My heated bed runs at 20 amps, MANUAL SAYS 10 Amps!

[mrbi11]

The resistance of my onyx bed heater is 0.6 ohms,
which i verified by measuring its current at 20 amps.

And it heats up pretty quickly, maybe 10 minutes to 100C

I gather others that take twice as long have the resistance the manual gives.

Not sure how much this matters, but the 14 gauge wires are not rated for 20 amps,
so it's good they heater does not run full time. That should require 10 gauge.

But as I read other postings, I gather other's beds are running with less power,
so my main question is am I going to fry my Rambo card?
I was pretty amazed it could switch 20 amps.
The connectors are not sized for 10 gauge wires, among other things.

Is this a new heater or a variation in manufacturing?

Should I replace it to protect my rambo card?

[Eric]

The real reason larger gauge wires are recommended is to minimize voltage drop over the length of the wire. In this case the run is under 2 feet, so 14 gauge should be fine for 20 amps at ~12 volts with minimal voltage drop. That gauge wire is capable of carrying over 30 amps before actually melting, so you're not pushing the limit. Of the wire, anyway...I can't say about the rambo and connectors.

[geneb]

I agree with Eric. The gauge I specified in the build manual is sufficient and safe for the task at hand.

I really only print ABS with my Orion and I've never run the heated bed over 80C. I've yet to have adhesion issues.

g.

[mrbi11]

The standards for wire rate 14 guage at 5.9 amps.
http://www.keywolf.com/American_Wire_Gauge.php

The wire may not melt till 30, but the insulation will melt at much lower current,
then short out and ruin everything.

My actual question is: Is my heater bed bad?, although I'm way too long winded.
The manual says it has a resistance of 1.2 ohms, so would run at 10 amps, which I beleive 14 guage can stretch too and survive.

My unit is running at 20 amps.

thanks for your feedback.

[Eric]

You're looking at a chart labeled for "power transmission", the first assumption of which is LONG wire runs, as in miles, not feet. If you look at the text at the bottom of the same page, you'll notice it mentions the NEC allows 14 AWG to be used for 20 amps in free-air applications, which is what you have. Not that the NEC even applies in this case, but it meets code.

Oops, I didn't link the right chart first try. This one is pretty nice:
[img]http://assets.bluesea.com/files/resourc ... hartlg.jpg[/img]

Now the question that just occurred to me. What's the size of the fuse on the Rambo, and why aren't you blowing it? I don't have a Rambo myself, so I may be ignorant here.

[Eaglezsoar]

Here is a chart I got from the Internet which shows allowable current at different lengths of wire.
There is a difference between a wire that is running inside romex and one that is running free.
14 guage wire can carry 20 amps AS LONG AS IT IS KEPT REASONABLY SHORT. Look for yourselves
at the various charts available. Everyone wants to use the rules for house wiring and that is not a
fair way to determine ampacity.

[Eaglezsoar]

The standards for wire rate 14 guage at 5.9 amps.
http://www.keywolf.com/American_Wire_Gauge.php

The wire may not melt till 30, but the insulation will melt at much lower current,
then short out and ruin everything.

My actual question is: Is my heater bed bad?, although I'm way too long winded.
The manual says it has a resistance of 1.2 ohms, so would run at 10 amps, which I beleive 14 guage can stretch too and survive.

My unit is running at 20 amps.

thanks for your feedback.

5.9 amps for 14 gauge wire makes no sense unless they are using it for transmission lines that can be thousands of feet long.
They use 14 gauge wire in houses fused at 15 amps and those runs can be hundreds of feet. See the other posts.
Your heater bed is not bad.

[626Pilot]

What's interesting to me is that the fuses are rated 5 amps...

[Eaglezsoar]

What's interesting to me is that the fuses are rated 5 amps...

The one that looks like an auto fuse is for the heated bed and is rated at 15 amps, I don't know how they are pushing 20 amps through it.
I use an SSR and 24 volts so that all the current goes through the SSR.

[mrbi11]

You're looking at a chart labeled for "power transmission", the first assumption of which is LONG wire runs, as in miles, not feet. If you look at the text at the bottom of the same page, you'll notice it mentions the NEC allows 14 AWG to be used for 20 amps in free-air applications, which is what you have. Not that the NEC even applies in this case, but it meets code.

Oops, I didn't link the right chart first try. This one is pretty nice:
[img]http://assets.bluesea.com/files/resourc ... hartlg.jpg[/img]

Now the question that just occurred to me. What's the size of the fuse on the Rambo, and why aren't you blowing it? I don't have a Rambo myself, so I may be ignorant here.

Thanks! That is helpful.

However, power transmission means building wiring, not transmitting across states.
No one uses small wires to run power for miles.
House wiring does NOT allow 14 gauge romex to be used for a 20 amp circuit, at any length (Thats code).
As you point out, with free air the wires dissipate heat better than romex,
the extra insulation actually lowers the current capacity,
because it slows down the heat dissipation.

I have never come across a table that ok's 14 gauge for 20 amps at any length.
So, I do feel better.

