Help diagnose micro rectifier failure, please

[peteski]

Peteski,
Everything you say is pretty much what I would have thought.  Sure is weird, though.  I suppose I could have gotten a bum batch of these.

If I had a bench supply that could go up to 60v, I would take another one from the package and start driving it with higher and higher voltage to see what the failure point is.  That would be very educational.

Meanwhile.... I found one other manufacturer that makes an SOT-363 package bridge rectifier (the same tiny footprint as the one I've been using).   They have an 85 volt reverse voltage rating.  I ordered a dozen from Mouser and I'll put one of those in there when they arrive.  Using a different part from a different manufacturer with a higher voltage rating will probably be a good thing.

Sounds like a sound plan.
My small rectifier of choice is: BGX50AE6327

[Point353]

Sadly, it's time to resurrect this thread.  The micro rectifier has failed again.
It *has* to be an emf kick-back from the motor, I would think.

Did you ever try adding a capacitor across the input to the bridge rectifier?

[mmagliaro]

Did you ever try adding a capacitor across the input to the bridge rectifier?

No, because I still don't understand why that would help.  If somebody would explain that to me, I would give it a go.
Space is very tight in this installation, and I don't want to just throw parts at the problem without understanding
the theory behind why it should work.  When the motor loses power, and kicks out its inductive spike,
how would a tiny capacitor mitigate that?   A very large one, yes (like 1000 uF or something?).    But a little .01 uF or something?  How would that shunt the spike away?

[peteski]

I'm kind of rusty on my AC theory but a spike is a very fast changing voltage. As such, even a relatively small value capacitor will become a conductive path for it and shunt it. But I really think that the danger described here is a bit overblown. But then again, I'm not the owner of a multi-year custom-built model.

[DKS]

I'm kind of rusty on my AC theory but a spike is a very fast changing voltage. As such, even a relatively small value capacitor will become a conductive path for it and shunt it. But I really think that the danger described here is a bit overblown. But then again, I'm not the owner of a multi-year custom-built model.

Correct. A large capacitor wouldn't even "see" a short transient spike, and it would get passed along as though the capacitor wasn't even there, while a tiny capacitor would catch it. (Ask me how I learned this the hard way.)

[mmagliaro]

Correct. A large capacitor wouldn't even "see" a short transient spike, and it would get passed along as though the capacitor wasn't even there, while a tiny capacitor would catch it. (Ask me how I learned this the hard way.)

Ah, okay.  So, thinking about the time constant when the capacitor charges, I am beginning to understand this.
And this is why larger capacitors can filter out low frequencies, but you need to use smaller ones to filter high frequencies.

But wouldn't the little cap only shunt the spike until the capacitor is fully charged up?   Isn't there a balancing act here to have a small enough value cap to "trap" the high-frequency spike, but still be large enough to absorb all the energy from that spike before the cap fully charges?

So, now, assuming I don't have a scope (which I don't), and assuming this is a coreless motor at a maximum of, say, 20,000 rpm, is there any reasonable way to predict the frequency range of that spike?

A .01 "or" 0.1 uF cap has been suggested.   Truth be told, if I were "guessing" I would have tried one of those, but I really don't have any basis for other than that those convenient values.

[peteski]

Max, do not think of a capacitor as simply a miniature battery (energy storage device). Think of it as a device which will conduct current if the voltage across its terminals is changing (as in a voltage spike).  The other important consideration of the type of a dielectric (insulator between the metal plates) used in the capacitor. That affects is AC properties. Specifically, ceramic type of dielectric is well suited for shunting voltage spikes. As long as it is a ceramic cap the capacitance is not that critical. That is why you can see 0.001 or 0.01 micro farads being recommended.

I'm not explaining this in excruciating detail because (1): I don't remember all the specifics, (2): I'm not a big math whiz, and (3): the mathematical details (which I would have to dig up) can be boring, especially to people that ate not electronic engineers. 

