Unknown diodes

[lti]

I want to know what the three diodes on this board are. If it's hard to see in the picture, D1 and D2 are marked W1 and D3 is marked 4D. I've found some 2.4V zener diodes marked W1, but these two diodes are being used as half-wave rectifiers in a 2.4V battery charger (D1 provides an AC detection signal for the IC). They're obviously not zener diodes.

If anyone is wondering, this board came from a toothbrush. I took apart the same model of toothbrush a few years ago, and it had worn motor brushes. This one also has a dead motor, but it took out Q1 (TDM2306), which is kind of impressive for something that runs from two 300mAh 1/2AA NiCd cells. I'm going to disassemble the motor because there is a position where I measure a short across the terminals.

[ReverendJones]

Re: Unknown diodes

W1 FES3A Si-diode Ultrafast Recovery, 50V, 3A, <50ns
4D MMSZ2V7 Z-diode 2.7V±5%, Izt=5mA, Zzt=100, 500mW

Info is from the Turuta SMD-Codes Databook, 2011 Edition.

[stj]

Re: Unknown diodes

i have that book, it also id's them as something else - a bit of a problem without drawing a schematic.

[lti]

Re: Unknown diodes

I think I found the W1 diode. It's a 1N4448W made by Semtech.
http://www.semtech.com.hk/script/pro...1421443877.pdf
There might be other diodes in the same package (SOD-123) with the same markings, but this makes sense. I have an older revision of this board (from the other toothbrush) that uses small MELF package diodes like 1N4148s. On the older board, D3 is a larger through hole glass diode.

If it helps, VE+ is the battery positive, VE- is the battery negative, and MTR+ and MTR- are the motor connections.

[stj]

Re: Unknown diodes

those in the foto look a bit big for a 4448

[lti]

Re: Unknown diodes

They're the same package shown in the datasheet.

I'll have to test the diode at D3 to see if it is a zener. The diode on the older board is marked C2V7, but it doesn't appear to be a 2.7V zener diode. That diode is in parallel with the motor.

[lti]

Re: Unknown diodes

D3 is a 2.7V zener diode on both boards. It is most likely the MMSZ2V7 that ReverendJones mentioned, but there is also an MM1Z2V7 from the same manufacturer (EIC) with slightly better tolerances. The through-hole diode on the other board may be a BZX79-C2V7 or a similar part from a company nobody has heard of.

[budm]

Re: Unknown diodes

D3, C1 are just snubbers to protect Q1 from back EMF from the motor.
Q1 drives (low side (sink) the load which is the Motor) the motor but if the motor has really low DCR, then lots of current will try to flow through Q1 and burnt it up.
Did you remove D3, C1 to verify if they are not shorted without motor in place?

[lti]

Re: Unknown diodes

Q1 drives (low side (sink) the load which is the Motor) the motor but if the motor has really low DCR, then lots of current will try to flow through Q1 and burnt it up.
Did you remove D3, C1 to verify if they are not shorted without motor in place?

I didn't think the little battery could continuously supply enough current to make that FET fail open. There is voltage at the gate (slightly lower than the battery voltage, as expected with the voltage divider formed by R4 and R9) when the motor should be running, so the IC is still working properly. D3 and C1 are not shorted. I tested that diode earlier to see if it was a zener or a normal diode. Rotating the motor shaft made the resistance increase to 5 ohms.

One of the motor brushes was bent and touching the wrong contacts. It looks like the brush wore down so far that it got caught in between contacts on the commutator and bent. The motor is similar to a Mabuchi FA-130RA.

I noticed that there is a plus sign molded into the plastic near one terminal. If that means that the motor should only turn in one direction (battery positive to the terminal with the plus sign next to it), then it was being used with the polarity reversed.

[fuhz2009]

Re: Unknown diodes

Thanks for share this info.