Danger, Will Robinson! .5Ω

General topics regarding the Tarantula
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Dave
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Danger, Will Robinson! .5Ω

Post by Dave » Tue Apr 25, 2017 11:28 am

I have read that some of the later Trantula beds standard size and large
are .5Ω, I can confirm that this is indeed true as I have a low resistance standard size bed to hand.

I believe that Tevo have stated that this is 'normal' ... is it heck !! ( about as normal as martians invading the planet earth) and for the unwary / uninformed / unprepared its all going to end in tears.

Sometimes I despair, other times I just shake my head :roll:
Here is a quick fudge workaround that will save purchasing a better psu & mosfet
The 12v connections on the bed are connecting 2 separate heater resistances in parallel (which lowers the overall resistance by half) the connections to the - side are linked / bridged together.... so the 'get out of bother trick' would be to snip / remove the bridge and thus only use 1 of the heater resistances.

In theory 1Ω @ 12v = 12 amps and that = 144 Watts which may be acceptable.

The up side is that if the heater in the bead wears out.. then the - connection can be swapped to the previously disconnected connection and all will be well again (its like having a free spare, lol)

JaY_III
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Re: Danger, Will Robinson! .5Ω

Post by JaY_III » Wed Apr 26, 2017 4:43 pm

can confirm, just got mine yesterday and 0.5 Ohms

Have not set mine fully up yet, so not 100% sure on how the device is controlled.
Does the MKS base Board provide current limiting? Or Voltage control?
Once the bed reaches temp, it might not be drawing the full 288Watts. If the MKS Base board is smart enough it will either limit the current or voltage, so a heat up time before print time might also be a good intern solution until a larger power supply can be sourced.

However if the MKS base simply cycles the bed either on or off to maintain temp, that is an issue. Not sure what the hot end pulls as i forgot to ohm mine out, but that on and the bed at the same time may be a cause for alarm.

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Charles
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Re: Danger, Will Robinson! .5Ω

Post by Charles » Thu Apr 27, 2017 5:41 am

By Default the MKS board will just turn the power full on. Then when it's up to temeperature it will be controlled. At 0.5 Ohms it will blow up the MKS board. An external mosfet is highly recommended. But at 0.5 Ohm its going to draw 24 Amps. Seems to high.

Their is a Max PWM setting in the firmware which can be used to limit the max power. The default is 255. If you change it to 127 you will get half power.

Charles

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Re: Danger, Will Robinson! .5Ω

Post by JaY_III » Thu Apr 27, 2017 4:18 pm

I have a mosfet, so no worries on that front.

Its PWM fully answers what I was looking for, and that is the worst case for the PSU. During its on portion of the duty cycle, the heat bed will try to draw full current. With everything else also drawing current, we could easily request more than the 25Amps the PSU is able to supply.

Has anyone opened it up and checked if it is fussed? Being a no name part from China, I wonder if a safety fuse is inside.

Either way, with any luck, ill have a test print done tonight and in the near furtur will be able to look into this more.

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Re: Danger, Will Robinson! .5Ω

Post by Charles » Fri Apr 28, 2017 4:36 am

These cheap supplies are not current limited. They just blow up if you put to much load on them. If fuse does blow it means it's stuffed.

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Re: Danger, Will Robinson! .5Ω

Post by stephen » Wed May 17, 2017 4:36 am

I got my Tarantula about two months ago with a 0.5ohm heatbed.
The fist time I power it up after almost 8 hours of assembly, it blew my power supply in sparks and smoke.

I quickly contacted Tevo and explained the issue. At first they told me it was the normal resistance for their beds. I argued with them that it would draw 24 Amps, way over the capacity of the flimsy stock PSU.

I made a deal with them to replace only the bed (with express shipping) and forget about the PSU as I had a better one on hand anyway (old Dell server PSU 750W 62 Amps).

They agreed to the deal and I am happy customer ever since :-)

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Re: Danger, Will Robinson! .5Ω

Post by stephen » Thu May 18, 2017 1:59 am

JaY_III wrote:
Thu Apr 27, 2017 4:18 pm

Has anyone opened it up and checked if it is fussed? Being a no name part from China, I wonder if a safety fuse is inside.
Yes there is a fuse inside but mine didn't blow in time apparently. Other components were damaged.

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Re: Danger, Will Robinson! .5Ω

Post by JaY_III » Thu May 18, 2017 4:00 am

That is sad to hear.
been rocking my large bed, so far no issues.
Once I am ready to get my octopi setup, ill grab an ATX PSU. Need that 5V for the Pi

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Re: Danger, Will Robinson! .5Ω

Post by Dave » Thu May 18, 2017 8:25 am

My advice is use a 'HP DPS-600P' server psu...

Heres a thread showing how to connect it

https://arcaded.discussion.community/po ... 1295699489

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john.price
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Re: Danger, Will Robinson! .5Ω

Post by john.price » Thu Oct 26, 2017 7:54 am

I know this is an old thread, however, today i stumbled on this section in the Marlin Tevo community firmware that is relevant.

configuration.h

// If your bed has low resistance e.g. .6 ohm and throws the fuse you can duty cycle it to reduce the
// average current. The value should be an integer and the heat bed will be turned on for 1 interval of
// HEATER_BED_DUTY_CYCLE_DIVIDER intervals.
// #define HEATER_BED_DUTY_CYCLE_DIVIDER 4

I've been having the same issue of power supply died, upgraded to 25A with over current protection.
Still trips out.
I enabled the above part and so far no problems.
My large heated bed for my tarantula measures 0.5ohm.

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