Bed takes forever to heat up?

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Hdale85
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Bed takes forever to heat up?

Post by Hdale85 »

So most people recommended me to print ABS with a bed temp around 100c, but it takes the stock bed FOREVER to get up to that. I'm curious if I grab that bed SeeMeCnC has on sale for 24v would it heat up faster if I convert over to 24v?
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Re: Bed takes forever to heat up?

Post by McSlappy »

What version bed are you using?

And yes 24v would greatly speed up heat-up times!
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Hdale85
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Re: Bed takes forever to heat up?

Post by Hdale85 »

The stock Onyx which runs at 12v stock. You can run it on a 24v psu but apparently you have to PID limit it which kind of defeats the point as you'll just be running it at 12v then.
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Re: Bed takes forever to heat up?

Post by IMBoring25 »

The listing says those boards are 1.2 ohms. At 24V, they'll draw 20A. I hope the conversion you're pondering involves a dedicated 24V power supply, SSR, and some very heavy wiring.
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nitewatchman
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Re: Bed takes forever to heat up?

Post by nitewatchman »

I am running mine at 20VDC. I did not have to touch the PID Loop other than rerunning the auto calibration routine.

The heat up is dramatically improved when you switch to 24VDC. I used a Meanwell 500Watt 24VDC Power supply, and a Crydon D1D40 SSR. The fans, hotend heater, steppers, and Rambo were left at 12VDC by using a 24VDC to 12VDC Voltage Regulator. I have the Rambo Fan, Hot End Fan, 3 layer fans, 2 80mm base cooling fans.

The power supply as a voltage output adjustment screw. Adjusting the output voltage to 20VDC makes the current draw a little more compatible with the 480W output. Been running almost everyday since December.

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Re: Bed takes forever to heat up?

Post by Jimustanguitar »

Hdale85 wrote:The stock Onyx which runs at 12v stock. You can run it on a 24v psu but apparently you have to PID limit it which kind of defeats the point as you'll just be running it at 12v then.
Who says? Lots of people run 24VDC at full 24. It toes the line on what the Rambo can handle, but it's definitely been done successfully.
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Re: Bed takes forever to heat up?

Post by Hdale85 »

Hmm, well that's interesting. I want to swap my steppers over to 24v as well though. Either way shouldn't be hard to adjust the PID max down a bit to run this bed a bit lower than 24v.

And as far as who said that it was in some info I saw from SeeMeCNC.

Nitewatchman, if I adjust the PID max down so it's not feeding 24v would I really need to run it via a separate relay? It should draw less amps at 24v, and the Rambo can handle 24v as well.

ImBoring, Yes I was planning on using dedicated PSU's and the listing also states you might have to throw a resistor to limit the amperage. But if I can get a lot better performance with the board I have I'd rather just do that.
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nitewatchman
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Re: Bed takes forever to heat up?

Post by nitewatchman »

You must watch the voltage and the current. The Rambo can handle and operate at 24VDC BUT if I recall correctly the Rambo Bed Heater is Fused at 15A. Thinking that the bed will pull fewer amps at 24VDC verses 12VDC is a common misconception. Remember that Ohm's Law states the P=E*E/R or the Power in a system equals the voltage squared divided by the resistance and A=P/E or Amperes equals the Power divided by the voltage.

So for 12VDC, P = 12*12/1.2 or 120Watts. A = 120/12 or 10Amps. Normal heating of the bed = Slooow :(

Or for 24VDC, P = 24*24/1.2 or 480Watts. A = 480/24 or 20Amps. Expedited heating of the bed = Fassst :D

Working this the other way if we limit the current to the Rambo's 15A fuse restriction. Maximum allowed voltage is E = I*R or E = 15*1.2 or 18VDC

So for 18VDC, P = 18*18/1.2 or 270Watts. A = 270/15 or 15Amps. Compromised heating of the bed = Kinda Fast :|

So you see that the biggest advantage of going to 24VDC is that the wattage output of the bed is increased by 4X. Running through the Rambo and limiting the current to 15A the wattage output of the bed is only increased by 2.25X.

I am sure that you could manipulate the power supply voltage and reduce the maximum on-time in the PID Loop to limit the current to below the fuse rating but this tends to defeat the purpose of moving to 24VDC. (2.25X verses 4X the heating capacity of the bed)

I run my 24VDC Heated Bed PID Loop limit at 255 (WAO) and adjust the voltage output of the 24VDC power supply so the power supply maximum current draw with everything running is about 19A to 20A (Meanwell 500W ps) measured with a Fluke Clamp-On Amp Meter. Use could also manipulate the screw until you hear the power supply start going into current limit and crowbarring down.

My Heated Bed hits 100C in 3 or 4 minutes for ABS, 60C for PLA in about 1 minute, 70C for PETG in 1-1/2 minutes.

Also remember all of these calculations assume that the proper size conductors are used based upon amperage. I would recommend a #12 copper wire as a minimum size for the 20A load and a #10 would not be overkill.

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Re: Bed takes forever to heat up?

Post by Hdale85 »

So the best way then is to use a 25 amp relay or so and run the heated bed directly off the PSU being triggered by the bed output of the rambo? That way I can get the 20amps it needs and run it at 480w?
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Re: Bed takes forever to heat up?

Post by nitewatchman »

That is what I did. I used a Crydom D1D40 SSR (100VDC - 40A) and even running at 100%, it doesn't even get warm. This also appears to be a common eBay SSR. Remember MOST SSR are AC units and not what you want. Be sure that it is DC input and DC output. As luck would have these are also more expensive that AC SSR's.

Beware that there are some counterfeits circulating around. Primarily the ones in the white cases. Apparently they can get very hot and lock on since there actual ampacity does not nearly approach the rating printed on the case. I have not heard of a bogus Crydom, most have been Fotec.
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Re: Bed takes forever to heat up?

Post by Hdale85 »

Why not an automotive style unit like one of these?

http://www.amazon.com/Vehicle-Relay-Swi ... s=24v+spdt" onclick="window.open(this.href);return false;

There are some 24v 100amp units even.
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Re: Bed takes forever to heat up?

Post by IMBoring25 »

Those are mechanical relays, not Solid State Relays (SSRs). They will not be as reliable as a good SSR for the number of cycles that will be involved on a heated bed and the clicking will drive you up the wall.
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Re: Bed takes forever to heat up?

Post by nitewatchman »

The power supply switching is likely so rapid that a mechanical relay would sound like an MG42 and fail quickly. The relay, especially a large one would never remotely keep up.

http://www.ebay.com/itm/NEW-IN-BOX-CRYD ... 419b7caf29" onclick="window.open(this.href);return false;

http://www.ebay.com/itm/CRYDOM-SOLID-ST ... 6e&vxp=mtr" onclick="window.open(this.href);return false;
Hdale85
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Re: Bed takes forever to heat up?

Post by Hdale85 »

Ugh, well adds a bit more money to the swap over.
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Re: Bed takes forever to heat up?

Post by teoman »

First things first,

Check the wires / connectors on your rambo to make sure they are not heating up or the connectors are not burning / melting.

Then try the dish towel trick. Where you place a small towel over the bed so it does not lose heat.

Then you could change the wires going from the rambo to the heated bed.

The stock psu (mine atleast) drops to 11 V at the bed terminals. Changing both the input and output wires brought it closer to 12V.

Finally you can run directly plug a 15v psu (12v cranked up) to the heated bed input and that will not blow the fuse on the rambo.

While doing this mod, i did place copper heatsinks behind the chips on the rambo to help with heat dissipation.
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