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D40 and D43 do nothing, right?
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D40 and D43 do nothing, right?

by cobey on Wed Aug 28, 2013 2:50 pm

So I was trying to figure out the x0xb0x power supply for an add-on and it became evident that D40 and D43 perform no function aside from looking like diodes. The junction of D40 and D43 is about 12VDC (plus ripple), so D40 is always reverse biased by about 12V. And the junction of D42 and D43 is about 24VDC (plus ripple again), so D43 is also always reverse biased by about 12V. Am I missing something here?
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Re: D40 and D43 do nothing, right?

by MMM on Wed Aug 28, 2013 3:44 pm

There had been at least one thread about the power supply topology (but I can't find it at the moment). As far as I remember it has something to do with C5 acting as a charge pump (or something like this). The output at the positive pin of C5 is much higher than just rectified voltage.
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Re: D40 and D43 do nothing, right?

by cobey on Sat Aug 31, 2013 12:52 pm

Right, I get the charge-pump / voltage doubler function, which is not obvious by how the circuit is drawn. But it looks like only D41 and D42 are actually used for the power supply. D40 and D43 need not be installed, since they're always reverse biased.
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Re: D40 and D43 do nothing, right?

by tempomaster on Fri May 20, 2016 2:28 pm

D40 and D43 are the second half of a full-wave rectifier circuit. Without them, you're only getting charge on half the cycle. It might work without them, but the output will be a bit less than with them. (about 6.8 volts without and about 9.7 volts with). Plus, without them, you need to do a lot more filtering to get rid of the extra ripple, aka a more complicated circuit.

Basically, D41 and D42 are allowing electricity to flow when the voltage at D41/D42 is positive ad D40/D43 is negative. Since we're feeding the x0x with AC, half the time the voltage at D41/D42 is negative and it's positive at D40/D43. When that is the case, the power supply receives its power through D40/D43. In the US, this happens 60 times per second (aka 60Hz or 60 hertz). Without D40/D43, the regulators are only getting power half the time (ignoring the smoothing effects of C5).
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Please be positive and constructive with your questions and comments.