PCB Heatbed not getting hot enough

Our PCB Heatbed (MK1) can't reach 100°C. After 10 to 15 minutes it reach 80°C maybe 85°C but I cannot recall if it ever goes over 90°C.

What's the most probable source of this problem?
• We have only 350W ATX power source with 14A cord. Is it too weak?
• Poor (air and cork) from underlying aluminium plate?
• Low ambient temperature 18°C
• Something else?

Thank you for ideas.

Here is the image. Click to enlarge.


Edited 1 time(s). Last edit at 10/19/2012 07:40AM by jakub.sturc.

You need at least 10 A from your +12 V leg in order to heat the bed. This is in addition to the current needed for the extruder heater, stepper motors and controller board. So a minimum of 16 A is required. To be safe, your power supply should provide more than this.

jakub.sturc Wrote:
> What's the most probable source of this problem?
> • We have only 350W ATX power source with 14A
> cord. Is it too weak?
> • Poor (air and cork) from underlying aluminium
> plate?
> • Low ambient temperature 18°C
> • Something else?

Probably a few things. Check your power supply with a meter and ignore the ratings. I have a 350W ATX supply rated at 15A@12V but drops to 10.5V pulling 7A. I also have a 450W Antect rated at 17A that drops to 11.5V at the supply. If you measure a voltage drop at your supply then you need a better supply.

Once you have a good supply you can measure the voltage close to your supply and at the heatbed. On my setup it is a bit over 1V which is significant. (about 20% loss of power at the heatbed)

I've just been testing my bed in response to another thread and with a 15.5C ambient, insulation under the bed but nothing surrounding the build I couldn't raise bed temp above 104C even after 40 mins, with an insulated cover for the heated bed I easily got to 110C but dropped to 104C after I took the cover off.

@Logrod: You're starting from a very low temperature.You'll need to turn on your room heater or put your printer in a box.

I've heard the PCB heat beds can be inconsistent and that has been my experience. Personally I've never had one that would heat to 110 c on 12v. However I haven't checked voltage under load so it's possible my psu is not suppling 12v like Gibbedy shared.

On my printers I run higher voltage to my heat bed. They both use RAMPS 1.4 which supports up to 35v if the 1N4004 diode is not used. I use a 24v switched power supply on one and the other runs 17v from to computer power supplies I wired in series (using 12v rail on one to the 5v rail on the other). They both easily heat to 110 c now.

According to net wisdom, a lot of ATX power supplies require a load on the +5V leg in order to stay on and provide enough current on the +12V. I have a 10 Ohm, 10 W wirewound resistor on mine.

I have found they need a load on the 5v supply to stay on.

I can't see why voltage regulation on the 12V supply would have anything todo with the load on the 5V supply. I would like to see a link to any atx design that does this if someone has come across this. I'm too lazy to look into it and dismiss it as rubbish.

In my opinion the best option would be miro87043's and just increase the voltage untill you get 12V at the heatbed. That solves the problem of shitty regulation and voltage drop. I'm guessing a cheap atx supply could be modified for this.

There's usually only one transformer with both 12V and 5V coils wound on the same core, so increasing output voltage to compensate for voltage drop on loaded 12V will cause overshooting on unloaded 5V rail.

@ brnrd

Yes I realise ambient is low and as soon as I no-longer need regular access to all the axies I'll be adding an insulated chamber to the printer to provide both a heated chamber plus a way of extracting fumes easily. At some point I want to start playing with chamber temperature (but I need to be able to print first!) so I've allowed for using +5Vand +3.3V to heat resistors. I wanted to see also if with a heated chamber +5V would be able to provide a reasonable bed temperature.

My current ATX supply (the last one was scrounged from an old computer but started smoking one day when I switched on the bed) has 3 separate 12V rails; I use one for my Sanguinolulu, motors, and hotend and a second via a relay for the heated bed. This one is a better regulated supply so doesn't need a permanent 5V load but I had to use one on my last supply.

Is there any reason to limit yourself to the capabilities of an ATX supply? Here's a good one that would handle the load you specify for under $40 US:

12V, 12.5A (150W)

That same supplier has similar models at 200W, 320W and 400W if you want more power overhead.

B-

12V at 12.5 A is not enough if you want a heated bed. You should get at least 20 A. The input power rating is not the critical spec although.

The power rating given is output, not input:

12V * 12.5A = 150 W

20A would mean 240W so the 320W and 400W models would be more than sufficient.

Turns out loading up 5v did improve regulation. Instead of dropping to 11.3V My crappy supply held at 11.7V. I got similar results on an antec earthwatts 500W. Which makes me wonder what you have to spend to get a supply with independant regulation.

You can buy 12V 20A switching power supplies from ebay for less than $30 (USD).

I have the same problem with my MK-1. It never gets hotter than 90o. My Power Supply is a 250W 12V 20A. So it should be enough.
I checked the voltage at the power supply and it´s very stable when a turn on hearbed. at the mosfets output its marking 12V sharp and in the heabed it´s marking 11.2V. SO I checked resistance. The resistance of the heatbed is 3.5 ohms and the resistance of the cable that connects the heatbed to sanguinololu is 0.8 ohms. Is it normal?

Just based on the resistance of the heatbed alone, I'd say there is definitely something wrong.

12v / 3.5ohms = 3.43A

12V * 3.43A = 41W

so even with full voltage the current is nowhere near the 11 or 12 Amps considered minimum for a bed heater.

For comparison, 12V @ 12A = 144W. To achieve this you would need a heatbed resistance of 1 ohm (12V / 12A).

