Running heated bed with relay?

Quote

So an SSR would do it?

Solid State Relays work, but make sure your firmware uses on/off aka BangBang.

PWM requires a MOSFET and gives slightly more constant bed temperatures.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

matty2013 Wrote:
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> All this stuff is new to me for my first printer,
> just wish there were some A-Z guides to run all
> 24v, or 12v for ramps and steppers and a 24v for
> the heated bed only.

If you have a RAMPS, then I think you can just the MOSFETs on the RAMPs. It has a separate power connection for heatbed, so you connect that to 24V.

On a different board, I used its MOSFET to switch an external 24V via an optoisolated relay for a heated bed. I changed the firmware to switch the output no more than once per second. I only replaced the relay with a MOSFET when I got irritated by the constant clicking.

bobc Wrote:
-------------------------------------------------------
> matty2013 Wrote:
> --------------------------------------------------
> -----
> > All this stuff is new to me for my first
> printer,
> > just wish there were some A-Z guides to run all
> > 24v, or 12v for ramps and steppers and a 24v
> for
> > the heated bed only.
>
> If you have a RAMPS, then I think you can just the
> MOSFETs on the RAMPs. It has a separate power
> connection for heatbed, so you connect that to
> 24V.
>
> On a different board, I used its MOSFET to switch
> an external 24V via an optoisolated relay for a
> heated bed. I changed the firmware to switch the
> output no more than once per second. I only
> replaced the relay with a MOSFET when I got
> irritated by the constant clicking.


I watched some tutorials on mosfets, and ideally would like to use one instead of an SSR based on how I've understood how they work.
Is there any type, or method used to select a MOSFET for my application?

Essentially set up is 12v 20a supply for the ramps
And a 24v 10a for a heated bed that draws approx 8amps.

The MOSFET would receive signal from the 12v coming off the board fromt the heated bed power wires that would normally heat the bed.

I am building a Lulzbot TK-0 with a 300x300 mm bed and bought a QU-BD silicone bed heater that draws as much as 20 amps. I am planning to use a Rumba board to control my printer. The Rumba board says the he bed connection is good for up to 11 amps, so if I connect the silicone heater I will cook the board. Should I use a relay as some suggest or a mosfet?

I know only the very basic concepts of electricity, enough not to get fried though that does happen on occasion, so please keep it simple.

If a Mosfet would work, which one and what would I have to do about the possible "inversion" issue?

davew_tx: your solution of the Bosch automotive relay seems simple enough, but I was concerned that the PWM in my board might cook a conventional relay. which Bosch relay did you choose or would anything that can control 12 volts and up to 30 amps work OK.

Sorry for my ignorance, but I am trying to learn.

Edited: Would this work? DC-DC relay

Thanks in advance

Edited 1 time(s). Last edit at 02/14/2013 10:20AM by roygpa.

That solid state relay will work fine. I use a SSR on my RAMPS1.4 to control an AC powered heater for my heatbed and it works fine with Marlin's PID.

Thanks for the reply stephenrc.

I plan on using the Marlin firmware with the Rumba.

Roy

I bought one of the relays in my post above, installed it... without disabling PID. Would leaving PID on cause the relay to get very hot and possibly limit the current through the relay? The relay was rated for 25 amps and I measured 13 amps. With the bed heater connected directly to the power supply I get 20.5 amps.

I have since disabled PID, but found the relay to have suffered a bit of a melt down. I have another higher amperage SSR on the way.

thanks.

Roy

Edited 1 time(s). Last edit at 03/01/2013 08:36AM by roygpa.

SSRs suitable for use with PWM are rated as such. In my experience, relays like the one you referred to in your post above is suitable only for switching speeds of about 100Hz and below, and that' presuming that it's a FET based SSR, which is uncertain from the product description shown in the ebay sales link. It's not even a real one from the mfg who they cloned it from...(looks to me to be a clone of a Crydom part actually)

The shorter answer is yes. Relays in that form factor generally have transition speeds (ON/OFF) measured in milliseconds or tens of milliseconds, which means that running in PID mode will use them as a defacto variable resistor. (and of course a resistor is little more than an electrical to thermal converter...)

Please note that while it is simply ignorant to state that SSRs cannot be used for PID or for PWM, they must be selected for use in that manner and the PWM frequency has to be kept within a given devices' limits of operation.

Switch over to bang bang and as long as the load current is kept well below its maximum rating, your heating issues should disappear. (note that max continuous load currents are generally based on use of external cooling, so keeping the actual maximum load to ~75% or less is a conservative approach that will serve you well in the long run by saving you money for replacements and time wasted...even more expensive in my experience)

Edited 1 time(s). Last edit at 03/01/2013 09:47AM by xiando.

Thanks for the quick reply Xiando, that's my plan. To use bang-bang and retire the SSR that melted down. I'll chalk it up as a learning experience. I have two new SSR's on the way a 40 and an 80 amp.

edited: I bought a heat sink too.

Thanks again.

Roy

Edited 1 time(s). Last edit at 03/01/2013 09:56AM by roygpa.

Sucks to learn that way but fortunately it was a cheap lesson. I've smoked a few in my time too.

Rule of thumb: If the t_on and t_off values aren't specified by a relay mfg, you should either use them as simple remote switches or choose another vendor. And keep that point about maximums in mind. It goes for FETs, voltage regulators, and other components as well. Max ratings generally refer to operation under ideal circumstance.

Got a heat sink and another relay. Learned another lesson. Pay attention to polarity when wiring an SSR.
When wired correctly, it hols temp between 109 and 111 deg C.

Take care.

Roy

Is a good idea to run the heatbed on a classic relay (not SSR)?

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Prusa i3 Rework - hotend IeC 0.4mm (bowden) - filamento 1.75mm - Repetier-Host - MarlinKimbra - RAMPS 1.4

If you're using bang-bang temperature control it's OK, but running PID regulates the temperature better, and an SSR will let you run PID control.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Ok, in bang-bang mode, using a relay the heated bed will heat up more quickly compared to the RAMPS mosfet?

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Prusa i3 Rework - hotend IeC 0.4mm (bowden) - filamento 1.75mm - Repetier-Host - MarlinKimbra - RAMPS 1.4

No. The only reason to use an external relay or SSR is because the bed takes more current than the MOSFET or PCB can handle. Otherwise, just use the MOSFET.

Use PID control if at all possible. There are some print quality problems that can be avoided by using PID control.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
Use PID control if at all possible. There are some print quality problems that can be avoided by using PID control.

That depends. I have 2 printers with completely different kinds of heated bed, and both work just fine with bang-bang control. Many other 3D printer users find the same. The only person I have ever seen on these forums who reports issues when using bang-bang control for the bed is digital_dentist.

The key is the amount of temperature variation you get. Both my heated beds remain within 0.2C of the set temperature using bang-bang control. If you have a badly-designed heated bed where there is substantial lag between the temperature of the heating element and the temperature read by the thermistor, that would lead to a greater bed temperature variation using bang-bang control. Using PID would alleviate the problem.

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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].

I installed the relay, the bed reaches the temperature of 55°C in 15 seconds less than using the mosfet

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Prusa i3 Rework - hotend IeC 0.4mm (bowden) - filamento 1.75mm - Repetier-Host - MarlinKimbra - RAMPS 1.4

This is a 12 volt SSR. It works perfectly and gets that load off of the printer board. Fan is hooked up to the input side and only cools when heatbed power comes from printer board. Got the relay from mpja.com for $30.00 I think.