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Using relays for hotend and bed gives unexpected results

Posted by harlandraka 
Using relays for hotend and bed gives unexpected results
May 12, 2017 08:53AM
Hi everyone!

I have a RAMPS 1.4 board. Today I connected all the wires, including the ones for heated bed and extruder, providing double power supply to the RAMPS shield.

I started a print, everything works fine but this morning I saw a little bit of smoke on the board while heating, so I decided to disconnect the heated bed and extruder and move them away with a dedicated relay connected directly to the 12V/30A power supply.
Disclaimer: I saw smoke on the double power supply connector, no component is damaged, I guess it was just too much current.

Everything works perfectly, but when heating the extruder Marlin sends intermittent on-off to the extruder, so the relay makes a lot of noise.
If I set it to 190°C, Marlin goes to 180°C, shuts down the extruder and begins powering it on and off, and it continues even if the temperature was reached. As you can imagine the relay at this point will make noise as it is being powered on and off very quickly.

It doesn't happen with the heated bed, if I set it to 60°C, it reaches 60°C and then shuts down the relay, which will be powered back on when the bed goes under 60°C. It just makes 1 click at time.

I know my explaination probably is not good enaugh (I'm not a native english speaker) so I'm attaching a video which demonstrates the issue.

Since the forum doesn't let me upload videos greater than 600K I'm providing the link on youtube: https://youtu.be/B_Z7ZK587SI

Is there a way to change this behaviour? Do I have to switch to a transistor instead of a relay?

Thank you

Edited 1 time(s). Last edit at 05/12/2017 08:54AM by harlandraka.
Re: Using relays for hotend and bed gives unexpected results
May 13, 2017 02:28AM
This is how PID works.... check your PIDTEMPBED settings

A real relay will
a) drive you batty with clicking
b) eventually due to sparking contacts it will one day fuse in one position.

a external mosfet driver on heat sink is a better option, there are several options now on the market
eg
[www.aliexpress.com]
or
[www.ebay.com]
Re: Using relays for hotend and bed gives unexpected results
May 13, 2017 02:34AM
Hi Dust,

I think I wasn't able to explain nicely the issue.

The issue is with the hotend. The heated bed behaves correctly, it gets powered on and off as expected without making noise.

The hotend gets powered on and off several times per second, and the relay will be noisy.

So the issue is with the hotend being powered on and off too much frequently, is there a way to avoid this?

BTW I just discovered there are SSR (solid state relay). They should be based on MOSFETs, so there shouldn't be noise at all when powering the circuit on and off, and they shouldn't suffer from powering on and off too frequently as there isn't a physical part which moves forward and backward.

Would you recommend that?
Re: Using relays for hotend and bed gives unexpected results
May 13, 2017 08:19AM
As Dust said, use a external mosfet on a heatsink, a relay WLL NOT WORK on the hotend.
Re: Using relays for hotend and bed gives unexpected results
May 14, 2017 02:14AM
Hi obewan, so a plain relay will not work, I'll get a SSR relay, which is made which has no mechanical parts. DC SSR Relays are based on MOSFETs, so I'll buy one of them instead of doing the circuit myself.

Thank you!

Edited 2 time(s). Last edit at 05/14/2017 02:14AM by harlandraka.
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