Is it ok to crank up the voltage of my 12V psu?
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Hi all. I wish I'd read enough to know how easy it would have been to build my Dbot around a 24v psu and heated bed, but I didn't, so here's the question:
Is there anything wrong with just using the trim pot on my Meanwell 12v psu to crank it up to 13v or 14v? I'm not sure how high it would go, but assuming it would go to 14v, is there a good reason I shouldn't do it?
I ask because my 12v heated bed will only hold around 96 C or so. I had a print I'd specified as 105 C in S3D, and I laid a spare cork board over the bed to insulate it on top while heating up to speed it up. After a good long while it actually hit 105 C and I removed the cork board and started the print. Over the next few minutes the temperature fell to around 96 C or so and stayed there. Apparently at 12V the heated bed just doesn't have the wattage to hold the 105 C. It's well insulated on the bottom with cork board.
My heated bed is around 1.3 ohms. If I could jack my psu up to 14v I could go from around 9A on the heated bed to around 10.5A. I know the Duet Wifi will run fine all the way up to 24V. I'm assuming that if I ran it at 14v that same 14v would be passed through the heated bed mosfet.
It would also be passed through to the 12v fans I'm using. I don't know how well they'd like that. If they would be harmed by seeing 14v, could I mitigate that by limiting the fan power to 90% or 95% or so on my PWM fan headers? None of my fans are hooked up to the always-on headers.
Have any of you tried this approach to improving the bed heating speed and top end temperature? If so, were their any issues?
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I've done it a long while ago on my i3 I used to have, running off a ramps board, ran it at 13.8v. Fans don't mind sure they'll run 15% faster which might shorten their lifespan by 15% but hey ho. Everything else will be fine, heaters are better at higher voltage even 13.8v instead of 12v makes a big difference.
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DjDemonD, thanks for your comments. I've never thought of doing that with my Di3, probably because the filaments I print on it don't need more than 70 C or so on the bed, and so it just hasn't been an issue.
I guess I'll have to get my multimeter out and do some adjustment.
Question: before you cranked it up from 12v to 13.8v, did you establish a maximum temperature that your bed would reach, and after going to 13.8v, by how much did that maximum go up?
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Not theoretically no, I just tried to get it to 110 degC which it did (MK3 aluminium backed PCB heater). But lots of insulation under it, and a sheet of foil bubble wrap over the top during heating. Its so frustrating using PCB heaters at 12v I don't have any now, I've got a 200x300 one on 24v (using the 12v connectors so it heats up fast, is limited to 250w by the PSU being only 250w) and a 240v 700w mains heater which is really the way these things should be done.
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Two things to consider before you turn up your power supply voltage:
1. Will your PSU be able to provide enough power? The higher the voltage you set it to, the lower the current you can draw safely from it. If it's a 200W power supply then you may already be close to its limits. OTOH if your PCB bed heater has a higher resistance than typical (which would explain why it doesn't heat very well), you may be OK.
2. 12V fans are normally rated to 13.2V maximum. If you go higher than that, you should put 2 silicon diodes in series with them to reduce the voltage a little. Each diode will reduce the voltage by about 0.8V. However, people often don't bother adding the diodes, and most 12V fans seem to survive running at 14V.
EDIT: to reduce the voltage to all the fans at once, make up a "jumper" with 2 silicon diodes in series (e.g. 1N4001) and use it as the V_FAN jumper.
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Thanks again for the comments, guys!
My pcb has a resistance of around 1.3 ohms (measured by me), which I don't think is atypical for a 200x300mm 12v pcb bed. My power supply is a 350 W Meanwell 12v power supply, so I think it should have the head room.
I will probably crank this up a little and see how it goes. I'll probably start with just bumping it from 12v to 13v. That should scale the power to the heated bed by around 17% or so. It'll be interesting to see what effect that has on heating speed and max temp.
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1.3 ohms is very high for a 200x300mm bed, no wonder it is under-powered. Your 350W PSU gives you lots of power to spare, so no worries theree. See the edit I made to my previous post about how to reduce the voltage to all the fans at once.
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1.3 ohms is very high for a 200x300mm bed, no wonder it is under-powered. Your 350W PSU gives you lots of power to spare, so no worries theree. See the edit I made to my previous post about how to reduce the voltage to all the fans at once.
Ok, I just pulled the XT60 connector off my heated bed and remeasured it, and it's actually 1.2 ohms, not 1.3. Is that still very high for this size of bed? I jammed the probes into the XT60 connector to make this measurement. I think when I measured before I was just touching the pads or something.
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It is difficult for multimeters to measure low resistances accurately.
What is the measured resistance when you touch the probes together?
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You could measure the voltage drop with heater on and off and then calculate the resistance.
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The voltage drop of what? The power supply? That wouldn't work/help, the drop there is a function of the PS regulation.
The voltage drop across the heater will be whatever voltage is applied to it.
Also for Ohm's law to let one calculate resistance, both voltage and current are needed.
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I adjusted my voltage to 13.06v (the pot changed tons for just a tiny movement, so I called it good) and my heated bed will now reach 105 C uninsulated by the cork sheet on top, so that's nice. It also heats up to my usually temperatures (60 and 70 C) noticeably faster. I think I'm going to call it good for now.
