[Solved] Relay and cables getting hot

Short version

My cables from PSU gets very hot and so does the relay used in heating. In order to prevent wires from heating what could be the cause of it?

Long version

I’ve build my 12” Prusa i3v from Makerfarm’s. Due to larger heatbed area it is heated through solid state relay which came along with the package. There’s 430W ATX PSU firing up the machine and I’ve attached 12 v cables from 24 pin ATX to the Ramps and 8 pin CPU cable to power up the heatbed through solid state relay (there was 3 12 volts cables attached to the positive and negative connections of the relay).

I’ve already made some prints with this until recently my heatbed wouldn’t heat up anymore. Upon searching reason for this I noticed that wire leading from relay to heatbed had melted a bit inside the other relay’s connector. I never smelled any burned plastic (other than heated abs or pla) so this might have happened during different prints or just this last one.

Since the relay had this bad connector I ripped it off to examine the damage on PCB. It seems that PCB has also some fried connections. So just to test out I rewired this bad connection from relay and fired up my heatbed. It started to heat up normally but the wires from PSU also got very hot and so did the relay (measured over 100 degrees of Celsius from relay and 70 – 80 degrees from wires before cutting the power into heatbed). I went a little further and soldered the other connections just to try if that would have helped with the heat problem. Wires and the relay still gets very hot during heatbed warming so no help in that. What could cause this kind of heating of wires and relay?

I had this other relay at my house so I tried it with the heatbed. It seems to heat up the heatbed without heating the relay. Wires warms up to around 50 – 60 degrees but nothing like with the relay from Makerfarm. Now the problem with this relay is that there’s no temperature control with it at all. It starts to heat up the bed as soon as I plug the power to it (without giving command to heat up). I even measured voltage from Ramps 1.4 (D8) and it’s flat zero so there’s no communication between relay and Ramps. Since I’m pretty much clueless with this kind of electronics I could really use some wise advice . Is this kind of relay suitable for warming the heatbed and if so what I’m doing wrong since I can’t control the heat? Would it be better to heat up the heatbed through Power expander like they are selling at Reprap.me?

Edited 2 time(s). Last edit at 07/21/2015 02:51PM by Narvi.

You've miswired that Songle relay board, and if you activate the relay then you will short out the power supply. Connect 12V negative from PSU direct to the heat bed. Connect +12V from PSU to K0, and connect K1 to the other heatbed terminal. Leave the other 2 terminals unconnected, they are connected in parallel with K0 and K1.

Edited 1 time(s). Last edit at 07/21/2015 07:39AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for your advice dc42. I'll try that out when i'll get back to home.

Looking at that first card, that's an awfully small relay, and from what I can find, is likely only rated for 5 amps - far too small for a heat bed, and an explanation for the heating. The second relay is adequately sized, so it runs cool. If the wiring is heating, it is because it is also too small for the load, and you are not only heating the wire, but losing power that could be going to the bed.

- Tim

Thanks for the answer Tim. Just wondering why would Makerfarm sell these along with the printer if that wouldn't be big enough for it. Not trying to argue about this since I'm totally out of my own expertise with these things. Just wondering.

And big load of thanks for dc42! You got my problem solved. The wiring was wrong and with your help I got my printer working again. In any case someone else is facing similar problems I made a visual guide for this kind of relay connections.



Edited 1 time(s). Last edit at 07/21/2015 03:29PM by Narvi.

Was it of Chinese origin? If so, you have your answer . . . quality control is job none . . . and maximizing profits is!

- Tim

In case you were talking of relay I have no idea about the origin but somehow I would not be surprised to hear that it actually is from China. Printer itself is US origin. Luckily through this heating problem I've become more aware about the possibility of fire hazard with these printers. Seems like there's extra efforts that needs to be put on safety due to electronic failure.

Edited 1 time(s). Last edit at 07/21/2015 04:48PM by Narvi.

Some US sources just kit up offshore components, and don't do a very good job verifying specs . . . not sure what's going on here . . . The good news is that you have a working solution.

- Tim

For as an electrician the problem already starts with the heatbed.
Let's face it: Copper is for good connection and low resitance but not for heating, this is where materials like Nichrome or today even Carbon comes into play.
For the same amout of heat energy produced a proper resistance wire would need only a fraction of the amps.
But it is easy to produce and why someone care how much electricity you waste because of the rong materials used...
Then there is the power consumption that goes along with the copper....
It is no problem to find heated bed that consume fr over 10 amps when heating at full capacity.
But the way they are connected to the power supply is simply scary!
Like in the above pic: 4 wires joined and twisted together to keep up with the load - still they get war to touch!
Especially the areas where they are joined is a constant concern as only soldered wires provide proper, low resistance - but if not done properly with screw connections there is only little surface making contact.
In most cases with srew connectiong here, the connectors are not even rated for the power....
Did anyone notice that these heated bed have no markings in regards to electrical safety or the common labels that they at least are conform to some standards?
You can get silicone heated beds for 240V on libaba for as low as 1$ if you buy 50 or more - so what do you expext?...
Sure, someone had a great idea how to make a heated bed the easy way - but for sure he was not thinking of electrical safety while doing it ....

