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Heated bed confusion

Posted by Via 
Heated bed confusion
November 07, 2018 02:00PM
I'm in process of putting together a Frankenstein printer using a duet Wi-Fi and an AM8 frame.

I have got a 24V psu and have a donor heat bed from a leshp Z1 (A8 6 pin connector style) and a recently purchased MK3 heat bed from eBay.

The eBay one is typical eBay quality and about as thick as a piece of cardboard. But most annoyingly half heats much faster / hotter than the other, putting thermistor on hotter side and setting to 50°c gives me 25°c on other half, putting on colder side and setting to 50°c gives me 110°c on hotter side (measured using infra red thermometer). I'm guessing this is just poor quality and may balance out once up to temp but not sure if I want to risk it as I could get either prints that lift on one half where it started when half of bed is too cold or one side getting insanely hot. (Wiring is correct for 24v, there are a fair few unpopulated solder pads on board, is anything meant to go on those? Labeled R1 etc... So guessing resistor spaces?).

When searching for a 24v heat bed I did see quite a few adverts for boards that look similar to leshp with ++(sensor)-- wiring advertised as 12/24v, can these be run at 24v? Resistance is showing as 1.5ohm.
Re: Heated bed confusion
November 07, 2018 04:41PM
You didn't say what side the bed heater is, an I am not familiar with the AM8. However, a common PCB bed heater about 220 mm square would normally have a resistance of about 1.2 ohms when wired for 12V (both sections in parallel), or 4.8 ohms when wired for 24V (both sections in series).

When measuring low resistance values, check what reading you get when you short the multimeter probes together. Then subtract that reading from the readings you get when you measure the bed heater.

Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet.
Re: Heated bed confusion
November 08, 2018 07:55AM

AM8 is the metal frame upgrade of the Anet A8, had a cheapie chinese clone of the A6 so have ended up replacing most components and building the AM8 frame for it.

It is 220mm2 and did a lazy measure of resistance including wiring loom, from the pads etc... does show ~1.2ohm.

Will stick with old bed but thrown in a 12v PSU and MOSFET for now until I come up with something more permanent.

Edited 1 time(s). Last edit at 11/08/2018 07:55AM by Via.
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