While reading some old (paper) books, I found this as an example but without further explanation:
Why does it use double diodes? I don't see anything about this elsewhere. Does this increase the handling current? Wouldn't it also drop more volts?
This is from an old and quite damaged book I have, Soviet, 1969. Electronic Problems for New Technicians.
this increases the handling current?
Yes, but it brings complications. The diodes would need to be in close thermal contact so that one couldn't get hot independently of the other (or it would "steal" current, get hotter yet, steal more current, then end up carrying the whole load).
Wouldn't (also?) drop more volts?
No, parts in parallel don't drop a higher voltage.
Why it uses double diodes?
Mostly you would do that if you couldn't get individual diodes that would carry the rated current, or if such diodes would be more expensive than diodes in parallel.
Which makes me wonder how old the book was, or how high a current the bridge was designed for. This was something that wasn't uncommon in projects from the 1970's (as I recall), but whose popularity was fading fast by the 1980's, even in amateur use.
I have implemented the double diode bridge circuit. It works well in various applications. I have used TO220 and TO247 dual Shottkey diode packages with good results. The diodes are very well matched and the voltage drops are much lower than a standard diode bridge which means cooler operation.
Looks like an example that will work with vacuum or selenium diodes (both still in wide use in 1969) and will pretty much fail with silicon or germanium diodes (rather new at the time) for reasons mentioned in the other answers.
It is unlikely to be vacuum-based design because the prices back then would dictate a center-tapped transformer winding and only 2 diodes. Vacuum diodes have a different symbol anyway.
The textbooks of the same era (I also had Soviet ones available) clearly stated that one should add equalizing resistor in series to each of the paralleled Si or Ge diodes.
Simple answer. It gives a vital redundancy for the circuit to handle power surges. With single diode rectifier, one power surge at most is enough to burn it. With double diodes, at least two such surges needed to burn it. Interesting quirk of single phase AC plays role at this, where the parallel diodes don't exactly split the power 50/50. Positive feedback loop playing with voltage/current being delayed one from another because of slight mismatch of diodes' impedances is the source of such peculiarities.
All of the reasons given earlier.
If designing for a safety or life support related function, designs are usually required to have redundancy in critical areas.
Two diodes in parallel can take more current than one but as mentioned earlier they can't be expected to share current evenly. Normally you would just go to a higher rated diode but if you had bucketloads of a particular dual diode in the stock room and wanted to use them up, you might go this way.
Doubling up on diodes will reduce the forward voltage drop a little bit, as would a higher current diode in the circuit, and this improves efficiency by a similar amount.
Interesting question though, because I can't recall the last time I saw something like this. .
