2
\$\begingroup\$

On a GPS device pcb, I found a small smd component that is marked as "22p V2". I found (tell me if I am wrong here) that it is a transistor like DDTA144WCA-7-F SOT-23-3 package, with built in biasing resistors R1 and R2 . So, If the last conclusion is correct, how to test this transistor with a DMM to check if it is ok ?

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Are you trying to measure R1 and R2 separately? This is a good question that you're asking :) \$\endgroup\$ – KingDuken Oct 15 '18 at 18:23
  • \$\begingroup\$ Basically I want to know if the transistor is shorted or it is working. If that test includes also the R1 and R2 then yes ! \$\endgroup\$ – Maverick Oct 15 '18 at 18:35
  • \$\begingroup\$ Turn the DIMM to the continuity test testing and probe between the terminals if you want to see if it's working. When you say "shorting", where do you believe the short circuit is occurring? \$\endgroup\$ – KingDuken Oct 15 '18 at 18:59
0
\$\begingroup\$

You might have the wrong decode, SMT topcodes can be tricky, for example 22p is not the same as P22, which is what the DDTA part is marked as. But a 22P (note the upper case) is a PMOSFET DMP2215L (also from Diodes Inc.) I don't know if your 'p' is meant to be upper or lower case, but it can make a difference.

In terms of measurement using a DMM there are only certain number of tests you can run to determine if the part is working, and interpretation depends on knowing what the part technology is.

The best way to do some diagnostic tests is to hook the part up to a bias circuit and see what you get, but a DMM can be used to some effect and you might get lucky.

With that in mind, with the datasheet in front of you to verify the pin functions, if you connect the DMM with positive on terminal pin 1 (base/bias), negative to pin 2 (emitter), and you measure something like 70K ohms, you can be reasonably sure that you have the DDTA part (the base-emitter junction is reverse biased in this case). If the resistance indicates open then you either have a bad DDTA, or a plain PNP transistor or you have a PMOSFET part (possibly). If you swap the leads, you should be able to forward bias the emitter-base junction, and read 47K plus some nominal resistance due to the Vbe junction (it sort of depends on what voltage the DMM uses to measure the resistance, you can try the diode setting to see if you can forward bias the junction as well, but a lot depends on the DMM probe voltages. If you measure an open then you may again have a PMOSFET, or a bad PNP transistor.

Another test is to measure (diode mode) between the D/collector (3) pin and the S/emitter (2) pins. If the part is a PMOSFET, you will get a reading on the body diode, and if you reverse the leads you should read open. If you read open on both ways then you likely have a PNP or a bad FET. It sometimes helps to draw up a table so that you don't have to remember all the different test results when trying to match the readings to the physics of the device.

NEW INFO,

https://www.nxp.com/docs/en/data-sheet/BF545A_BF545B_BF545C.pdf

Looks like it's a NJFET, BF545C at least from the topcode match. This also means that you would get different results from the above tests. Since it's a JFET, the gate is not insulated from the channel.

\$\endgroup\$
  • \$\begingroup\$ Hi, Thanks for your answer. The actual mark is "22p V2". I did not find any other datasheet of what the heck is that component. \$\endgroup\$ – Maverick Oct 16 '18 at 6:03
  • \$\begingroup\$ Hi, It seems that you have right. I did a quick test to an Ohm meter but I found about ~1K from base to emmiter and vice verca. It seems it may be a NJFET. I did not complete my tests since I had to go, but as soon as I verify that is indeed a JFET I will let you know. \$\endgroup\$ – Maverick Oct 18 '18 at 6:06

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.