Converting to 24VDC on the heated bed

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Demolishun
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Converting to 24VDC on the heated bed

Postby Demolishun » Tue May 20, 2014 3:45 am

Okay, I have to fess up. I have not converted to 24VDC. I actually dialed a 24V supply down to 20V. Now I know this subject has been covered before, but I believe it was not covered very well. Because I am not using an external SSR to do this. I am using the RAMBo board to make this happen. I am also running at 12 amps...wait...12 amps...how can that be? 20V/1.2 ohms is 16.7 amps...wait...what?

Exactly! We are not using the features of the board to be able to use a 24V supply right from the board. The Repetier firmware has some nice features. First lets look at the RAMBo board features: http://reprap.org/wiki/Rambo. Look in the Features section under Power. It says plain as day the 1.2 version can handle 24VDC and 15 amps. Well, not unless we throttle the 24VDC. So how do we do that? That is where the Repetier firmware comes into play. Note: that I am only changing the voltage for the heated bed in this discussion. When changing voltages for other rails on the RAMBo board you need to be aware of all the devices on that particular rail. See the warning later in this post about the other rails and potential problems.

Now go to page 18 of this manual: http://www.repetier.com/w/wp-content/uploads/2013/12/repetier_fw_user_interface.pdf. Now I have no idea what version of the Repetier firmware this is from, and the section it describes is for the extruder PID. However, it told me what I needed to know. You have the ability to adjust the maximum PWM output for the PID loops. Since it is an 8 bit value you can put in 0 to 255. By default this output max is 255. So any power supply you put on there will be output at max value. That is why you cannot hook a 24VDC supply for any part of the machine unless you adjust some of the max outputs. Now I will explain how this is done for the heated bed.

First we have to understand the math here. If you don't understand this part it could still cause issues. Lets do some calculations for a 12VDC heated bed.
I measured the resistance of my heated bed at 1.2 ohms. I have one of the newer beds with the SeeMeCNC name and logo on it. I suggest you disconnect your heated bed and measure the resistance if you are not sure what it is. I believe most of the heated beds from SeeMeCNC are right around 1.1 to 1.2 ohms. Now since mine is 1.2 I will use that number. It also makes the math easier...

12VDC / 1.2 ohms = 10 amps (see why the math is easier?)
Now what happens when I do this for 24VDC?
24VDC / 1.2 ohms = 20 amps

Okay, now that is way too much for the 15 amp limit on the RAMBo board. Now what would happen if we cut that value in half? But how can you do that? Well lets look at the power of each. Power or wattage is calculated as Voltage x Current. So lets get the wattage for each scenario.

12VDC x 10 amps = 120 watts
24VDC x 20 amps = 480 watts

Wait?! I only doubled the voltage. How can the power be quadrupled? Well, son, that is just the way it is. Now how do we cut that power in half? I know, lets only turn it on half the time! Remember that setting in the Repetier firmware? It controls the duty cycle of the PWM output. This means it can control the max amount of time the output is on. This setting is called "Bed PID Max Value [0-255]". The default value is 255. So lets cut that in half and set to 127. Now the max PWM output from the PID loop is only on half the time. That means the heated bed has a maximum duty cycle of 50%. So only half the active time the 24VDC power supply is conducting 20 amps. This will effectively cut the wattage to 240 watts. The average current draw is 10 amps. See warning later in post about changing the extruder voltages.

Now, doubling the heated bed output is pretty cool. This takes the bed heat-up time down immensely. I was able to go from 20 degrees C to 60 degrees C during a PID loop tuning session in about 4 or 5 minutes. That is the main reason to pursue this for the heated bed.

Now, lets go back to my actual voltage. I am running my 24VDC power supply at 20 VDC. Why would I do that? Well, I want to give the PID loop more fine control of the output. So I chose a lower voltage that I could tweak my power supply to. The numbers ended up like this:

20V / 1.2 ohms = 16.7 amps
My target amperage is 12 amps (Gives the power supply and the RAMBo board wiggle room, always a good idea. My power supply is rated for 15 amps @ 24V)
So 12 amps / 16.7 amps = 0.72 or 72 % (about)
Now I use this find the value of the max output:
0.72 * 255 = 183 (about)
This is the number I put into the EEPROM value for "Bed PID Max Value [0-255]".
This gives me 20V x 12 amps = 240 watts. I still double my wattage, but put less stress on components by only surging to 16.7 amps max. I still get the average lower current that is well withing the tolerance of the hardware.

