New 24v system for heated bed not heating any faster?

Hey, I just got my 24v PSU installed and I wired it directly to my heated bed.

Originally, through my ramps at 12v, the bed took 13 min 50 seconds to get to 110C and would not sustain that temperature. I took all necessary steps to convert to a dedicated 24v PSU to run the MK3 on the 24v circuit. I measured the resistance of the 24V circuit and it was around 5.4 ohms.

I hooked my PSU directly to my heated bed, bypassing my ramps, just to see how fast I could get it to warm up. It is taken longer then the 12v through the ramps. What is going on? I soldered all connections, use heavy gauge high strand cable and for this test I am not even running it through a relay. I am completely puzzled.

Well, considering it is 5.4 ohms, thats only 104 watts. So, what should I do to put down more power. Can I run the 24v through the 12V circuit in the bed? Or if that seems too high, how about 16 or 18v through the 12v circuit in the bed? I am unsure what is causing the 24v circuit to be a flop, any help is appreciated.

Thank you,

Just hooked the 24v system to my 12v circuit and it gets to 110c in 2 minutes 30 seconds LOL,

Ok, that works.

Ohms law, Those heated beds have two resistors for all intense and purpose. if wired for 12v or 24v does not matter, they will put out nearly the same wattage. What I did to my own bed is I run it at 14v. Dropped the heat up time to to about 7 minutes to 105c. I am planing on running it around 30v and wiring it up for 24v to ease the load on my DC-DC upconverter as its basically limited to ~10A. At this point its putting out more than 10A on the bed. With that said, I run an unusually large power domain on my printer, ~700w at 110v or 1300w @ 220V. That gives me enough room to run everything else including a heater for my chamber and my web cam and a bunch of other accessories.

To put the math to work.

if your bed is like mike a MK3 heated bed which has a resistance of 1.4-1.6ohm at 12v or 5.0 - 5.4ohm at 24v

Here are the calculations
12.3v - 1.4ohm = 108W = ~8amps on the rail
14.0v - 1.4ohm = 140W = 10Amps on the rail
24.0v - 5.0ohm = 115.2W = 4.8Amps on the rail
24.0v - 1.4ohm = 411.4W = 17.1Amps on the rail
Running a mild boost should bring up the wattage while keeping the wires from going up in smoke.
35.5V - 5.0ohm = 252.05W = 7.1Amps on the rail.

It is much more friendly to be able to control the voltage on this kind of resistive load, as a bed that was meant to put out ~120w of heat running over 400w should have a very short life cycle. Even the mild boost I stated above would be taxing. I will probably set mine to run around 30v and keep the bed in the 180w-200w range.

As it stands my printer spikes to 475w out the wall socket when running full tilt and heating everything up.

Well what I did was run the 24v supply through the 12V circuit. This thing gets hot, real fast.

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Originally Posted by DBFIU

Well what I did was run the 24v supply through the 12V circuit. This thing gets hot, real fast.

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I would be scared to do that for very long. But I bet it would be ok for a minute or two to speed things up....

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Originally Posted by Roxy

I would be scared to do that for very long. But I bet it would be ok for a minute or two to speed things up....

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It's been running like this for a solid week now. It only sees the full 13 amps during warm up, then the relay cuts on and off every 5-8 seconds and stays on for maybe 2-3 seconds. Sounds like a 25% duty cycle once hot, not too bad. Surviving!!

The thing is that it's not drawing 13amps.. more like 17-20amps. You could have a fire on your hands...

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Originally Posted by jaguarking11

The thing is that it's not drawing 13amps.. more like 17-20amps. You could have a fire on your hands...

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The power supply is 24v 350watt, 14.6 amps max. Not sure if the PSU has a built in failsafe if it exceeds that current it shuts down or limits itself? I put a 15 amp fuse in line and it hasnt popped yet. I also used heavy gauge wire to power the bed. I am always near the machine when it is heating up.

Fuses are not a safe way to protect electronics. Your transistors will have been to Tibet and back before a fuse blows.

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Originally Posted by Mjolinor

Fuses are not a safe way to protect electronics. Your transistors will have been to Tibet and back before a fuse blows.

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Im powering the bed through the PSU and a mechanical relay, not through ramps.

Did you mean my PSU's internal transistors would go long before a fuse?

They would, fuses are horrendously complex things and their design is really complicated and difficult to understand but whatever the fuse type given the choice of a fuse and a transistor with both in overload state then the transistor will always blow first when the fuse is the sort with a burnable link.

What do you suggest I do?

I love how fast this bed gets up to temp.

I would just leave it. The PSU should be able to look after itself so if you are using a relay and it has been working so far then leave it. The only thing to watch with relays is the back EMF from the collapsing magnetic fields but I think that has been mentioned already.

If it fails then you just look at it as an opportunity to learn something new while you fix it, a plus in anyone's book. :)

Cool thanks for the tips.

By the way, I bought a bunch of Fotek SSRs, and they dont work, none of them. Yet the reprap forum says the Fotek SSR's should work as a DC relay. Mine say they are for AC, and dont work in DC power circuits? Not sure, do you happen to know what gives with the contradiction?

AC SSRs probably use thyristors for switching, DC SSRs will use MOSFETS.

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Originally Posted by Mjolinor

AC SSRs probably use thyristors for switching, DC SSRs will use MOSFETS.

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I think what this means is: The Solid State Relay will turn on... But it won't ever turn off because it needs the current to reverse to turn off the Silicon Controlled Rectifier. Right?

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Originally Posted by Roxy

I think what this means is: The Solid State Relay will turn on... But it won't ever turn off because it needs the current to reverse to turn off the Silicon Controlled Rectifier. Right?

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The thyristor needs the current to drop to zero before it will open circuit once it is short.

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Originally Posted by Mjolinor

The thyristor needs the current to drop to zero before it will open circuit once it is short.

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Right! That is what I thought. But I used the wrong term. I said 'Silicon Controlled Rectifier'. A Thyristor is two of those in parallel but pointed in opposite directions.