But my question is STILL
IS MY HEAT BED DRAWING TOO MUCH CURRENT FOR THE DESIGN? (Is my heat bed faulty?)

THE MANUAL SAYS THE HEAT BED SHOULD DRAW 10 AMPS, mine is drawing twice that.

[Eric]

The POTS system is a good example of small wires carrying power for miles. Classic phones are powered by the phone line itself, and many homes installed phones long before they had any other kind of power.

As for your bed/rambo combination, we're confused at this point. We don't understand how the 15A fuse on the rambo heater circuit isn't blowing if you're really drawing 20A. If the answer is that you aren't really drawing 20A, then we need to identify the measurement error. You did disconnect at least one of the heater leads from the rambo when measuring the resistance of the onyx? How about current...did you insert an appropriate meter in the heater circuit for the current reading?

[mrbi11]

I used a mag clamp ammeter, which works for ac and dc.
It was 19.something amps.
I did that because I tested the resistance at 0.6 Ohms. That I confirmed with 2 somewhat pricey meters.
And the 4th test was into my 15 amp 12 volt supply, on which the meter pegged, so i disconnected it quickly.
So 4 tests say 20 amps ish.

Ironically, I did the testing the first place fo i could see if i really needed a wire that BIG--
to see if i really needed to go to the store to buy 2 feet of wire not included in the kit.

Thanks for telling me about the fuse. It probably will blow at some point.
Fuses are basically resistors with some thermal mass.
Neither fuses nor breakers really go at when current exceeds their rating for short while, or
blow if the current is exceeded "just a bit" They are analog power devices. They have to heat up.
Even magnetic breakers have a fast and a slow way to trip, by design, not to drive you crazy tripping
when you flip on the fluorescent lights.
They blow quickly if you put 10x the current, like a short, soonish for 2x, but 30% quite a long time.

The LCD shows it never stays at 100% more than 5 minutes or so.
But if the fuse is at 15 amps, it will go if the rambo sticks in the on state.
(Which makes me comfortable actually)
I'm happier replacing fuses than slagged printers.

Now light bulbs and some heating elements, by design, have resistance that increases with temperature.
A kind of self regulating mechanism. Usually this means really hot "glowy" temperatures, so i don't think the onyx bed
does that unless it is some atypical conductor. In any case, watching the HOSt graph and my amp meter
at the same time, current doesn't drop till it starts switching on and off.

Thanks for the feedback.

[cope413]

The fuses don't blow because they're not quick blow fuses.

I don't know what the fuses are on the board, but my guess as to why they're not blowing is because they are slow fuses and need to be at a current for a specific amount of time. You may be peaking at 20a, but there's no way it's staying at 20a - you're machine would be melting.

[mrbi11]

20131218_223903.jpg (14.25 KiB) Viewed 10617 times

After satisfying myself the heater i got is indeed pulling 3 times normal heater power,
I realized that means if it works I wait 1/3 as long to start.

So I added some solid state switches (which I already had).
Plural because they are rated at 12 amps.
Triple because if 1 failsI don't want the others to,
and because they were gathering dust anyway.

tastes great.

[Eaglezsoar]

Have you contacted SeeMeCNC to see if they created new Onyx boards that are .6 ohms?
If they haven't then you must have some shorted traces within the board.

[Polygonhell]

The reason the auto fuse doesn't blow is because they are rated for current at more than 12V, usually 32V I think, so the 15A fuse won't actually blow until there is close to 40A going through it at 12V. Power is what blows fuses, not current, the amp rating is only meaningful at the rated voltage.

You'd probably damage the board before you blow that fuse.

[Eric]

The reason the auto fuse doesn't blow is because they are rated for current at more than 12V, usually 32V I think, so the 15A fuse won't actually blow until there is close to 40A going through it at 12V. Power is what blows fuses, not current, the amp rating is only meaningful at the rated voltage.

You'd probably damage the board before you blow that fuse.

No, you're thinking wrong here. The voltage rating on fuses is the maximum safe voltage they are designed for, and even then they're mainly talking about how they behave WHEN they blow, not before.

It's not the voltage of the total circuit that matters, it's the voltage drop across the resistance of the fuse that matters. That 15A automotive fuse probably has a resistance in the neighborhood of 5 milliohms (check the spec sheet for whatever brand fuse you actually have for an exact number), so the voltage drop at 15A would be V = I * R = 15 * 0.005 = 0.075 volts. That fuse is going to behave almost identically in a 30V circuit as a 12V circuit, since the voltage drop isn't changing. It really is the current that matters.

[mrbi11]

Have you contacted SeeMeCNC to see if they created new Onyx boards that are .6 ohms?
If they haven't then you must have some shorted traces within the board.

I have spoken to them and got no mention of that.
It is a defective part, whatever the cause.
I'm sort of a magnet for defects. My ezstruder would not let filament go thru, the aluminum chunk was off center.
My 0.7 mm replacement tip ordered came un-drilled or rather an 0.0000000 mm tip.

They sent replacements for the parts i asked them to. I just worked around the heater bed and fixed the ezstruder (mostly).
I may need to replace the bed heater at some point, but I'm more interested in printing some stuff than messing with it.

thanks.