PSU cuts out with MK1 heatbed

Hello,

My power supply unit keeps cutting out when connecting my new Prusa MK1- PCB heatbed.
I've been told that it's due to the unit not detecting any resistance due to the low resistance of the heatbed.

I have now bought and tried three types, and they are getting progressively more expensive.
I have also added a few resistors. A high powered resistor would work but it would be too hot and unsafe.

Can anyone suggest an ATX type PSU that works or a way to prevent this?

thanks!

Mike

Where are you adding the loading resistors, on the 12V connection? If so move them to the +5V and/or +3.3V rails
ATX PSU's usually only become stable after the lower power rails have some load on them

Leave the Heatbed on the +12V.

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[richrap.blogspot.com]

Personally, I'd move across to a proper PSU. It sounds as if your overloading the 12V line.
Option 2 is to power the bed from a separate ATX supply.

There are plenty of 30A 12V CNC/LED power supplies on ebay right now. Perfect for the job

Rich: thanks for the tip, i'll try this this evening.

Cynar: for safety reasons i'm using PC PSUs, one is a fairly high spec. There will be children/young adults in contact with the machine eventually. I'm not sure what you mean by overloading, I think the PSU just thinks its shorting due to the low resistance of the bed - at the moment i'm attaching the PSU directly to the heatbed.

A computer PSU is designed to put most of it's power down the 5V rail. It varies a LOT how much power they can push on the 12V rail. The stability of the while PSU tends to go down when there is no load on the 5V rail. We use one at work to run some 12V sockets and, under load and 8A load, I've seen it drop from 12.75V to 9.6V. It's rated to 22A on the 12V line.

Also, just so you know, all the PSU sees is the current being drawn on the line. If the current gets to high the PSU shuts itself down. A short is a form of overload to the supply.

Have you checked to make sure the heat-bed does in fact have more than 1ohm of resistance? It's always possible there is an actual short drawing more than 18A.

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www.Fablicator.com

Most PC PSUs only have one regulator for all the rails. If you load the 12V rail heavily and don't load the 5V rail the 12V rail will drop and the 5V rail will rise. This can cause the supply to shut down due to the 5V rail being too high.

You need to load the 5V rail with a power resistor and it will get hot.

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[www.hydraraptor.blogspot.com]

Thanks for your help.

Rich: I tried the resistors, however this did not work.
Andrew: The heatbed has 1.5ohms, I'm not sure what its supposed to be.
Cynar/Nophead: thank you for the explanation, I'll have to go down the power resistor route

I have a 15R 25W AL clad resistor on my 5V rail and a 2R7 25W on the 3.3V rail. They get too hot to touch but they are mounted without heatsinks on acrylic and it does not melt.

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[www.hydraraptor.blogspot.com]

To get a PC power supply to work as a general purpose power supply, you NEED to put a load on the 5 V line (e.g. permanently solder in a 10 ohm / 10 Watt power resister between the 5 V (Red) and ground (Black) wires. You also NEED to connect the Brown "Power Good" line to a low voltage supply (e.g. an Orange 3.3 V line).

The heatbed on its own should be drawing something like 8 to 12 Amps (100 to 150 Watts), on top of the demand of the steppers and hot-end etc - make sure your power supply is rated at 250 Watts / 12 V or better if attaching a heatbed to it; otherwise it will overheat and shutdown.

See the full instructions here: [www.wikihow.com]

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

julianh72 Wrote:
-------------------------------------------------------
> To get a PC power supply to work as a general
> purpose power supply, you NEED to put a load on
> the 5 V line (e.g. permanently solder in a 10 ohm
> / 10 Watt power resister between the 5 V (Red) and
> ground (Black) wires. You also NEED to connect the
> Brown "Power Good" line to a low voltage supply
> (e.g. an Orange 3.3 V line).
>
> The heatbed on its own should be drawing something
> like 8 to 12 Amps (100 to 150 Watts), on top of
> the demand of the steppers and hot-end etc - make
> sure your power supply is rated at 250 Watts / 12
> V or better if attaching a heatbed to it;
> otherwise it will overheat and shutdown.
>
> See the full instructions here:
> [www.wikihow.com]
> r-Supply-to-a-Lab-Power-Supply


I've came across the same problem as Michaelc, but nothing here seems to do. I've connected one 10 ohm, 10 watt resistor between many red wire and many ground on my PSU, and it didn't worked. I've also tried a second resistor between many orange wire and ground. Didn't worked either. The only way I could get something is by adding a 10 ohm 10 watt resistor in my bed circuit : This way, the PSU didn't shutted down, but my heated bed wasn't heating (The resistor was). The block heater of my extruder is working properly if that can help.

My heated bed is a MK2, I use a RAMPS1.4 and a 350 watt ATX PSU.

Is there something I am missing? Thanks!

I use a 12v, 50w downlight as a 5v load on mine, works perfectly and doubles as a work light

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Wooden Mendel
Teacup Firmware

What Is the current rating of the PSU on the 12V rail? With a heated bed you are going to need around 16A.

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[www.hydraraptor.blogspot.com]

Some PSU's have multiple 12V lines, and the max current is calculated as the sum of all lines. So you might be overloading one line. Although if you tried multiple PSU's that is unlikely. I have pretty expensive PSU, with two 12V lines. Each for one segment of the heated bed.

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reprapworld.com
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It is 17, I think this should be enough. And while making my test, the block heater is not working.

Atfer Wrote:
-------------------------------------------------------
> It is 17, I think this should be enough. And while
> making my test, the block heater is not working.

If your hot-end is now not working then either the PSU has failed or check your PTC fuses on RAMPS are not shutting down, they may not be the correct rated parts fitted?

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[richrap.blogspot.com]

Ok, thanks for your help, but I found that my problem was a stupid short in my connecter between the bed and the ramps. Yesterday I read 1.5 ohm on the bed, but I got 0,4 today. Now everything is almost finished!