RAMPS 1.4 heatbed and fuse problem

Hello. Since last week, when I power on the heatbed after 20 seconds more or less the fuse stops working (if I touch the fuse is VERY hot), after one minute and the fuse rearms, the heatbed start heating for another 10-20 seconds and same problem. Obviously I have a fan attached to my RAMPS, but its impossible to cool the extreme temperature that the fuse reaches in few seconds.

I tried to connect the heatbed directly to 12v and works perfectly. What's happening to my RAMPS?

Thanks!

I had the same problem. It worked at first. Then it would sometimes work, then only come on for a second. The fuse was too hot to touch, and it was fan cooled.

In the end it turned out I was running my power supply at the limit and it would no longer provide enough current. I switched to a beefier PSU and the fuse is cold and everything is perfect. As I'm now providing a lot more current I guess the fuse has to work less.

If you connect your supply directly to the HB then it will always work, just not at full powere - but you'd have no way of telling

Edited 1 time(s). Last edit at 04/04/2014 06:54AM by Dodgey99.

Quote
Dodgey99
In the end it turned out I was running my power supply at the limit and it would no longer provide enough current. I switched to a beefier PSU and the fuse is cold and everything is perfect. As I'm now providing a lot more current I guess the fuse has to work less.

That makes absolutely no sense. Fuses don't work more or less.

Polyfuses (the standard resettable fuses RAMPS usually comes with have a current rating. When the current flowing through the fuse reaches that rating, they start to heat. The heat transforms the material from a crystaline state to an amorphous state and in the process, increasing the distance between the conductive carbon particles. The farther away the particles are, the higher resistance. Once it's "tripped" the resistance is so high it's effectively an open connection until the fuse cools to the point where the particles are close again lowering resistance.

If your power supply can't handle the current, it can't handle the current. A beefier supply will not make the fuse run cooler or a marginal supply run the fuse hotter. In fact, it's more likely the opposite would happen. A beefier supply potentially could supply more current enabling the fuse to trip faster.

As to the OP's problem, I'd look for poor connections both at the ramps and at the heat bed that might increase current. Measure the resistance at the screw terminals for D8. The resistance for a standard bed should be 1-1.2 ohms. "Out of spec" beds may be +/- .2 ohms of that. What is the resistance reading? If it's really out of spec then it's possible the bed is just drawing too much current. I = V / R where I = current, V = voltage, and R = resistance. If you measure .8 ohms for instance, then when the bed is running at 12V, it is drawing 15 amps when the polyfuse is only rated at 11 amps continuous.

Polyfuses are designed only to work with a voltage that doesn't vary by too much, and the ones on RAMPS are only designed to work with DC.

If the voltage fluctuates a lot (eg: your supply produces a varying voltage due to being right at the current limit), the PTC fuses will heat up more than normal, and will trip earlier.

The Ardunio regulator should work fine down to about 7V, so as long as the voltage stays above 7V (but varies between 12 and 7V) the PTC fuses could be affected without the Arduino resetting. Of course, if you're using 5V power for the Arduino (eg: PC PSU with 5V standby), then you'd only see a reset when the 5V line shuts down.

Quote
Cefiar
Polyfuses are designed only to work with a voltage that doesn't vary by too much, and the ones on RAMPS are only designed to work with DC.

Wrong. A Polyfuse is a electric component which resistance increases with higher temperatures. The Yellow Square is nothing more than a heatsink. The more Amps you pump through the component the hotter it gets. As it is getting hotter the resistance incrases until it gets an resistance so high to cause the current to drop.

Quote
Cefiar
If the voltage fluctuates a lot (eg: your supply produces a varying voltage due to being right at the current limit), the PTC fuses will heat up more than normal, and will trip earlier.

The Polyfuse doesnt gives a damn about Voltage as long its below the rated Voltage ... any Voltage above the rated one will cause the Polyfuse to Spark through the insulation to the other components.
The only thing a Polyfuse cares about is Current

Edited 1 time(s). Last edit at 02/11/2015 05:05PM by ~phoeny~.

Quote
~phoeny~
The Polyfuse doesnt gives a damn about Voltage as long its below the rated Voltage ... any Voltage above the rated one will cause the Polyfuse to Spark through the insulation to the other components.

Not so. The reason that the polyfuse has a voltage rating is nothing to do with the insulation. It's because in the event that the load goes short circuit, the full supply voltage appears across the polyfuse. The higher the supply voltage, the hotter the polyfuse will get - even though its resistance increases with temperature. Above the rated voltage, the polyfuse will overheat with a short-circuit load, resulting in damage to it, and possibly fire.

---

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].