Hotbed problem

Hello,
I use RAMPS 1.4 with MARLIN.
It has gone well until yesterday.
When I warm up to 110 degrees, "hotbed" looked normal at first but at 60 degrees stopped voltage and the temperature dropped?

I felt with the finger on the fuse F2, it was very hot (I got a blister on the finger).

What has happened?

/Bertil

The fuses on RAMPS are PTC fuses, which (supposedly) automatically reset.

IMO these fuses are crap, and due to the way they operate they are actually dangerous

They work on heat. Once they get to a specific internal temp, they trip, which cuts the power. The current limit for F2 is 11A. I have seen first hand the results of what happens if you draw too much current through one - they catch fire (quite pointless way of protecting your circuit from a high current load).

Some of the problems with PTC fuses are:
A 12V heatbed draws very close to 11A, sometimes more, depending on various factors (tolerances, temperature, etc). If you use 13.8V (becoming common), it will draw even more current than 11A.
A change in temp around the RAMPS board itself changes just exactly where it will trip, which means it's not reliably going to trip at the 11A mark.

I personally replace the PTC fuse with a standard automotive 15A fuse (either blade type, or standard M205 or 3AG) in-line on the 11A input, after replacing the PTC fuse on the RAMPS PCB with a piece of fairly thick wire.

Notes:
If your PTC fuses are touching, one will trip before the other as they operate at different temperatures. Also you definitely need a fan on a RAMPS board, specially if you are using PTC fuses, as they really do require active cooling (along with the Pololus/Stepsticks/whatever).
If you run voltages over 15V for the heated bed, then MUST remove the PTC fuse F2. The max voltage the PTC fuse F2 supports is 16V.

What You are saying, is T2 tripped !
Is possible that F1 and F2 where in contact.
I am going to get automotive 15A fuses.

/Bertil

Sorry to bring back an old thread. But did you ever figure out what happened? and what you did to fix it? I just started having this problem today after cleaning up some wiring. Every time I turn my heatbed on it the F2 fuse gets super hot. The bed temperature reading on pronterface gets to about 42 degrees then slowly goes back to room temperature. I've tried enabling and disabling PID bed control. Will replacing automotive fuses solve this problem? I've seen some people mention a bad D2 diode? Forgot to add, I have been running it this way for about 6 months already with no issues(that i've noticed)
Please let me know,
Thanks!

Edited 1 time(s). Last edit at 12/16/2013 08:48PM by wumbo6.

I got automotive 15A fuses, then it works good.

/Bertil

Check your current drain before you blame the stupid PTC fuses. I'd dump them regardless of what you find. You might save yourself the price of a 15A fuse...

I don't have an ammeter on my multimeter But so far i've checked that all the connections are tight and not anywhere they aren't supposed to be. I'm getting 0.9 ohms across my heatbed. And i'm getting continuity on my F2 fuse when the heatbed is turned off. One thing i've noticed is that I am getting a 260 ohm reading on my d2 diode in both directions. I bought the 15A fuses but havent installed them yet. Also could the mosfets have anything to do with it?
Thanks

So I replaced the fuse with a piece of wire and put the 15A automotive fuse on the power line of the 11A input. Still getting the same thing. The mosfet is getting crazy hot although the hotbed is getting to a higher temp. Presumably since the fuse isnt limiting it keeping it from turning itself off. I didnt dare to test how high I could get it. So now I guess the only options are the heatbed or the mosfet? I've ordered a new ramps 1.4 board just in case because I have some upcoming projects I need to be printing for. Any suggestions on how to test the mosfet/heatbed that doesnt involve desoldering the components?
Thanks!

If your DVM has a 200 mv scale, you can make a perfectly adequate current shunt to read what your power supply is doing. You can buy a pre-made shunt for < $5 delivered.

Unless you have a 4 wire ohm meter and understand how to use it, making low resistance measurements can be a challenge. It's quite possible that your "0.9 ohm bed" is really a 0.7 ohm bed.

Even at 0.9 ohms, you may be putting 14 or 15A through the FET. You will need a heat sink on it to keep it from getting fairly hot.