AC for PCB print bed

I have a PCB print bed from Romscraj, and it appears well made, but it heats very slowly. From about 20C ambient to 60C, it takes an hour. It has 12V at the board, but when I check resistance, it shows 6 ohm. So 12V is only generating 24W. No wonder it heats slowly.

I have a 24V transformer, which would give 96W. Not ideal, but a lot better. I was just going to do it, but then I considered that running AC into a bunch of traces might create a whole bunch of signal that wouldn't be a good idea. Does anyone out there use AC for a PCB print bed? Any observed problems? It isn't a big deal to rectify the AC and smooth it a bit, but I prefer simple, if it's possible.

Thanks;
Rick

Hi Rick,

I'm heating my bed with 230VAC - but with a SSR, that is switched with the 12V-heatbed-MOSFET of the RAMPS at maybe 10mA

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

VDX Wrote:
-------------------------------------------------------
> Hi Rick,
>
> I'm heating my bed with 230VAC - but with a SSR,
> that is switched with the 12V-heatbed-MOSFET of
> the RAMPS at maybe 10mA

Thanks, Viktor: if you're doing it at 230V, then I should be able to do it at 24V without any problems.

Rick

I have a 24V transformer, which would give 96W. Not ideal, but a lot better. I was just going to do it, but then I considered that running AC into a bunch of traces might create a whole bunch of signal that wouldn't be a good idea.

50Hz won't create nearly as much electro magnetic noise as DC PWMed by fast MOSFETs with no slew rate control.

---

[www.hydraraptor.blogspot.com]

VDX Wrote:
-------------------------------------------------------
> Hi Rick,
>
> I'm heating my bed with 230VAC - but with a SSR,
> that is switched with the 12V-heatbed-MOSFET of
> the RAMPS at maybe 10mA
Viktor,

I am thinking of doing something similar but am confused as to the specs for the SSR that I need. DO you have the specs, or perhaps a manufacturer or source, for the SSR you are using. What I don't understand is that some of them say they switch AC and some of them say they switch DC. Drawing my my knowledge of a normal magnetic relay, I don't see why AC or DC would matter.

Thanks

... it's mostly for the type of primary switching semiconductor - for "DC-switching" it's a transistor or MOSFET type and will drive the current when the input voltage exceeds the threshold value (mostly +3V).

"AC-switching' is done with a thyristor - this device switches on when the input voltage changes from negative to positive and blocks when from positiove to negative ... so when activating with +5V it would drive the current, but for blocking you'll need a negative input voltage and then again a positive level for activating, what's really simple with an AC input, but limits the switching speed to the 'zero-passes' of the AC waveform ...

---

Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

jbernardis Wrote:
-------------------------------------------------------
> VDX Wrote:
> --------------------------------------------------
> -----
>
> I am thinking of doing something similar but am
> confused as to the specs for the SSR that I need.
> DO you have the specs, or perhaps a manufacturer
> or source, for the SSR you are using. What I
> don't understand is that some of them say they
> switch AC and some of them say they switch DC.
> Drawing my my knowledge of a normal magnetic
> relay, I don't see why AC or DC would matter.
>
> Thanks

Here's an overview from Omron, describing the variations.

VDX Wrote:
-------------------------------------------------------
>
> "AC-switching' is done with a thyristor - this
> device switches on when the input voltage changes
> from negative to positive and blocks when from
> positiove to negative ... so when activating with
> +5V it would drive the current, but for blocking
> you'll need a negative input voltage and then
> again a positive level for activating, what's
> really simple with an AC input, but limits the
> switching speed to the 'zero-passes' of the AC
> waveform ...

Ouch: I didn't consider this, also Nophead's comment about PWM without slew-rate control. I'll have to look at the firmware, see if I could make the on/off times longer while keeping the duty cycle.

Re: AC switching delay.

At least with the Marlin firmware I'm using (v1, Durbie branch), the bed isn't PWM'ed or PID'ed. The bed is checked on a 2-second interval, and turned on or off, as needed.

So the zero-cross SSR looks reasonable. For someone who wants something cheaper, a triac should work with this firmware.

NOTE: if you're using two switch-mode power supplies without isolation transformers, or one supply without transformer to drive a triac to control mains voltage, make sure you provide some kind of isolation between your electronics and the triac.

Edited 1 time(s). Last edit at 06/14/2013 08:05PM by rick55.