Is this a diode? - Page 1
Forum section: electronics / beginners

author: TheGenesis
author_rank: Contributor
timestamp: May 26, 2018, 02:45:21 pm
content: I try to repair a Transmission-ECU from an Audi Quattro which has been under water for a few days.Most parts have survivied, but this one on the picture is gone and I dont know what it is.Can soneone identify this part?

author: jmelson
author_rank: Super Contributor
timestamp: May 26, 2018, 03:39:02 pm
content: Looks like it might be a Tantalum capacitor.Jon

author: lordvader88
author_rank: Frequent Contributor
timestamp: May 26, 2018, 04:25:24 pm
content: well does it look like a diode to a multimeter ?I'm very new to SMD stuff but usually that stripe at 1 end is the cathode

author: james_s
author_rank: Super Contributor
timestamp: May 26, 2018, 06:00:06 pm
content: That's almost certainly a tantalum capacitor.

author: janoc
author_rank: Super Contributor
timestamp: May 26, 2018, 07:39:14 pm
content: The question is - how do you know it is bad when you don't know what it is?

author: james_s
author_rank: Super Contributor
timestamp: May 26, 2018, 08:23:44 pm
content: The terminals look all nasty and corroded, that's a pretty good indication that it should be replaced.

author: chris_leyson
author_rank: Super Contributor
timestamp: May 26, 2018, 08:31:19 pm
content: The package looks more like diode than a tant cap, the sides look as if they are tapered whereas I would expect straight sides on a plastic mold tant package. Maybe DO214AC or SMA package, some dimensions would help.

author: Zero999
author_rank: Super Contributor
timestamp: May 26, 2018, 08:39:45 pm
content: Quote from: lordvader88 on May 26, 2018, 04:25:24 pmTantalum capacitors exhibit diode like behaviour, which can be used to determine the anode from cathode. When connected backwards, a tantalum capacitor will conduct like a diode, dropping a small voltage. When connected correctly, the capacitor will charge, then block the voltage when fully charged. The same is true for other polarised capacitors. The aluminium capacitor was developed from the electrolytic rectifier.Now in this case, if it's dropping about 600mV, when forward biased, then it's most likely a silicon diode. If you're unsure, you could try it on a capacitance tester: it should read below 100pF. If it were a tantalum capacitor, then the capacitance would be much higher >100nF.

author: TheGenesis
author_rank: Contributor
timestamp: May 26, 2018, 10:52:08 pm
content: The dimensions are 4.5mm x 2.6mm.I know that it is faulty because one terminal is completely gone.Found another two of the same item on the pcb (see pic) and measured them in-circuit. Resistance is slowly increasing regardless of the polarity. Then it should be a capacitor and not a diode, right?How do I find out there specification?

author: james_s
author_rank: Super Contributor
timestamp: May 27, 2018, 01:01:03 am
content: You could reverse engineer enough of the circuit around the part to try to figure out what it does, the value may not be all that critical. You could also remove one of the good ones and test it.

author: janoc
author_rank: Super Contributor
timestamp: May 27, 2018, 08:46:38 am
content: Quote from: james_s on May 26, 2018, 08:23:44 pmGiven the "potato quality" of that photo, those "nasty and corroded" terminals could well be a flux residue or some other contamination, especially given that it is a desoldered component.

author: TheGenesis
author_rank: Contributor
timestamp: May 27, 2018, 12:38:22 pm
content: I have desoldered one of the "good" ones and China says it's a diode (see pic).And no, I haven't just laid the part on the contacts while measuring. Instead I pressed it down with a tweezer and also checked the opposite polarity. The china thing always comes to the same conclusion.But dont know how much voltage this thing can/must take and why does the china thing told me a capacitance of 15pF?

author: Gyro
author_rank: Super Contributor
timestamp: May 27, 2018, 01:05:17 pm
content: That's the junction capacitance it's showing. It is indeed a diode - an ordinary Silicon (not Schottky) one too.Edit: There's a small chance that it might still be a zener or TVS with a breakdown voltage greater than the tester uses (about 5V). If you have a variable bench psu and a series resistor (~1k) you could wind it up, reverse biased with a DMM across it and see if it clamps at some higher voltage.

author: gamalot
author_rank: Super Contributor
timestamp: May 27, 2018, 02:03:07 pm
content: Looks like 1SR154-400 from Rohmhttps://www.rohm.com/datasheet/1SR154-400(picture from RS)

author: Gyro
author_rank: Super Contributor
timestamp: May 27, 2018, 04:31:01 pm
content: Well spotted

author: james_s
author_rank: Super Contributor
timestamp: May 27, 2018, 04:34:51 pm
content: Hey look at that, yep, definitely a diode. I've seen some capacitors that looked very similar.

author: TheGenesis
author_rank: Contributor
timestamp: May 27, 2018, 07:30:16 pm
content: Thank you all ... I'll try and post the results

author: hydrolisk1792
author_rank: Contributor
timestamp: May 28, 2018, 09:46:17 am
content: Quote from: james_s on May 27, 2018, 04:34:51 pmAren't the caps in that package usually orange?

author: james_s
author_rank: Super Contributor
timestamp: May 28, 2018, 03:43:09 pm
content: I've seen orange, yellow and black. I've got some black ones I used on some boards I built, they do have obvious voltage and uF ratings printed though.

author: TheGenesis
author_rank: Contributor
timestamp: June 07, 2018, 08:36:54 pm
content: Haha ... I scraped a bit of the podding around the point where the terminal was attached to and managed to solder a jumper wire on to it.Today we tried the module on the car and everything is working fine againThanks again for your suppoert.Cheers