate value for the resistance of the heat bed.

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Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

This doesn't happen to be a 24v compatible bed, is it? Measuring the end points of such a bed would measure ~2 ohms for 24V use, while measuring from each of the ends of the trace and the center pad would measure ~1 ohm.

Edited 1 time(s). Last edit at 11/18/2014 05:11PM by cdru.

Thanks everyone. @dc42 nailed it; I hadn't taken the step to measure the resistance at the probes only.

When I touch the probes together the multimeter reads 1.0ohm steadily and drops briefly to as low as 0.8ohms but won't stay there. Since the bed measures between 2.0-2.1ohms the actual resistance varies between 1.0 and 1.2ohms; I guess about where it should.

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?

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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?

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.

My bed has a .8 ohms resistance and I've yet to see my RAMPS polyfuse trip, and that would be with pulling 15 amps.

So that 11A is the holding current? What amperage will it trip at? If your polyfuse didn't trip pulling 15A then mine shouldn't at 12-ish, granted polyfuses are all the same.

But say I did wire in a 15A car fuse. It would not trip till 15A but what would get hot before it did? The copper traces on the PCB? At what amperage will other stuff on the board definitely start getting fried?

The cheap bed has LEDs so I will eventually wire it up and test it.

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Garry Bartsch
So that 11A is the holding current? What amperage will it trip at?

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.

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If your polyfuse didn't trip pulling 15A then mine shouldn't at 12-ish, granted polyfuses are all the same.

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.

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But say I did wire in a 15A car fuse. It would not trip till 15A but what would get hot before it did?

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.

Thank you for those answers. Sorry I am slow to reply.

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.

Are all screw terminals created equal? Or could better ones be soldered in to the RAMPS?

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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.

Unfortunately, the datasheet doesn't define what it means by the trip current. But my guess is that it is guaranteed to hold at 11A (i.e. remain in a low-resistance state while passing 11A) and to go into thermal runaway eventually (resulting in high resistance) at 18.7A. So it might trip at anything between 11A and 18.7A. It might trip at less than 11A if the temperature around it is higher than 25C.

Edited 1 time(s). Last edit at 11/28/2014 03:39AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Perfect explanation dc42. Thank you!

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Yes better screw terminals can be soldered to RAMPS. The ones that ship with many boards are the cheaper end of the spectrum.

Note: I've successfully used Phoenix removable screw connectors (the two-part ones) in place of the standard screw connectors, but to get it to fit on a standard RAMPS board I had to grind the edge of one of the connectors off to make it fit (runs up against the power connectors if you're also using Phoenix connectors for them, and the overlap is about 1mm). This makes very easy to plug/unplug without disturbing the screw terminals, as excessive screw/unscrewing damages the wires, and the screw terminals themselves can eventually fail (especially if they're cheap - eg: the screw threads wear, etc).

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Cefiar
Note: I've successfully used Phoenix removable screw connectors (the two-part ones) in place of the standard screw connectors.

Thanks. I searched the Phoenix website but there are a lot of connectors so I wasn't sure which one you mean. But I could see the Pheonix were more robust looking connectors.

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.

I wonder how many percent more a controller would cost if used only the very components? I'm betting the final price would not have to be drastically higher.

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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.

That datasheet doesn't explain what the ratings mean, but from another one:
I_hold = Hold current: maximum current device will pass without tripping in 20°C still air.
I_trip = Trip current: minimum current at which the device will trip in 20°C still air.

In other words, in a 20°C room, you can expect to get at least I_hold but not more I_trip on a sustained basis.

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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.

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.

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.

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cdru
In other words, in a 20°C room, you can expect to get at least I_hold but not more I_trip on a sustained basis.

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.

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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.

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.

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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.

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.

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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.

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.

Thank you cdru. That is more good information and is really helping me get a grasp on these issues. I read the catalogue and looked up some of the alternate connector manufactures you mention. Electronics is a big field!

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