Why not using AC to heat?

[gregwar 1]

Maybe this is a stupid question, but why getting a 250w DC power supply when a thyristor based device could use AC (with solid state relays?) to achieve this?

I mean, 75% of the UM2 power goes to the bed and nozzle heating, right?

And a 30$ low cost oven can reach 250°C and is like 1/3 the price of just the UM2 PSU

[gr5 2,271]

The UM2 power supply is designed to work with both 120VAC and 240VAC. If it was an AC heated bed you would have to either make 2 beds or use a transformer to lower the current. Also the USA Military determined that truly safe voltages must be under 60V DC or 30V AC. To make the bed AC power and also safe you might want to lower the voltage to 30V max. But now you have a heavy, expensive transformer to achieve this (and with a switch for 120V or 240V mains which adds more danger/complexity) and now it's just cheaper to use a DC power supply.

This is my guess.

[mastory 44]

I am sure mine is not the only heated bed that runs on separate 120vac supply (driven with solid state relay). I don't feel it is at all more unsafe than other household appliances. As gr5 pointed out though, even higher voltage may not be safe. In Europe where a lot of UM's are sold, the AC line voltage is 230v and you'd be stuck running your heaters at that or add a costly step-down transformer.

On the other hand, I don't think I would want my hot end to run on AC because of the nature of the head motion (wire travel and fatigue), and that I am touching the head quite often. If the nozzle heater were to short out, it would be easy to get shocked. Otherwise, the DC powered hot end and bed are already included in the new machines as they are shipped. I think only the UMO lacks a HB as stock.

[gregwar 1]

Ok but, there is a lot of things working with AC that I feel safe with

Think of your boiler, hairdrier (/hot gun), soldering irons which are almost like a nozzle or electric radiators which are almost like a heated bed...

There is also hot glue gun, which is basically like an extruder, no expansive DC psu with this

Don't think military US want all that to go <30V :-)

[avogra 68]

Think of your boiler, hairdrier (/hot gun), soldering irons which are almost like a nozzle or electric radiators which are almost like a heated bed...

 

All those devices have to comply with a bunch of regulations. Those regulations are extra strict, if there are metal parts that could be touched. The devices you mentioned are all equipped with ground connections of metal parts, extra isolation and more.

One can certainly modify the heated bed and/or the extruder to meet those regulation. But that means a lot of extra weight, cost in development and compliance testing and quite some production cost. For example, you might have to completely isolate the aluminum heating plate from accidental touching and you have to make sure, that even if this isolation brakes you still can't get shocked. That means a lot of extra material and engineering.

Edited January 4, 2016 by Guest