Now, be careful here. If you ever wipe your firmware you need to change the max output of the PID so you don't melt your connectors or burn up components on your RAMBo board. You may even want to modify the parameters in the code you use for your firmware to make sure you cannot mess this up. If you are really paranoid and know you might reset the firmware settings to default then by all means use a SSR instead.

The place to change this in the Repetier firmware is in the configuration.h file:

Code: Select all

// maximum time the heater can be switched on. Max = 255.  Overridden if EEPROM activated.
#define HEATED_BED_PID_MAX 183

I went ahead and changed mine so I don't mess stuff up if I change the firmware.

This should help people wanting to use higher voltage power supplies with the RAMBo board with the Repetier firmware. Other firmwares may support the max output so it should be applicable to those firmwares as well. This should help reduce the cost of using alternate voltages as well. I was somewhat driven by frustration when I bought DC-DC SSR that did not work properly. The output was on regardless of the state of the input. I then returned the SSR realizing it was defective and ordered 3 other models of different brands. I then started wondering why the RAMBo board had the specs it showed. That began my quest for parameters that might change the output duty cycle. I have successfully tuned the PID using these settings and am quite pleased with the lack of time it takes to warm up the bed now. I hope you get similar results and don't have to watch grass grow waiting for the heated bed.

Warning!
If changing the extruder voltage (we only focused on the heated bed here) it changes the fan voltages:
Power
Three independent power rails for flexible input power configurations
  1. Heated Bed, 15A 12-24V (version earlier than 1.1 limted to 14A 16V due to fuse rating, see rambo_development for hacks)
  2. Extruders and fans, 5A 12-24V (fuse can be exchanged for up to 10A capacity) 10V-26V input voltage
  3. Motors, 5A 12-24V (fuse can be exchanged for up to 10A capacity) 10V-26V input voltage
Built in SMPS for 5V generation

That means you will have to change your fans if messing with the extruder voltages. If you don't want to do that then use an SSR for the extruder.

Update:
The results are that I can heat up the bed from 30 deg C to 100 deg C in 15 minutes. Before I could only reliably get to about 80 deg C and that took 30 to 40 minutes. So this is a vast improvement for my machine!
Last edited by Demolishun on Tue May 20, 2014 10:32 pm, edited 2 times in total.
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Re: Converting to 24VDC on the heated bed

Postby Eric » Tue May 20, 2014 4:35 am

Yes, that's what PWM is all about.

The thing you didn't mention that MUST be remembered is that increased voltage to the Rambo will apply to ALL the Mosfet outputs, not just the heater bed. People have blown their 12V fans forgetting that. Of course you can do the same PWM throttling on all, but sometimes it's easier to use the SSR solution to avoid the side effects.

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Re: Converting to 24VDC on the heated bed

Postby Demolishun » Tue May 20, 2014 11:14 am

Power

Three independent power rails for flexible input power configurations
  1. Heated Bed, 15A 12-24V (version earlier than 1.1 limted to 14A 16V due to fuse rating, see rambo_development for hacks)
  2. Extruders and fans, 5A 12-24V (fuse can be exchanged for up to 10A capacity) 10V-26V input voltage
  3. Motors, 5A 12-24V (fuse can be exchanged for up to 10A capacity) 10V-26V input voltage
Built in SMPS for 5V generation


Did not realize that. Thanks for pointing that out. I have not messed with the extruder power so far though. Glad I did not burn anything up! I will edit the original post to make it clear that messing with extruder voltages will affect the fans.

Great catch Eric! Thanks
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Re: Converting to 24VDC on the heated bed

Postby bvandiepenbos » Tue May 20, 2014 11:41 am

Thanks for the detailed explanation
It is really helpful. I will be trying out the 24v supply I have now. (I adjusted it down to 19v)
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Re: Converting to 24VDC on the heated bed

Postby Eric » Tue May 20, 2014 2:00 pm

You're right, I wasn't completely clear. There is an independant bed input, but was thinking of those that just cranked up/replaced their sole power supply rather than adding a second. The motors can deal with voltage, as the drivers are current-limiting devices already. The logic is fine too as the +5V is regulated on-board from the input. The rest is up to the user.

