Four diodes, chosen on a number most people do not look up, wired in an arrangement that decides what voltage the rest of the machine lives at.
What it is and how it works
A diode passes current one way and blocks it the other. Four of them in a bridge send both halves of an alternating supply the same way round, so what comes out never changes sign. Behind them sits a bank of capacitors, and the two together make a direct voltage out of an alternating one.
The part that is not obvious is when the diodes are actually working. The capacitor holds a voltage. The supply is a sine, so for most of every cycle it is below what the capacitor is already holding, and while that is true the diodes are reverse biased and off. Only near the top of each hump does the supply exceed the capacitor, and only then does any current flow through them at all.
So the current through a rectifier is not a smooth trickle. It is a series of short, tall bursts, one per hump, and the average of those bursts is a small fraction of their height.
What it does in a coil, and what you decide
The bank behind it is what actually holds the bus up during a bang, and sizing that is its own page. This one is about the four diodes and how they are wired.
Which arrangement
From 230 V mains:
- A plain bridge gives about 325 V. Both half cycles, four diodes, nothing clever.
- A Delon doubler gives about 650 V. Both half cycles again, and twice the voltage from the same wall socket.
- A half wave gives about 325 V for half the period. Fewer parts, and worse in the way that matters: for the other half of the cycle nothing refills the bank, and residual voltage left on the bus is what makes the arc branch at the start of a ramp.
The reason to reach for the doubler is not the 650 V. It is what four times the stored energy does to the shape of a ramp, and that argument belongs with the bank, because the bank is the part it is an argument about.
Which diodes
By peak current, not average, for the reason in the drawing above. A part sized on the average figure is sized on a number it never sees.
Look for I_FSM, the single half-cycle surge rating, and the repetitive peak alongside it. The average forward current is the last of the three to consult, not the first.
The numbers
- Plain bridge, 230 V mains: about 325 V.
- Delon doubler, 230 V mains: about 650 V.
- Half wave, 230 V mains: about 325 V, half the period.
- The rating to size on:
I_FSMfirst, then the repetitive peak, then the average.
What will get you
Balancing and bleeder resistors are still wanted across those sections, but for the spread in electrolytic leakage current and for discharging the bank afterwards. Not for dividing anything.
What goes wrong
- A diode fails early with everything apparently within rating. It was sized on the average forward current. The peak at the top of the sine wave is several times that.
- The arc branches in the first few milliseconds of a ramp. Residual bus voltage from the previous bang. A half wave supply leaves it; a doubler pulls the bus practically to zero every cycle.
- The two halves of a doubler read different voltages. They are not meant to divide; check the bleeders and the leakage spread, not the capacitance matching.
Where next
- The bus bank, which is what these four diodes are filling, and where the doubler argument actually lives.
- A high bus and a low bridge, on using 650 V without putting 650 V on the devices.
The rectifier is the second box on every department diagram, between the mains and the bank.