Upgrading Heater Bed

[addisonElliott]

Hello,

I am planning on buying a Rostock Max v2 within a couple of weeks here and I just want to prepare for some upgrades. I have read a lot of reviews about the Rostock Max v2, and I have heard good things. I had a couple of questions regarding upgrading the heated bed.
I am going to buy a 24V power supply to hook directly into the bed. The heated bed outputs 130W of power, so I figured 300W would be more than enough for me. I found a Meanwell NES-350(350W) for $52.99 on amazon. Is this a nice model? Is it going to fail on me?

I understand that the heated bed will draw ~22A and the PSU can only handle 14.6A. This means I will need to use PWM to bring it down, but doesn’t that still draw 22A just in little spurts? Is this going to break my power supply?

Since my goal is to output 300W from this power supply, this amounts to 12.5A. This is ~86% of the maximum current, and I have read that it is best to stay under 80%. Should I upgrade to a better power supply? Will running at 85% destroy my power supply in the long run?

I have also taken a look at SSR on the web. I noticed that these can be expensive since the relay has to dissipate large amounts of heat due to the large currents. I have found a 12V automotive relay that can handle 30A for $7. This uses an electromagnet to activate the switch, which requires no dissipation of heat such as a MOSFET or phototransistor would require. Is there any negative side to using an electromechanical relay instead of a SSR?

The only issue I currently see is that the automotive relays are meant to output 12V but I will be outputting 24V. If they don’t have enough spacing, then arcing could occur. I will have to dig around in the datasheets to see what the dielectric strength is.

I plan to upgrade the 12V power supply to a Corsair 430M and the dimensions of the 24V power supply are 215x115x50mm. Would these both be able to fit underneath of the printer? I would like to keep it out of the way. Would I need more fans for cooling?

I have a side question. I have heard from numerous people that it takes 15-20minutes for the heated bed to heat up to 100degC. My old printer(Prusa i3 rework 8” heated bed) took the same amount of time, and I measured 100W output for it, including voltage drop. When I increased the power to 120W and added some insulation, it only took 10minutes for it to reach 110degC. However, the rostock max heated bed should be receiving 120W as well, but it takes just as long as my 100W heated bed. I am neglecting that the rostock has a larger heated bed, no insulation, and the voltage drop across the terminals, but does that really make that big of a difference?

For others who have upgraded their heated bed, how much power and how quickly does your bed heat up?

Thanks for any help.

[nitewatchman]

I think that there may be some fundamental problems here. I think that the 300W power supply is a bit small. It will likely be running in current limit during initial heating and not give you the results you want. I used the Meanwell SP-500-24 power supply to supply everthing (single p/s located in the base. A very good guide for this change would be the post http://forum.seemecnc.com/viewtopic.php ... 9&start=50. I did mind in the same fashion and the performance has been flawless.

Using a electromechanical relay also is not such a good idea. If it is being driven by the Rambo in PWM made, it will sound like an MG-42 (Hilter's Buzzsaw from WW-II) and not give very good results with a short life if it works at all. In band-bang mode it may work but you loose the PID control of the heated bed. Additionally, if you use a mechanical relay make sure that it has a diode across the coil to snub any reverse voltage flow due to EMF when the coil opens.

I shopped my parts on e-Bay. I found the SP-500-24 for $36.36 delivered. The Crydom SSR (more about this) for $9.99 delivered new in the box and the 24VDC to 12VDC p/s for $1.99 + $5.00 delivery. Be cautious with SSR's, there are fakes and grossly over rated devices floating around. This seems to be particularly true of the Flotek brand. My SSR is mounted to a 1/2" aluminum plate bolted to the side of the P/S (doesn't have to be that thick, it was what I had). During operation, the relay does not even get warm to the touch. The 24VDC to 12VDC p/s gets slightly warm.

This works well and heatups are fast. 2+ minutes for PLA, 4 to 5 minutes for ABS/HIPS.

[addisonElliott]

I didn't consider the fact that I was using PWM with a electromechanical relay, so that's definitely out of the question now. It seems like you got really good deals on your items. I cannot find a 500W meanwell for less than $70 on ebay.

Also, what model is your crydom SSR? That sounds like a great deal to me.

[jdurand]

PWM or not, the max current draw of the power supply shouldn't be exceeded. Not unless you like replacing power supplies. Many won't trip when driving a PWM because the pulse is too short for them to see, they'll just burn out.

Also, very very few "regular" power supplies current limit. They trip off, or if they're cheap they go poof.

Second, with the current bed you need a 24V supply that puts out at least 24 Amps , so that would be a 600 Watt supply.

[nitewatchman]

I used a Crydom D1D40 (http://www.ebay.com/itm/CRYDOM-SOLID-ST ... 1c4db08079) (http://www.ebay.com/itm/Lot-of-6-CRYDOM ... 43d7373314) (http://www.ebay.com/itm/MEAN-WELL-SP-50 ... 4d2c6ffcde).

The 500W unit is good to go if the maximum output voltage is dialed down to ~20VDC using the voltage adjustment pot on the end of the power supply. The p/s fan rarely runs at high speed and only then when the bed and hot end are both heating at max rate.

I got the deal on the p/s by making an offer, it was listed at $59.00 originally.

I originally built my printer with a Corsair RM750 p/s, about $125.00. It worked well but heatup was still a little slow. In hindsight the single 24VDC p/s done in this fashion should be doable for the same amount or for me less. Depends on what is for sale on that day.