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Re: Converting to 24VDC on the heated bed

Postby Demolishun » Tue May 20, 2014 10:35 pm

Spot on Eric. I made the assumption in my post that people knew I was only changing the heated bed rail. I just added a note early in the post that indicates this. Thanks for finding another issue that might cause problems for someone. It is those pesky assumptions that cause other people to swear sometimes.
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Re: Converting to 24VDC on the heated bed

Postby teoman » Sat Nov 01, 2014 4:17 pm

So, what is indicated is that, one should change the fans because they will be receiving the input voltage on the rail.

What about the motors? As they are current controlled will their PWM values need to be changed aswell? Also, for the fans can i change them in the firmware? Or must I change them?

I am in the process of building an enclosure for my rostock. And I will probably be removing the power supply from the unit. I had the idea of getting one of these:
http://www.skyrc.com/index.php?route=pr ... uct_id=167

EDIT: YOU CANNOT SET THE MAXIMUM AMPS THIS POWER SUPPLY DELIVERS AS I INITIALLY THOUGHT, SO IT DOES NOT HAVE ANY SIGNIFICANT ADVANTAGE OVER OTHER ANY OTHER POWER SUPPLY + AND AMP METER.

It is an 18V 30A power supply. I would prefer not to have a 2 power supplies, so if i can hook it all up to a single power supply that would be beneficial for me.

My idea is to encase the whole unit, remove the power supply, seal the electronics bay and modify the side panels on the rostock to integrate ducts so cooler air can be brought to the Rambo.
Last edited by teoman on Thu Nov 06, 2014 6:39 am, edited 1 time in total.
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Re: Converting to 24VDC on the heated bed

Postby Eric » Sat Nov 01, 2014 7:01 pm

A fan can be handled in hardware by adding an appropriate value resistor to the fan circuit to drop the extra volts (use a potentiometer if you want easy adjustment). Or you can handle the fan in software by never allowing the fan to run at 100% speed.

On the motors, when you turn the voltage up you may need to turn the current settings down a bit. But so long as motors aren't getting too hot, you're good.

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Re: Converting to 24VDC on the heated bed

Postby Earthbound » Sat Nov 01, 2014 7:28 pm

Demolishun wrote:Update:
The results are that I can heat up the bed from 30 deg C to 100 deg C in 15 minutes. Before I could only reliably get to about 80 deg C and that took 30 to 40 minutes. So this is a vast improvement for my machine!


I wonder if the thermal performance you get is limited in some other way. I've been reading and considering a 24v heated bed option myself. A few minutes ago I measured a baseline for my 12v setup. My Max v2 has a stock ATX power supply and a rev6 Onyx bed. The machine is in a slightly drafty area of the basement. At the start of the test the ambient temperature readings for the hot end and bed were 23.2 +/- 0.1. I set the bed target to 100C and reached it in 10:51. That suggests a faster heat up than you are now seeing with your 20v configuration. The only modification I made during assembly was to run three parallel 18 gauge wires to each power pad on the Onyx bed, instead of the single 18 gauge wire shown in the instructions.
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Re: Converting to 24VDC on the heated bed

Postby teoman » Sat Nov 01, 2014 7:59 pm

I am a big fan of these wires. They are very very flexible and the copper inside is multi strand.
http://www.banggood.com/1-Meter-14AWG-o ... 21159.html

Image

rated to 200C

PS: sorry for posting a lot of links from that site. They have free shipping and i purchase a lot from them.
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Re: Converting to 24VDC on the heated bed

Postby teoman » Sat Nov 01, 2014 9:44 pm

Eric wrote:A fan can be handled in hardware by adding an appropriate value resistor to the fan circuit to drop the extra volts (use a potentiometer if you want easy adjustment). Or you can handle the fan in software by never allowing the fan to run at 100% speed.

