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Heat bed output voltage quite low?

Posted by Phizinza 
Heat bed output voltage quite low?
December 16, 2012 07:17AM
So while waiting quite a long time for my heatbed to heat up each time I start it from cold I thought I'd measure the volts to it to make sure it's getting full PSU voltage. Now under the load of the heatbed my PSU was delivering 11.8 volts, but on the output of the MOSFET there were only 10.5 volts. Is this usual? It seems like quite a drop and I can really see why it takes so long to get the bed up to 110 degrees, and how it won't go much over 115 degrees.
Re: Heat bed output voltage quite low?
December 16, 2012 07:29AM
Quote

Now under the load of the heatbed my PSU was delivering 11.8 volts, but on the output of the MOSFET there were only 10.5 volts. Is this usual?

Yes, this is expected. MOSFETs have, like all semiconductors, a voltage drop in forwarding direction.


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Re: Heat bed output voltage quite low?
December 16, 2012 11:01AM
You shouldn't be losing 1.3V though. That would dissipate 13W @ 10A so would require a big heatsink and the bed would not get hot enough. A suitable MOSFET should drop less than 0.1V.


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Re: Heat bed output voltage quite low?
December 17, 2012 04:17PM
So why not use a NOTO delay relay instead of a MOSFET? Or is the MOSFET there for much more sensitive quicker heating heatbeds?
Re: Heat bed output voltage quite low?
December 18, 2012 07:46AM
If you prefer a relay, use a relay. Nobody enforces you to actually use the MOSFET.


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Re: Heat bed output voltage quite low?
December 18, 2012 09:35AM
... if you can/should use a reay, depends on the estimated switching counts - realys are specified for some 100k cycles, MOSFET's to several times more grinning smiley


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