Prusa i3 Hephestos+ printbed issue

Hello everyone. I have some problems with my heatbedbed. I'm using Prusa i3 Hephestos 3D printer with 2 separate power suplies. Both have output 12V 8A. At first I had connected heatbed MK2A. For first few days there was no problem with it. After some time it started to have problems with heating. Sometimes it heated in 50% and then stoped, sometimes it heated correctly but after hour of 2 of printing it started droping temperature. And after next few days it was dead. I thought that problem might be with faulty heatbed so I connected another one this time model MK2B it was working without any issues for next few days and then it started again. Right now its dead. It is heating only to 27C and dont want to go any higher. Any ideas what might be the problem? During first instalation of heatbed I've done everything acording to this manual [www.3d-printerstore.ch]

How are you connecting your power supplies? If you are using one for the bed and the other for everything else, then it's under powered. Expect the bed to draw roughly 11A, and spec the power supply for 15 to 20, depending on how well built you think it is. Now, if you are wiring the supplies in parallel it might work. Maybe. I would strongly suggest not doing that though. It's far safer to just get a suitably rated supply.

Yes. I got 1 ower supply for heatbed and 1 for everything else. I gues I'll need to get 1 stronger, and check it out. Thank you for reply.

Measure the resistance of the heatbed heater and discuss it with Ohm...his law will help you out. My bed is .42 ohms and at 14.2 volts eats up about 34 amps!!! I am feeding the bed on its own 50 amp DC supply and running the rest on its own supply. I can run the whole printer off of the 50 amp supply but the current draw on the bed causes the rest to stutter and it can be seen in the prints.

Already bought new power suply 12V/24V 30A. Now just need to wait till it's delivered. Im just surprised that I was able to print before even with not enough of juice. I basically printed like 1kg of PLA on those 2 heatbeds.

Just connected new power supply, but no luck. Any other suggestions? My best bet would be buying new ramps, but there might be other way around.

Edited 1 time(s). Last edit at 06/09/2015 05:29PM by ExiledDiclonius.

As Itchytweed suggested, time to get back to basics and Ohms law. Can you measure the resistance of the heatbed and the voltage at the bed terminals when the heater is on? From that we will have a better idea of what's going on. You also said that you were getting a 12/24V 30A power supply, which doesn't quite make sense - can you post a link? And a photo of the connections to the heatbed would be good too.

Thank you for reply. Sorry that it took me so long, but I had to borrow multimeter from a friend Resistance at bed is 1,5ohm and voltage 6,2V. If it comes down to power supply I have voltage output set to 12V.

Here is link to power supply I have [www.ebay.co.uk]

And here are pictures of connections: [imgur.com]

Edited 2 time(s). Last edit at 06/11/2015 01:34PM by ExiledDiclonius.

Quote
ExiledDiclonius
If it is necessary I can set it up to 24. I didnt do it yet since I dont know if it is good idea.

You either have a 12v PSU or a 24v PSU. You cant turn a 12v up enough to make it a 24v.
The link you posted for the PSU shows you can select 12v or 24v options for purchasing.

I purchased 12V

There's a switch on the side of the PSU to select the input voltage, it sounds like you have it set for 240 when you are running it on 110. If that's not the case, check the voltage at the output terminals of the psu - it should be close to 12V under load. If you have 12V at the psu and 6 at the heat bed you have a problem with your wires (but it doesn't seem likely as to drop that much voltage they'd probably have melted the insulation and caught fire by now).

Edited 1 time(s). Last edit at 06/11/2015 01:56PM by JamesK.

Thanks for help. Will need to buy new wires. Voltage output on power supply was 12V while heatbed 6V.

Really? Wow, I would have put money on that not being the case. I was just looking at your photos and the bed wires look pretty reasonable in gauge. Is there any sign of the ramps board over heating near the connectors? I'm wondering if there's a high resistance at one of the connectors. The other possibility is a bad mosfet that isn't turning fully on, in which case you'd expect the green heatsink to be getting really hot. Before you replace the heat bed wires you could try disconnecting them at the RAMPS end and connecting directly to the PSU, then measure the voltage at the heat bed end again. If the voltage is OK at the bed when connected directly to the PSU then that would point the problem at the RAMPS board. Swapping out the mosfet isn't hard if you have a soldering iron, otherwise a spare ramps board would be useful to test with.

One more thing to check, if you are using Marlin there's a configuration option for reducing power to the bed that would probably confuse the meter:

// This sets the max power delivered to the bed, and replaces the HEATER_BED_DUTY_CYCLE_DIVIDER option.
// all forms of bed control obey this (PID, bang-bang, bang-bang with hysteresis)
// setting this to anything other than 255 enables a form of PWM to the bed just like HEATER_BED_DUTY_CYCLE_DIVIDER did,
// so you shouldn't use it unless you are OK with PWM on your bed. (see the comment on enabling PIDTEMPBED)
#define MAX_BED_POWER 255 // limits duty cycle to bed; 255=full current