Diodes in parallel

Nope. since paired diodes need to be match perfectly to insure both share an equal load it's not practical. one diode may start conducting before the other, and this happens the other diode normally does not hardly even or at all conduct. thus one is doing all of the work.

Whether the power supply can provide more amperes is not just dependent on the diodes. But to address the diode question, paralleling with a similar type will allow a bit higher current without destroying the diodes, but not twice as much. The diodes will not share the current exactly and each will heat the ambient air near he other, so each will get hotter for its share of the current than if it were operating at that current without its partner near by.

Even alone, a 1 amp diode is not normally expected to deliver 1 amp average, except under the most ideal of conditions (leads heat sunk to

I didn't say anything about a bad thing. I said that two 1 amp diodes in parallel and also in close proximity should not be expected to deliver 2 amps. They will deliver somewhat more than one diode would be capable of, just not twice as much.

Forced air definitely redefines close proximity.

Blown diodes are normally short circuits, so they will not take turns as you describe. A better way to protect the alternator might be to add a resistor between the regulator and the armature winding, so that the excitation current was limited to a value slightly less than the design maximum. Since the alternator normally uses way less than the maximum possible excitation, this would affect its operation only at lowest speed and during overload situations.

I am working on a new regulator for my motorcycle that includes an adjustable current limit for the excitation, but for a completely different reason.

There would be less heat in any one of the parallel diodes than if only one were carrying the whole current. The problem is that the hottest diode tends to have the lowest forward drop and hog most of the current, making sharing worse. So all sets need similar heat sinking.

Just increasing the diodes will not increase the capacity of the psu. You must also increase the rating of the other components.

John... This fellow's question brings to mind a similar concept I had over on an automotive group - Alternators are fairly tough little beasties - Most of the guts of them are all but indestructible short of physical damage. But a very common failure mode for them is for the diode trios to barf if/when they're asked to try to charge a mostly-dead battery.

I had the thought not too long ago that paralleling multiple diodes per "trio position" should give at least some added "anti-fry" protection in terms of how dead a battery the alternator can charge without burning itself (or more specifically, its diode trios) up.

Since it's such a common failure mode, and cheap to repair (compared to "buy a new/reman alternator and replace as a unit") the benefit would potentially be huge to the person doing such a mod to their alternator.

Now, it sounds like you're saying that rather than improving the situation, such a setup would likely be an actively BAD thing?

Is the "extra heating" issue still a significant problem in a forced-air environment such as what I'd have in my particularly alternator? (diode trios in it are heat-sinked, and are the first thing that the air, being actively pulled in through the back of the alternator by its built-in fan, encounters)

It would seem to me, at least at first glance, that "stacking" multiple diode trios (let's say to three diodes per phase and polarity, for a total of 18 discrete diodes (versus the standard 6 -- two, one for each polarity, on each of three phases)) in this beast would be beneficial, if only in terms of "Well, that one over there cooked, but I'm still doing fine, and so is the one next to me" style backup - should one fail, the remaining two for that phase pick up the slack. (Of course, with such an arrangement, if there's an overload big enough to blow one of the diodes, I think it unlikely that the other two would survive as they were forced to pick up even more of the load that managed to burn out one of the three already...)

Which doesn't even start to address the whole "Since three of them are sharing the load (however unevenly they might be portioning it out amongst themselves) there should be little or no reason for one to fail in the first place" concept...

Gimme a sanity-check on this idea, would ya, John?

OK, clarification noted. So does it make sense to expect that "unloading" any single diode by splitting the load across (as per my previous example) two more diodes wired in parallel with it would have a positive effect as far as "diode-death prevention" is concerned?

Basically, I'm working under the concept of "A draw that would kill a "lone" diode should (generally speaking, and for the sake of discussion, ignoring the possibility/effects of uneven load division/thermal runaway that you mention later on) be insignificant when three diodes are splitting the load."

Do I need a new concept?

Kinda figured it would :) Seems to me that heat would become (at least for practical purposes) a non-issue in such a situation.

Hmmm... not a concept that I had thought about - That would tend to throw a bit of a wrench in the works, wouldn't it? Again, though, it comes back to the whole "Shouldn't the load that would blow a single diode be seen as effectively insignificant by three similarly rated/heat-sinked/etc diodes wired in parallel?" - If the diode is unloaded enough by paralleling multiple "copies" of itself in its position in the circuit that even under ridiculaous loading, no single diode of the group should fail in the first place, it's pretty much irrelevant that a given diode "usually fails shorted" when it dies, no?

May I be so bold as allow my curiousity to run wild long enough for me to enquire about what this completely different reason is?

Ahhh, yes... Good ol' thermal runaway.

Makes good sense, now that I consider it. That would seem to indicate that one would want to carefully match the diodes being used to construct the "trio set" - A task with the potential to be anywhere from "trivial - I did it three times while sleeping last week" to "outright impossible", I imagine...

A positive effect, yes. A multiple of the current capability equal to the multiple of diodes, no.

No. Paralleling can be some use. But a bigger diode and more effective heat sinking may do better.

If paralleling prevents failure, then, yes, you don't have to deal with failure.

I have a motorcycle that has its idle speed loaded variably by the alternator. If I adjust the idle when the battery is under heavy charge, then it races after the battery is fully charged. If I adjust the idle when the battery is fully charged, it has trouble stalling the next time the battery needs charge. I am going to make the regulator a switcher for improved efficiency and include a current limit so that the low RPM maximum torque load can be reduced. I also expect it to keep the battery more accurately charged, compared to what the electromechanical regulator accomplishes, now.

No. Minute differences between the diodes will cause one to handle all or nearly all of the current. The "parallel second diode" will not conduct much at all. Use 1N5402 diodes, even if you have to "stand 'em up" off the PCB. Some clever fiddling should get 'em all-four in place!

Right. Got that part. Pessimistically assuming that you'd actually LOSE some capacity to the paralleling (For the sake of discussion, let's pull a number out of the air and say each diode would only handle 75% of its rated load) that would still give 125% more "head space" over that which is available from a single diode in that position when the group is considered as a unit. One would HOPE that an alternator rated for 55 amps is never going to be asked to supply nearly 125! Of course, should that ever happen with the "ganged" diodes, the failure point may well end up being the coil cooking open, rather than a diode frying...

No argument there. "How much better, and for what price - in both dollars and difficulty?" probably becomes an issue quickly. As of yet, I haven't figured out what the price on one of the diodes from the trio would be. (For that matter, I haven't yet figured out what the installed diodes actually are.) I would expect that, as is usually the case when messing about with rectifiers and regulators, moving up in current capacity implies moving up to a bigger price tag.

Sounds like a fun mess with the idle... NOT!

Electromechanical? As in the old-style "solenoid and gap" type? Oi, vey... those things always were a mess. I gotta give 'em credit - They worked amazingly well. But keeping them working correctly... Wotta headache...

"Jack// ani" wrote in news: snipped-for-privacy@o13g2000cwo.googlegroups.com:

Listen to Nike, just do it. You "can easily put the diodes in parallel" so do it and take current reading thru both your 1n4007 diodes at the same time to see if one conducts more or not. Post your results, sounds interesting.