Circuit Analysis of the 1.8kW Induction Hotplate

Let’s start at the input.  AC in is bridge rectified and fed to an 8uF cap at a node called TOP.  TOP is the input to the work coil, the high-side screw lug.  This 8uf cap is one of the super heavy duty MKPH’s we saw in the last article as it is subjected to a whole lotta ripple..  It’s job is to supply the peak resonant current to the work coil, NOT to filter the 60Hz line voltage.  The voltage at TOP will rise and fall with the AC input, and the work coil current (as well as the peak resonant voltage) will also rise and fall.

Fig 2 – Input Stage – AC Rectification and Blocking Choke

This is a really good thing, since it means that the work coil will always be driven at a power level proportional to the input voltage.  Therefore, the device loads the power line as if it were a perfect (resistive) load and can therefore suck the maximum amount of power from the AC line before it trips the breaker.   We already blabbed about this in the first article, so we’ll just notch the belt in favor of the Burton and be done with it.

COOL THING #1 – Free Power Factor Correction

COOL THING #2 – LOW Capacitance on AC line filter.

Yes, low filter capacitance (not having to buy a 600v, 680uF cap) is definitely an advantage.  But it means that the input voltage (as far as the work coil is concerned) is going to be varying all over the place.  Usually that makes things harder, but in this case the circuit takes care of all of that for us.  And it does it all with a single LM339 comparator.

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