Play heated bed: alternating power supplies

[__Vinz__]

Hi everyone,

I have a Play and I got the Y axis upgrade with a heated bed. So far, I have never used a heated bed and I got that upgrade because the Y axis one now only comes with it (I got the upgrade via the recent discount on the PB website). I figured this is a good opportunity to learn to use one and maybe try some new materials.

I am writing here because my main usage will be without the heated bed. I am very wary about security with the heated bed (I am browsing the various related threads, esp the 'Printrbot on Fire!' one do not want to have a fire).

Here are my questions:
1. Even if the heated bed is wired in, can I safely use my old power supply if I do not use the heated bed?
2. If by mistake I enable the heated bed while using the old power brick power supply, what happens?

I have contacted Printrbot support and asked those questions. Their answers were:
-> for #1: this is safe to do. I need to be sure to disable the heated bed option in the Cura 'Machine settings' section. What does that option do btw? As of now, without it, I have a thermistor reading in the bed temperature section of Prontrface. Is this about gcode generation?
-> for #2: it could fry the power supply. There is a difference between 'fry' and 'burn/cause fire'; I'd like to know which one.

Could you elaborate on those answers?

Thanks a lot for your help, sorry if the info is already in the forum threads, I am slowly reading all the related posts.

Vincent

[RetireeJay]

If you have a Rev F board, then you have screw terminals for the heated bed and your incoming power supply to the board has six pins instead of four. These changes were made specifically to avoid stressing power handling connectors, and they make the board much safer relative to the Rev D board in the "Printrbot on fire" thread. Many people successfully used the Rev D board for years with heated beds, so they weren't really unsafe, but you did have to pay attention. Some of us changed the way the heated bed was connected to the board, for example by soldering the wires directly to the board or by installing beefier connectors.

If you have the heated bed disabled in your slicer, then there should never be a command given to the board to turn on bed power without your knowledge. But the safest way to guarantee that the heated bed never draws power is simply to not plug it in to the board! Don't plug it in until you have a power supply that can handle it.

There are thousands of different power supplies, so it's impossible to authoritatively answer the question of what will happen if you overload your particular power supply. But high-quality, well-designed power supplies will automatically protect themselves from overheating by shutting down. In rare cases, this means shutting down permanently (which might be called "getting fried"); usually the shutdown or brownout is temporary. A Printrboard that loses power unexpectedly can, in some cases, suffer some damage to the internally-stored firmware, so it's not a good idea to push your power supply to its limit. Power supplies with sufficient power are not terribly expensive, so why not just install one rated for 350 watts or more and stop worrying?

[__Vinz__]

Hi Jay,

Thanks for your reply and your guidance. I am glad to see I have the rev F printrboard (sig updated with some info). I concur with you saying the safest is to either have the bed not connected or get a more powerful power supply and stop worrying.

My electrical science is a little bit rusty but I understand a 350W PSU will only use as much as it is requested i.e without the heated bed turned on, I will still use as much as the power brick, right (my kill-a-watt measured it at around 50W when printing)? I wanted to keep the power consuption low if possible. I will use a recycled ATX power supply from work to that end (I have seen the multiple forum threads about how to make that work).

Finally, for the record, the heated bed setting in Cura makes a new dialog box appear with the heated bed temperature under the extruder one (the Printrbot for the upgrade did not mention that, ticket on its way). That in turns generates a single M190 g-code command to turn the heat on for it at the beginning of the gcode file . So indeed, little risk to have that turned on by mistake if the option is not ticked.

[RetireeJay]

[__Vinz__]

Great, thanks again Jay! I will let you know how it goes. I plan to recycle a PC power supply from an Optiplex PC that is lying around. Otherwise, I have indeed seen that a good ATX power supply is reasonably priced (~$40 for a Corsair one) so indeed not a lot for the price of safety/peace of mind.
Thanks also for the guidance on the pin compatibility, that is very useful to know.