Thanks for the knowledge. You have a valid point here. As I said this is out of my own expertise so surely there can be some stupid mistakes I'm making.

What comes to heatbed itself Makerfarm do sell US made heatbeds. Kit had prewired heatbed along with. The wires to the bed are heavy and they are not made out of copper (not sure of the material used). As I've made these previous changes to the connections wires from PSU does get a bit warm but not hot to touch anymore. What you can see from the image above the connection part at the relay did melt which can indicate poor amount of connection between tightening screw and the wires. There was 2 or 3 wires connected from 8 pin CPU wires at the time. Now I've added fourth. However to make sure I won't repeat my mistakes seems like I have to reconsider my wire connections.

Screw terminal is easy to be replaced with some other form of connection and soldering seems to be easiest (and the laziest) for me at the moment. On the positive wires from PSU I could share the four clamped wires to the relay's two separate K0 connections thus giving more connection surface between screw and the wires.

Quote
Downunder35m
For as an electrician the problem already starts with the heatbed.
Let's face it: Copper is for good connection and low resitance but not for heating, this is where materials like Nichrome or today even Carbon comes into play.
For the same amout of heat energy produced a proper resistance wire would need only a fraction of the amps.

I stopped reading at this point because that is just plain wrong.

The truth is that Heat energy produced = electric energy in = Volts * Amps.

No matter what material the wire is made of, you get the same amount of heat for the same voltage and current. To get the same amount of heat from a fraction of the amps, you've got to increase the voltage proportionately. And high voltages are at least as dangerous as large currents... lots of energy just waiting to be released.

Nichrome wire could be 1/50th the length of the copper, having 50 times the resistivity, for the same resistance and therefore the same current and therefore the same heat produced.

You are a bit wrong assuming heat energy produced equals the power consumption.
If I take a given length of nichrome wire and use it to heat to a certain temp it will use far less current than copper getting to the same temp.
Take a 1mm copper wire (for ease o calculations) and @12V increase the current until it reaches a temp of 100° C.
Now a 0.2mm copper wire and do the same.
You will notice the thinner wire requires less current as the resistance is higher.
With Nichrome you also have the benefit of the greater surface area (copper with the same resistance is much thinner) - and most people would agree heat transfers better using big surface instead of small spots.

Or why do you think no electric heater actually uses copper for the heating elements? - Please do explain!

As long as you don't employ a heat pump the produced heat equals the power consumption.
Copper isn't use because it is inefficient to use huge amounts of low resistent wires if a lot less higher resistance nichrom does the trick just fine. It takes a lot less space and uses much less very expensive copper.
Your example is flawed, you are measuring temperature, not amount of heat. You can have very high temperatures in a very small volume with very little energy (think of piezo lighters).
Just think of a lit match and a tub full of hot water. The match has a much higher temperature, but the tub holds a lot more energy as heat.

Edited 1 time(s). Last edit at 07/22/2015 08:26AM by Srek.

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Quote
Downunder35m
You are a bit wrong assuming heat energy produced equals the power consumption.
If I take a given length of nichrome wire and use it to heat to a certain temp it will use far less current than copper getting to the same temp.

It can only use far less current if it takes far more voltage. Heat out = power in, no matter what the material. The reasons for using nichrome wire for heaters are that is is stable at high temperatures, it has a higher resistivity making it easier to build high voltage low current heaters, and its resistance doesn't change much with temperature, These factors are largely irrelevant when making a 12V bed heater, because we want low voltage/high current, and the temperature we heat the bed up to is relatively low.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
Downunder35m
You are a bit wrong assuming heat energy produced equals the power consumption.
If I take a given length of nichrome wire and use it to heat to a certain temp it will use far less current than copper getting to the same temp.
Take a 1mm copper wire (for ease o calculations) and @12V increase the current until it reaches a temp of 100° C.
Now a 0.2mm copper wire and do the same.
You will notice the thinner wire requires less current as the resistance is higher.
With Nichrome you also have the benefit of the greater surface area (copper with the same resistance is much thinner) - and most people would agree heat transfers better using big surface instead of small spots.

Or why do you think no electric heater actually uses copper for the heating elements? - Please do explain!

You will also note that the surface temperature of the element is not the power output . . . so the comparison between 1mm and .2mm is irrelevant . . . the larger wire has greater surface area, so is radiating the exact same BTU output as the smaller, but since it has more surface area, can do it at a lower temp.

Try as you might, Ohm's laws are not subject to modification by rant. V=IR, P=IV, P=I^2R P=V^2/R are absolute. For a given amount of current and voltage, you *WILL* get the same power delivered . . . . It never said the same temp, which is not power . . .

- TIm