Heated bed resistance/continuity?

Hi guys - just a quick question re' heated bed, ie if it should give straight continuity across the terminals when tested, or not?
And if it should have a certain resistance etc? I think I've got a fault with my PSU potentially after rebuilding my printer, but cannot find a reason why. Am just wanting to check if the heated bed could be at fault or if this is normal for them?

Thanks!

---

Prusa Mendel i2 - Melzi V2

Yes, put your test leads across the two solder tabs on the pcb (better to disconnect wires from bed or D8 first) - you should get around 1 to 1.2 ohms.
You didn't say what problem you're having?

---

_______________________________________
Waitaki 3D Printer

The exact problem was I rebuilt my heated bed/platform (turned it so the element and writing were on top), reassembled, replaced the carriage, then tried a print.
When I connected the printer I was able to home the axes and it all moved ok, but when I then ran the print (ie the bed tries to heat) the PSU turned off and won't come on.

Interestingly my friends PSU died recently also but he hadn't touched his printer, and he got a replacement sent to him.
The symptom is the same as his dying and same kind of generic printer PSU, but I'm not going to try to claim a replacement as I suspect it could be a mistake on my part.
I just can't figure what!

So to reiterate if had the bed & platform separated, then assembled again.

I'd wondered if there could have been a short on the bed but:
1) it measures continuity across the bed terminals and presumably normally does anyway due to tiny resistance
2) would a short do anything with the bed in reality since the resistor/bed element is effectively also a short anyway?!

I can't find evidence of anything wrong with what I've messed with, but want to figure out the cause so if I repair the PSU or buy another it doesn't happen again instantly.

Appreciate any suggestions.

---

Prusa Mendel i2 - Melzi V2

The bed isn't a short on the scale of PSU that can deliver in excess of 10A, it is ~1.2 Ohm resistor. The current that flows with a direct short might easily be higher than the PSU rating. A good one would have short circuit protection and shut down. It should recover but some times it has to be powered off for minutes to allow the capacitors to discharge before it will attempt to start again. A poor one might be damaged or need a fuse replacing.

---

[www.hydraraptor.blogspot.com]

Makes sense cheers.

It's been off some time and won't come on still. Onboard fuse is ok.
No sign of anything burnt etc. or smell.

It's one of the silver box type switching power supplies that people sell specifically for prusas. Pretty much identical to this:
[www.ebay.co.uk]

I still can't tell how there could have been a short though but obviously something has gone wrong.

Edit - especially since the specs for these PSUs state they have overload and short circuit protection.

Edited 2 time(s). Last edit at 06/25/2014 04:41PM by Mr Joshua.

---

Prusa Mendel i2 - Melzi V2

Just to be clear, does the power supply work if connected to something else (like a PC for example)?

If so, the clue seems to be your heat bed being flipped over. You may be shorting it unintentionally somewhere. I would remove it and set it down beside the printer and try powering it again.

---

_______________________________________
Waitaki 3D Printer

It doesn't work at all, regardless of being hooked up to any device, or to nothing.

I found the cause: my own stupidity! Thought it would be one way or another but nice to know I can stop this happening when I buy a new PSU.

I put screws (intentionally!) through the bed' tracks on either side, into the plywood behind the bed.
No problem in that as such, even though it's a bit rough!

Except, that unbeknown to me the reverse side of the bed has a copper sheet covering it for some reason. I didn't spot it since it's a uniform layer and covered in the red covering.
So when the screws came through it created a circuit because of the copper layer on the back and shorted the bed.

A dumb mistake really so I'm kicking myself but I thought the beds were single sided. What's the purpose of the copper on the reverse since it's electrically isolated from the heater (until you put screws through it!!)?

So I need a new PSU. I've ordered one but it's a bit annoying that these claim they have protection against shorts when that's clearly not the case.
I doubt there's much wrong with it as nothing is charred or burnt as such, but my electronics diagnostics skills aren't sufficient to ID the faulty component.

Lesson learned and own head slapped!!!

---

Prusa Mendel i2 - Melzi V2

Here's a cheaper one with free shipping ..

[www.dx.com]