On the motors, when you turn the voltage up you may need to turn the current settings down a bit. But so long as motors aren't getting too hot, you're good.


So basically fiddling with these two right? That is unless I rewrite the firmware which would kill the fan and probably blow the fuse for the bed.
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Re: Converting to 24VDC on the heated bed

Postby bdjohns1 » Sun Nov 02, 2014 1:47 am

teoman wrote:So, what is indicated is that, one should change the fans because they will be receiving the input voltage on the rail.

What about the motors? As they are current controlled will their PWM values need to be changed aswell? Also, for the fans can i change them in the firmware? Or must I change them?



You don't set PWM on the motors actually - the value is scaled 0-255, but you're changing a digital "knob" that turns up/down the current. I'm being pedantic a little, I know.

I've since moved over to a Smoothieboard, and I can tell you that there's no problem running our steppers off a 24V supply. I'm running 1.5A on mine.

I actually got a rather beefy 24V supply (rated to 750W) - I have a 120W 12V step-down converter for the hotend and my fans. If I were on a Rambo, I'd use a 2nd 120W step-down for the board+steppers.

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Re: Converting to 24VDC on the heated bed

Postby teoman » Sun Nov 02, 2014 10:43 am

What about the fan that cools the rambo?
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Re: Converting to 24VDC on the heated bed

Postby teoman » Tue Nov 04, 2014 7:44 pm

Hnm. If the power supply can be current limited. Then I think one would not need to fiddle with the max pid values.

I wonder if the power supply is fast enough to react. And how will the pid react with another controller kicking in.
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Re: Converting to 24VDC on the heated bed

Postby Eric » Tue Nov 04, 2014 9:39 pm

You can regulate the voltage OR the current, not both. Current limiting means effectively reducing the voltage to get the desired current for the load. The entire load...there's no way for a single power supply output to have different characteristics for different parts of the same load. You either have to modify or replace the non-compatible parts of the load or have separate power supplies. In the case of small computer fans, adding a current-limiting resistor is often the easiest approach (that's what the speed adapters that come with some computer fans are...an in-line resistor).

If your only reason for increasing the voltage is for better bed heat, the solution that avoids all the complications is to keep the original power supply and use the new one only for the bed input.

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Re: Converting to 24VDC on the heated bed

Postby teoman » Wed Nov 05, 2014 12:49 am

Yes, when I was talking about current limiting the power supply, I had in mind that it would be dedicated tho the heated bed.
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Re: Converting to 24VDC on the heated bed

Postby Eric » Wed Nov 05, 2014 6:29 am

Ah, I was fooled by your earlier mention/context of wanting only one power supply. So we are back to the original subject of this thread, which is how to take a higher voltage power supply and current limit it via software to remain within the 15A limit of the Rambo bed input. Essentially making it look and act like a lower voltage power supply so far as resistive loads and fuses are concerned.

If you don't already have the power supply, get one with a lower voltage and the issue goes away entirely. Current limiting by math and design, if you like. Remember ohms law, V=IR. If your bed resistance is 1.2Ω, stay under 18V. If it's 1.1Ω, 16.5V. Power = V*V/R, so it's still a pretty good wattage jump.

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Re: Converting to 24VDC on the heated bed

Postby teoman » Wed Nov 05, 2014 8:04 am

Thank you for the feedback.

If you have a current limiting power supply, you gain some advantages. Such as:
1- When your bed heats up the resistance goes up, but as you are current limiting, the voltage of the power supply also goes up so you are still able to pump some energy to the power supply.
2- No need to change the software at all, (not that this is a problem, but if you update your firmware and forget, it can prevent unplesant surprises).
3- For people ignorant of electronics etc. This can be a very plug and play solution.

The power supply i linked to previously is a tad expensive, but it does have voltage and current control. (18V and 30A limited).
EDIT: IN FACT IT DOES NOT HAVE THE ABILITY TO LIMIT THE CURRENT :(
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Re: Converting to 24VDC on the heated bed

Postby critical_limit » Wed Nov 05, 2014 8:25 pm

teoman wrote:Thank you for the feedback.

