Heat bed output voltage quite low?

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.

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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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Generation 7 Electronics	Teacup Firmware	RepRap DIY

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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[www.hydraraptor.blogspot.com]

So why not use a NOTO delay relay instead of a MOSFET? Or is the MOSFET there for much more sensitive quicker heating heatbeds?

If you prefer a relay, use a relay. Nobody enforces you to actually use the MOSFET.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

... 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

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Viktor
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