[mmagliaro]

Max, do not think of a capacitor as simply a miniature battery (energy storage device). Think of it as a device which will conduct current if the voltage across its terminals is changing (as in a voltage spike).  The other important consideration of the type of a dielectric (insulator between the metal plates) used in the capacitor. That affects is AC properties. Specifically, ceramic type of dielectric is well suited for shunting voltage spikes. As long as it is a ceramic cap the capacitance is not that critical. That is why you can see 0.001 or 0.01 micro farads being recommended.

I'm not explaining this in excruciating detail because (1): I don't remember all the specifics, (2): I'm not a big math whiz, and (3): the mathematical details (which I would have to dig up) can be boring, especially to people that ate not electronic engineers. 

Right.  It conducts when the voltage is changing, much like a signal amplifier or audio filter does.   I have been envisioning this spike as being just a single pulse.  But now that you mention it, I suppose that is no different than any other AC signal.  And now that you mention it, it's common to find .01 or .001 caps wired across things like power switches inside audio gear to shunt out the turn-on spikes from the switch. 

Okay.  You guys have convinced me it's a reasonable solution, and most likely a better one than just trying to use a bridge rectifier with a higher PIV rating (or hoping that the ones I have are somehow defective).   A little ceramic cap will go in there tomorrow (along with a new bridge rectifier).

Thank you all for thinking through this with me.

[peteski]

Right.  It conducts when the voltage is changing, much like a signal amplifier or audio filter does.   I have been envisioning this spike as being just a single pulse.  But now that you mention it, I suppose that is no different than any other AC signal.  And now that you mention it, it's common to find .01 or .001 caps wired across things like power switches inside audio gear to shunt out the turn-on spikes from the switch. 

Okay.  You guys have convinced me it's a reasonable solution, and most likely a better one than just trying to use a bridge rectifier with a higher PIV rating (or hoping that the ones I have are somehow defective).   A little ceramic cap will go in there tomorrow (along with a new bridge rectifier).

Thank you all for thinking through this with me.

Exactly, a spike (or even a square pulse) is considered an AC signal as far as a capacitor is concerned. So would be a single cycle of a sine wave, or even just part of a single sine wave. As long as the voltage is changing. Of course this is a very basic explanation, but it illustrates that all of those types of signal will pass through a capacitor.

I still think that is is an overkill here, but if you have the room for a capacitor, it will not be detrimental to add one. 

[mmagliaro]

Exactly, a spike (or even a square pulse) is considered an AC signal as far as a capacitor is concerned. So would be a single cycle of a sine wave, or even just part of a single sine wave. As long as the voltage is changing. Of course this is a very basic explanation, but it illustrates that all of those types of signal will pass through a capacitor.

I still think that is is an overkill here, but if you have the room for a capacitor, it will not be detrimental to add one. 

Well, *something* has to change, since I keep blowing bridge rectifiers.  I got the 85v ones in the mail today.  I suppose I will put one of those in since I have it, and I'll throw the cap in as well, now that I see a legitimate way that it could snub the spike.  Of course, if it never happens again, we'll never know if it was the rectifier change or the cap, but I will blissfully and unscientifically go on without knowing.   

[peteski]

Of course, if it never happens again, we'll never know if it was the rectifier change or the cap . . .   

That is exactly how I see it. 
But then like you said, you will be happy as long as the problem doesn't reoccur (regardless of what fixed it).

[Doug G.]

Max will put both new parts in, it will go two years without blowing, and Max will be forced to take out the capacitor and try it again to see if it was that or the heftier Rectifier.

That's what I would do.



Doug

[mmagliaro]

Max will put both new parts in, it will go two years without blowing, and Max will be forced to take out the capacitor and try it again to see if it was that or the heftier Rectifier.

That's what I would do.



Doug

Curse you, Doug, and your accurate personality assessment! 

[peteski]

Max will put both new parts in, it will go two years without blowing, and Max will be forced to take out the capacitor and try it again to see if it was that or the heftier Rectifier.

At least by that time the model might even be painted!   

[Doug G.]

I have done too many multi-component fixes in my life, wanting to know what the real cause was, to just be satisfied it was fixed. I think it's a common trait among troubleshooters.



Doug