If you have a current limiting power supply, you gain some advantages. Such as:
1- When your bed heats up the resistance goes up, but as you are current limiting, the voltage of the power supply also goes up so you are still able to pump some energy to the power supply.
2- No need to change the software at all, (not that this is a problem, but if you update your firmware and forget, it can prevent unplesant surprises).
3- For people ignorant of electronics etc. This can be a very plug and play solution.

The power supply i linked to previously is a tad expensive, but it does have voltage and current control. (18V and 30A limited).



teoman,

don´t get me wrong, but as Eric said. there is no current AND voltage Regulation at the same time.
Please try to understand ohms law and you will get it.

You mentioned a Power Supply that is at 18V and limited to 30A. Ok, That´s not a big deal. But if you follow Mr. Ohm, and that´s why it´s called a LAW, U=R+I.
For us , and only for that is my example, this Law means:

U (=18V) = R (1.1ohms) * I

so I is:

I= U/R

18V / 1.1ohms = 16,262626 Amps !!!!

So it doesn´t matter if your Power supply is rated to 30A. That only means, that your power supply can handle more than your heatbed is asking for and is capable to provide the 18V you set.

And as I remember right, 16.3636amps is out of Rambos spec (15Amps). You will blow the Fuse!

What you need is a Power Supply that is not capable to supply more than 15Amps and due to the specs of a switching power supply it will reduce the Voltage to not overshoot the 15 Amps.
Can you follow?

And yes, due to our Heatbeds are PTC (positive Temperature Coefficent) the Voltage will raise the hotter the bed is. But that´s difficult to understand without understanding the basic of Ohms Law.

The real thing what we are interested in is the Power (Wattage) our Bed is delivering. And that´s what Eric said before: P = (U*U)/R or: P=U*I

So please try to understand the basics before you give any recommendations to the members which Power Supply they should use.

So Folks:
Don´t use a Power Supplies for you Heatbed that is capable of delivering more than 15Amps! This is out of Spec and will blow your Fuse in short term and will damage your Rambo in long term means.

The only possible solution to save your Fuse and your Rambo Board and deliver more than 15 Amps To your Heatbed is to use a SSR !!


And again yes. I know that some of you are using more than 15Amps with reducing the PWM of the Mosfet. But that´s a different question and needs some mods to your Firmware and/or eeprom settings. And as Teoman said, once you forget about this setting while updating your firmware or clearing your eeprom, you will blow in best case your Fuse, in worst case your board.

And beside that. If you are willing to modify and use a SSR: have a look at the switching Frequency your SSR is able to handle! ;-)

For all Newbies:
Don´t fiddle with your Power Supply unless you completly understand the Ohms Law and you know what you are doing!

Just my 2 cents....

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Re: Converting to 24VDC on the heated bed

Postby teoman » Thu Nov 06, 2014 6:37 am

I believe we are having some kind of communication problem. I may be a language barrier. I am all too familiar with ohm's law and electric/electronics.

I did just notice that i have misinterpreted the link to the power supply i posted. I was under the impression that it was MAX voltage AND MAX current adjustable. Like the lab power supplies. But it turned out to be only voltage adjustable.
SO, DO NOT GET THE POWER SUPPLY THAT I LINKED, IT DOES NOT DO WHAT I THOUGHT IT DID

So let me retry with what I had in mind:

Considering you have a lab grade power supply that is capable of delivering 20A or so, you can then set the maximum voltage to 24V and maximum current to 12.5A. In this case, it should never deliver more than 12.5 Amps and therefore the RAMBo and it's fuse should be happy all the time. As the amps are regulated by the power supply itself, changes to firmware would not be necessary in my opinion. This kind of setup would be slightly expensive. But it does have one advantage. The lab grade power supply does have a closed loop control over current. Therefore, it is still able to deliver the same current even when the bed temperature rises and the resistance along with it. I have not done calculations or tests, so i do not know by how much the resistance rises, or if it is desirable to get the bed to such a high temperature to observe significant resistance increase which would necessitate higher voltages.

I hope that does make what i am trying to explain clear.

And if it could be done for cheap, this would (again in my opinion) create a plug and play solution to bed heating as all you would have to do is set the current and connect it to the RAMBo.


@critical_limit: I edited my posts to reflect my previous error.
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