<p>Recently the connection between the heated bed and my board blew out. I was able to solder a new connector onto the board and it worked for a day, but now it seems the SMT pad has come off the board bottom.</p>

<p>Another option I have seen to have the heated bed is to put it directly into the 12V power supply. But when I do this the bed gets very hot almost as soon as I turn it on.</p>

<p>I would like to be able to have the bed not get hot as soon as I turn the printer on. Ideally, it could be directly put in to the power supply if:</p>

<ul>
<li>I could find a mechanical switch capable of handling that much current</li>
<li>Or, I could use a TIP31 transistor and a servos pin to control the bed.</li>
</ul>

<p>I know if I add resistance to the wire then all the heat will just end up in that part of the connection (right?). So I'm wondering if it's even possible to use something like a switch or a transistor. Will all the bed heat just go into the transistor or mechanical switch that's soldered on? Should I be using some kind of high temperature solder for this?</p>

<p>I am <strong>not</strong> trying to "control" the heated bed so much as just "Activate" it. It is annoying to have the heat blasting up from the bed when I want to do a simple thing like just move the motors. But it is convenient to have the bed remain hot when I do something else like disconnect a USB or move into upper layers of a print.</p>

<p>It seems like the TIP31 transistor (<a href="https://www.hobbytronics.co.uk/datasheets/TIP31.pdf" rel="nofollow noreferrer">https://www.hobbytronics.co.uk/datasheets/TIP31.pdf</a>) can only handle up to 5A of current? or maybe 1A? In that case, it wouldn't work without adding a resistor, which would make the heated bed itself nonfunctional? It seems like the heated bed currently has a resistance of 0.7 Ohms. What is the ampere limit for the power supplies that most 3D printers use? (The silver box used by the Lulzbot Mini as well).</p>

<p>And then do I need to wire the GND of the servos pin to the heated bed pin as well? Won't that send heat to the board?</p>

CC BY-SA 4.0
2019-07-16T13:44:10.170Z
If you look where the SMT pad is, can you see any other component which is on the same circuit that you could solder the wire to? If you tell us what the control board is, we might be able to find a photo of the PCB and a circuit diagram - or you could link to them.
Andrew Morton
CC BY-SA 4.0
2019-07-16T13:50:48.637Z
The temperature of the bed is likely controlled by switching the heater on and off frequently. That can be done with an FET transistor which will have very low "on" resistance when driven with the correct sufficiently high voltage. With a control feedback loop, the on-off intervals can be set in a [PWM](https://en.wikipedia.org/wiki/Pulse-width_modulation) fashion.
Andrew Morton
<heated-bed><wiring>
Activating heated bed through a switch, or D-pin and TIP31 transistor
K Mmmm
# Activating heated bed through a switch, or D-pin and TIP31 transistor

(Asked by: username_0 on 2019-07-14)

Recently the connection between the heated bed and my board blew out. I was able to solder a new connector onto the board and it worked for a day, but now it seems the SMT pad has come off the board bottom.

Another option I have seen to have the heated bed is to put it directly into the 12V power supply. But when I do this the bed gets very hot almost as soon as I turn it on.

I would like to be able to have the bed not get hot as soon as I turn the printer on. Ideally, it could be directly put in to the power supply if:

* I could find a mechanical switch capable of handling that much current
* Or, I could use a TIP31 transistor and a servos pin to control the bed.

I know if I add resistance to the wire then all the heat will just end up in that part of the connection (right?). So I'm wondering if it's even possible to use something like a switch or a transistor. Will all the bed heat just go into the transistor or mechanical switch that's soldered on? Should I be using some kind of high temperature solder for this?

I am **not** trying to "control" the heated bed so much as just "Activate" it. It is annoying to have the heat blasting up from the bed when I want to do a simple thing like just move the motors. But it is convenient to have the bed remain hot when I do something else like disconnect a USB or move into upper layers of a print.

It seems like the TIP31 transistor (<https://www.hobbytronics.co.uk/datasheets/TIP31.pdf>) can only handle up to 5A of current? or maybe 1A? In that case, it wouldn't work without adding a resistor, which would make the heated bed itself nonfunctional? It seems like the heated bed currently has a resistance of 0.7 Ohms. What is the ampere limit for the power supplies that most 3D printers use? (The silver box used by the Lulzbot Mini as well).

And then do I need to wire the GND of the servos pin to the heated bed pin as well? Won't that send heat to the board?

**Tags:** heated-bed, wiring

> **Comments:**
> **username_1**: If you look where the SMT pad is, can you see any other component which is on the same circuit that you could solder the wire to? If you tell us what the control board is, we might be able to find a photo of the PCB and a circuit diagram - or you could link to them.  
> **username_1**: The temperature of the bed is likely controlled by switching the heater on and off frequently. That can be done with an FET transistor which will have very low "on" resistance when driven with the correct sufficiently high voltage. With a control feedback loop, the on-off intervals can be set in a [PWM](https://en.wikipedia.org/wiki/Pulse-width_modulation) fashion.  

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## 0 answers

