Help identifying diode replacement for cordless drill.

[schnagglesworth]

Greetings everyone!  This is my first post here, so I hope I have chosen the correct section for my inquiry.

I will try to make this as short as possible:  I have recently come into possession of a barely used 18V Lipo cordless hammer drill.  Evidently it died/stopped working after one use.  This is not a name-brand tool, but i don't think its the cheapest either (although i could be wrong in that it died so quickly).

First, I checked the batteries and they seem to be fine so i decided to disassemble it.

After disassembly I found that the trigger seemed faulty.  I took it a apart and it appears to have a small PCB with a small controller (I'm guessing for PWM), a mosfet, and a diode (I'm assuming it is what is called a flyback diode).  I have tested the mosfet and it has continuity between all pins, from what I have read this seems to mean that the mosfet is bad.  Additionally I get continuity in both directions on the diode, so I believe this also means that the diode is bad.

For the mosfet I am able to find a replacement by searching for the markings on it.
It is this one: https://datasheetspdf.com/pdf-file/1148508/Ruichips/RU75N08R/1
And i am planning to replace it with this one: https://cdn-reichelt.de/documents/datenblatt/A200/QNSTM_FET-STP75_EN.pdf
(Im pretty sure on the above replacement, but let me know if I have missed something...)

But for the diode I am having a bit of trouble, it has the markings 6A4 and ZG1603g.  Searching for ZG1603g seems to return nothing but the 6A4 seems to imply 6 amps at 400 volts, but I am not sure. Could someone chime in here and let me know if this is correct?

Also, should/can I replace the diode with a higher amp rated one.  What are the rules with upgrading a diode, can they be replaced with one of both a greater amperage and voltage rating?  Is there a specific type of diode I should select for this application? (from my research this appears to be what is called a flyback diode)

Additionally, I am curious on what could have happened here?  Which one of the components would have failed first? I assume one failed and then caused the other to fail.

Note: I have tested the motor by connecting it directly to a 12V 5A power supply and it turns fine.

Regards,
Schnagglesworth

[madires]

Yep, 6A4 is a silicon diode rated for 6A, 400V and a high surge current of 400A (8.3ms single half sine-wave). I'd suggest to replace it with a diode with similar ratings and the same package if you can't get a 6A4. Diodes with higher ratings will most likely have a larger package and need more space.

[schnagglesworth]

Thank you for the reply madires!

If space is not an issue, would replacing it with one of higher voltage/amperage ratings be problematic?

For example I have found this one (PX1500M): https://cdn-reichelt.de/documents/datenblatt/A400/PX1500_ENG_GER_DS.pdf

Thanks!

[Kleinstein]

I higher rating would not be a problem. There is absolutely no need for higher voltage rating. A slightly higher current rating may even be good.
Depending on the frequency used for PWM, too slow a diode could be a problem. However  most of the regulators use a relatively low frequency, where a slow diode is not an issue.

[bdunham7]

The 6A4 is a 'standard recovery' rectifier.  The other info is just the logo and such from the company that made it:

http://www.cz-zg.com.cn/

What you have is some sort of initial defect and unless there is some known history or a common problem with the device, I wouldn't go to much trouble to upgrade it as the design is likely OK and there was juts a bad part.  For the diode just use whichever one can be obtained cheaply and conveniently--the one you listed should be fine.  The MOSFET looks OK too at first glance.  I'm not sure what exactly the circuit looks like nor why both components would fail shorted (did the tool stop or keep running??), but you might want to order extras in case you have an additional problem somewhere. 

[schnagglesworth]

@bdunham7  When pulling the trigger it would basically move the motor a tiny bit then nothing.

Thanks for the help everyone!  I am going to order the parts and give it a go... I will report back with success or failure (hopefully there isnt something else broken, but i should know soon enough)