@edu_gomes:

Those resisance measurements are not consistent with the voltages. If you have a 0.8V drop through 0.8 ohm resistance leads, then this gives only 1 A of current. With 11.2 V across the heated bed at 1A current, this would give a resistance of 11.2 ohms for your MK1. Did you subtract the resistance value that you read from your meter when you connect the two test leads together? Perhaps your meter is not accurate in reading low resistance values.

Anyway, a drop of 0.8 V through your leads indicate that your wires are not thick enough or you have a bad connection somewhere. You should use 14 gauge wires.

I actually dont know a thing about electricity. So i didnt understand what you said
Maybe I'm doing something wrong. I put the meter in the 200 ohms position and put each probe in a terminal of the pcb heatbed. It gave me the 3.5 ohms reading. The same with the cable. Aprobe in each end of the cable.

If your bed is getting to 90C then it's not a simple question of your power supply not having enough oomph (aka amps) - the bed takes no more power at 90C than it does at 40C. There are two possible issues:

1) Your bed is putting out the design wattage (ie no resistance or voltage issues) but that wattage isn't large enough to compensate for the heat loss to the environment above 90C (the heat loss to the environment increases with bed temp so you get to a point where losses balance maximum bed output)

2) Due to increased resistance in bed and/or supply wires or low voltage supply from your power supply your bed isn't capable of delivering design output (again the actual output being equal to the losses to envirnoment at 90C). In this case you need to solve the resistance issue both in the supply wires (use thicker wires) but also in the bed itself as DCG Partworks calculates 41 Watts will not be enough to heat your bed (for what its worth I'd be surprised if your output is as low as 41 W and you're still getting to 90C , this sounds like an error in measuring the resistances. One thing you should make sure of is that you disconnect the bed from the control board (ramps/sanguinolulu) before you measure the resistance. My bed is approx 1.3 Ohms resistance so my 12 volts supply generates 12x12/1.3 = 110 Watts which gets me to 104C (but I do work in a fridge!)

I will try to get another multimeter to verify the pcb and contact the company a bought the pcb from. Thank you guys.

Connect the two multimeter leads together and measure the resistance. Ideally, this should be 0 ohms. But usually, you'll get a non-zero reading. Remember this value and subtract it from all readings taken at this scale setting.

edu_gomes Wrote:
-------------------------------------------------------
> I actually dont know a thing about electricity. So
> i didnt understand what you said
> Maybe I'm doing something wrong. I put the meter
> in the 200 ohms position and put each probe in a
> terminal of the pcb heatbed. It gave me the 3.5
> ohms reading. The same with the cable. Aprobe in
> each end of the cable.

Finally we did some measurements and heated bed is under 11.2V so it seems that in out case it's not root of the problem.

On the other hand aluminium plate under HB (see photo above) is getting real hot. So I am starting to believe that heat is leaking through it.

Try using nuts or spacers to put dead space between the PCB and the cork. Also, if you didn't put a load on your +5V rail,you should try that too to get more voltage from the +12V leg.

Quote
Logrod
If your bed is getting to 90C then it's not a simple question of your power supply not having enough oomph (aka amps) - the bed takes no more power at 90C than it does at 40C. There are two possible issues:

1) Your bed is putting out the design wattage (ie no resistance or voltage issues) but that wattage isn't large enough to compensate for the heat loss to the environment above 90C (the heat loss to the environment increases with bed temp so you get to a point where losses balance maximum bed output)

2) Due to increased resistance in bed and/or supply wires or low voltage supply from your power supply your bed isn't capable of delivering design output (again the actual output being equal to the losses to envirnoment at 90C). In this case you need to solve the resistance issue both in the supply wires (use thicker wires) but also in the bed itself as DCG Partworks calculates 41 Watts will not be enough to heat your bed (for what its worth I'd be surprised if your output is as low as 41 W and you're still getting to 90C , this sounds like an error in measuring the resistances. One thing you should make sure of is that you disconnect the bed from the control board (ramps/sanguinolulu) before you measure the resistance. My bed is approx 1.3 Ohms resistance so my 12 volts supply generates 12x12/1.3 = 110 Watts which gets me to 104C (but I do work in a fridge!)

comment was about using resistors. did 4 vishay 3.3 6w resistor and paralled the board. got up to about 70c and i stopped it at that point. the solder connecting the vishay's started melting. so my thought of using resistors is not feasable. would have to use a resistor with a heat sink. since i do not have one at this time. i do not see that working long term for heating the bed up faster. how ever in my experiment it did heat the bed considerably faster. it went from 25c to 70c in about a minute. it measured out about 1.26ohms in parallel with the head bed.

Edited 1 time(s). Last edit at 08/10/2015 02:45PM by batchkrazy.

Quote
batchkrazy
Has anyone tried adding a 3.3 or 10 ohm resistor to thier bed to drop its resistance. 3.3 would bring it down to around .8 and the 10 would drop it down around .9. this would help your bed heat up faster and get to higher temps. not sure what it would do to the rest of the system.

It doesn't work that way. The circuit's total resistance might be what an ideal resistance would be, but the actual PCB bed's resistance would remain the same plus you'd have a resistor heating up somewhere, wasting power. You could attach the power resistor to an aluminum bed somehow, but then you'd get a hot spot localized. The only time where adding a resistor would work easily is if you were already using an array of them.

If the bed's resistance is too high and you can't adjust the voltage, the only solution is to get a different bed.

i agree. it was a scary experiment.