horizen96 wrote:I have a roomba 550 that worked just fine and then it would not even turn on light up or any thing ... I quickly found the problem. There are 2 diodes in the charging circuit on the PCB.
Maybe not in the "charging circuit". Look at this photo:
...and study the two diodes next to the toroidal inductor. You will see D47 and D55 next to each other. If those are the diodes that blew out on your board, I can tell you that neither one carries battery charging current.
... i need to know what kind. The label on the diode was the destroyed so i can't see what it was. Is there any way i can get a circuit diagram ,,, .
The circuit you need is that of the 5XX Buck Converter, (looking something like this circuit draft):
... which accepts either +21.5Vdc (approx) from a charging power supply, or battery_voltage, minus about one volt, to then create Roomba's +5Vdc regulated power form.
You can see where input diode D47 taps into those power sources by reviewing this post: viewtopic.php?p=81792#p81792
Of course, the attached +5VREG schematic shows you that D55, SR240 (found by TechGuy) is the converter's flyback diode.
I do show D47 as a "1N4001" in the schematic, partly because I read "1N4..." on its case, but I also think a 1N4001 could serve there.
D55 is a Schottky diode, whereas D47 need not be.
You have been forewarned by Mike, regarding probable damage to other components, so simply replacing a couple diodes can't be expected to make this board functional.
When power diodes behave as fuses, you really need to find out what caused the damage! Then see about tending to that area first.
Granted, it is feasible to think that D55 simply suffered a latent semiconductor fault which resulted in it turning into a low-resistance path. D47 would then have been faced with delivering as much battery-current as it could to a switched ON Q7 MOSFET and through the shorted D55. OR, if the 550 was in charging mode at the time D55 failed, then D47 would have been subjected to continuous 1.3Adc current that would eventually (hours) smoke it.
Normally, Q7 is not continuously ON, it is rapidly pulsed ON/OFF as directed by controller U3. U3 also monitors current being delivered to five-volt loads. If that current assessment has an upper limit, then U3 could have shut down ops before D47 got blown. You will have to read the U3 data sheet and app-note to see about its current limiting capability. If there is no current limiting I can see how U3 voltage regulator may cause U3's Q2 to be given maximum ON-time, to then pull Q7's gate low and keep it maximally conducting throughout the fault period.
If D47 blew due to a high battery current coursing through it, the battery's fuse would have been stressed, and the parasitic diode in FET Q18 would have been stressed. But, if it was the charging PSU that heated D47 to destruction, you might get away with simply replacing the two damaged diodes.
Seems to me that you have quite a bit of work to do! You might even have to buy another 5XX PCA, any model, just so you can dismount D47 to identify it. That board could also be used as a test device for verifying the normal power path (backwards) through Q18 & Q17 to D47A, as well as trouble-shooting the Buck Converter.
If you are not up to that kind of "fun", then buy a replacement 550 PCA.