PCB Heatbed how to connect?

[red]

Granted I'm not experienced in electronics, but being able to connect the wires to an upgraded 30A power supply and measuring VREFs with a multimeter after watching or reading a tutorial, I don't think this should be hard for me to grasp.
However, I just can't find a tutorial explaining this anywhere.

Basically the first 3d printer I got and am using right now (Solidoodle 4) uses an undersized (15x15 for 20x20 bed) silicone heat pad and provides uneven heat distribution. BTW, I don't care about heating speed.
This causes warping/lifting/unsticking with almost every object I try to print with all the adhesion techniques (kapton tape, glass with hairspray/glue stick, etc).

So I ordered a geeetech 214x214 PCB heatbed with the LED and thermistor presoldered.
But I can't find a tutorial on how I should connect it, only tutorials on soldering the power wires.

I'm wondering if I can just connect the power wires to the connector on the motherboard the current silicone heat pad is connected to.
Some people say that's fine, others say they tried doing the same with larger silicone pads and burned their motherboard, and suggest to use a relay while I can't find any info on what/how that's used for in a 3d printer.

So am I good connecting to the existing connector used by the silicone heat pad? Again, I don't know electronics.

[soofle616]

Relays are not 3d printer specific which may be why you can't find info. Basically what you want to do (if you use a relay) is connect the coil terminals of the relay to the motherboard output that used to be connected to the silicon heater. That will cause the relay to close whenever the motherboard tells the heater to turn on. Now connect one terminal of the switch side of the relay to your power supply v+ and the other switch terminal to one terminal of the new heater. The other terminal of the heater gets connected to gnd/common/v-/Vcc/0V etc (based on your power supply).

How this works:
Motherboard has no idea what's actually connected to it, it just sends power to the heater output pins whenever it sees that the heater is too cold. Currently that causes power to get sent to your silicon heater which causes it to warm up. In the new setup the motherboard still has no idea what's connected to it so as before, whenever it sees the heater is too cold it will output power to the heater pins. Only this time the heater pins will turn on the relay instead of the heater. A relay is nothing more than an electrically operated switch. instead of you flipping it on and off, it does so on it's own based on whether it's powered or not. Power applied, switch turns on allowing power from your power supply to run to your heater. When the heater reaches temp, the motherboard turns off the heater output pins, the relay stops getting power so the switch turns off, power is cut off to the heater. So start and end conditions are the same, the only difference is that the new heater receives it's power directly from the power supply which isolates it from the motherboard and prevents any possible overload scenarios.

Clear as mud?

For your purposes an automotive 12v relay from radioshack will probably work but I don't know the specs on your new heater so I can't say that for sure.

[red]

Thanks for the explanation. I think I'm starting to get it.

Then I get this question: do I really need a relay and connecting the heatbed to the power supply if I don't care about speeding up the heating process?
Is there any reason not to connect directly to the motherboard as the current silicone heatpad is if that is not an issue?
I don't know what overload scenario you mentioned there can be.

here's the heatbed: http://www.geeetech.com/mk2b-heated-...del-p-802.html
here's the motherboard: http://reprap.org/wiki/Printrboard

Again I'm not very experienced with electronics so sorry if this is a very simple question.

[old man emu]

Red,

Here is a picture of the heat bed relay fitted to my Prusa i3 printer. The type of printer doesn't affect the way the relay works, just where it is located.

Heat Bed relay labelled.jpg

You can see that the relay is a 12V 30A one. I bought it from an auto parts shop, or you could get one from and electronics shop.
The wire that comes from the PSU to the relay is some thick speaker wire that I had around the workshop. The reason it is so thick compared to the other wires is that using thick reduces the resistance to the current flow between the PSU and switch. I don't lose as much voltage over this "transport" stage as I would if I used thin wire. The +ve wire going to the heated bed is attached to the terminal post beside it. Notice that I am only connecting the +ve output of the PSU to the relay. You can see the -ve wire from the heated bed creeping back to the PSU from beneath the printer.

The wires pointed out by the blue and the yellow pointers are the wires that originally ran from the controller board to the heated bed. Now they are being used to create the magnetic field within the relay that pulls a bar into contact with the two terminals that the thick +ve wires are connected to. When that bar touches both terminals, current flows from the PSU to the heated bed, with a much reduced loss of voltage because the current does not have to deal with the resistance within the controller board.

From the heated bed, a thick wire comes back to the -ve output of the PSU. Make sure you use the same thickness of wire in this physical circuit from PSU to heated bed and back to the PSU. If you use a thin-ve wire, you are increasing to resistance of the circuit and defeating the purpose of installing a relay.

Simply because the use of the relay eliminates resistance in the circuit, your heated bed will heat more quickly. You might not appreciate this at the beginning of your career as a 3D printer, but you will soon begin the hate the waiting time, but there's another story to tell about heating procedures.

IMPORTANT CAUTIONARY WARNING

Do not use a relay in any other circuit on your printer, unless you want a sparks and smoke display to rival the New Year's fireworks in Sydney, Australia.

Old Man Emu

[Roxy]

I would suggest you put a diode (reverse biased) across the coil connection of the relay. Otherwise, you can destroy the electronics driving the relay from the inductive kick of the coil when the coil is powered down.

[GOC]

I would suggest you put a diode (reverse biased) across the coil connection of the relay. Otherwise, you can destroy the electronics driving the relay from the inductive kick of the coil when the coil is powered down.

- Always learning something new, I love it. Thanks for the tip, I was unaware that relays had an inductive back-feed. With that bit of knowledge I will be patching a few my projects later in the week with a few rectifier diodes. This might explain some other phenomena that I had occur as well.

Great tip!

[Roxy]

- Always learning something new, I love it. Thanks for the tip, I was unaware that relays had an inductive back-feed. With that bit of knowledge I will be patching a few my projects later in the week with a few rectifier diodes. This might explain some other phenomena that I had occur as well.

Great tip!

Well... Inductive kick is caused by collapsing magnetic fields. And of course you know how a relay does it's thing: It has a strong electromagnet that pulls the switch contacts one way or the other. If you turn off the power suddenly, the magnetic field is going to collapse very quickly. And with all those coils of wire inside the relay, it is going to generate a voltage.

Just searching quickly... There are a million posts about it on the net:

http://elecraft.365791.n2.nabble.com...td2588382.html

[red]

does the relay need a cooler?

I would suggest you put a diode (reverse biased) across the coil connection of the relay. Otherwise, you can destroy the electronics driving the relay from the inductive kick of the coil when the coil is powered down.

First time hearing this.

[soofle616]

does the relay need a cooler?


First time hearing this.

No the relay does not need a cooler unless you pick one that isn't rated for the power you will be running through it in which case, pick a different relay

First time or not, it's a true statement and good advice.

[red]

Someone suggested to use an SSR instead of car horn relay.

The downsides:
- You MUST disable PID (enabling Bang-Bang) in the configuration.h or else you will be using PWM which will cycle that mechanical relay like crazy, eventually burn it out and most likely in the closed or ON position. Not safe!!
- Bang-Bang control results in a slower heat control cycle which will likely cause a vertical squishing (banding) effect.
At the end of the day, a lot less toil and trouble and way more safe to just use a SSR!