0.1ohm resistors for heated bed?

Hmm, might have got the wrong bits here...

I've just got 10x 10w 0.1 ohm aluminium resistors that looked great for a heated bed, however looking at ohms law they will draw about 145 amps at 12v when wired in series.

Anyone tried these or are they junk (only £2 delivered, probably as there is no popular application).

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Andrew

Based in Slough, United Kingdom

AndrewD Wrote:
-------------------------------------------------------
> Hmm, might have got the wrong bits here...
>
> I've just got 10x 10w 0.1 ohm aluminium resistors
> that looked great for a heated bed, however
> looking at ohms law they will draw about 145 amps
> at 12v when wired in series.
>
> Anyone tried these or are they junk (only £2
> delivered, probably as there is no popular
> application).

All of them in series 10 * .1 ohm = 1 ohm @ 12v = 12 amps.

This is more than your motherboard can supply but with a relay it may still work.

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver

... the MOSFET's can switch lower voltages too

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Viktor
--------
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Hmm, I made it 1 ohm but I'm not sure if the listing was clear, still I'll wait until they come and go from there, I have a tame engineering shop so I might knock up a few ally beds to recover some of my costs. There's not much on eBay over here?...

Sublime Wrote:
-------------------------------------------------------
> AndrewD Wrote:
> --------------------------------------------------
> -----
> > Hmm, might have got the wrong bits here...
> >
> > I've just got 10x 10w 0.1 ohm aluminium
> resistors
> > that looked great for a heated bed, however
> > looking at ohms law they will draw about 145
> amps
> > at 12v when wired in series.
> >
> > Anyone tried these or are they junk (only £2
> > delivered, probably as there is no popular
> > application).
>
> All of them in series 10 * .1 ohm = 1 ohm @ 12v =
> 12 amps.
>
> This is more than your motherboard can supply but
> with a relay it may still work.

Quote

This is more than your motherboard can supply but with a relay it may still work.

A decent electronics can deliver 12 amps directly

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Traumflug Wrote:
-------------------------------------------------------
> This is more than your motherboard can supply but
> with a relay it may still work.
>
> A decent electronics can deliver 12 amps directly
>

Your totally correct. I don't know what I was thinking about. The FETs on RAMPS and Sanguinololu which I have both of are rated to 55A.

---

FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver

Well I grabbed what was left on eBay so for less than £10 delivered I have 70!

Plan is to make 7 heated beds and sell 6

---

Andrew

Based in Slough, United Kingdom

Quote

Traumflug Wrote:
-------------------------------------------------------
> This is more than your motherboard can supply but
> with a relay it may still work.
>
> A decent electronics can deliver 12 amps directly
> winking smiley

Your totally correct. I don't know what I was thinking about. The FETs on RAMPS and Sanguinololu which I have both of are rated to 55A.

You were probably thinking about track widths, bed connector sizes, power supply connector sizes, etc.. Each of them has to be sufficient, the chain fails at it's weakest link.

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

Sublime Wrote:
-------------------------------------------------------
> Traumflug Wrote:
> --------------------------------------------------
> -----
> > This is more than your motherboard can supply
> but
> > with a relay it may still work.
> >
> > A decent electronics can deliver 12 amps
> directly
> >
>
> Your totally correct. I don't know what I was
> thinking about. The FETs on RAMPS and Sanguinololu
> which I have both of are rated to 55A.

Yes. 1 ohm should work fine with a 12V supply. The Prusa PCB heated bed is around 1.1 ohm at room temperature. Note that I've found that a heat sink on the mosfet is a good idea on RAMPS. I installed one on v1.2 and it kept mosfet cool to the touch when operating. I noticed that the latest RAMPS (1.4) now has a heatsink.