Please help me identify these SMT components!

[Lomax]

I'm repairing an IR thermometer that has a non-functioning LCD backlight (2x3 LEDs). Upon inspecting the PCBs, it became clear that the inductor (33 µH) for one of several DC-DC converters (MAX 856) had died a death and would need replacing. Considering that all other functions on the thermometer appear to be working as they should, it seems reasonable to assume I have found the fault. But what caused the inductor to fail? Since this is a pretty high-end device (Raytek Raynger MX4+), I doubt that the inductor is underrated, so something else has probably failed as well. I haven't measured it yet, but I suspect the output diode and/or the output filter cap. But when I look at the board, I am unable to identify the diode (it only has a single digit "6" - or "9" - printed on it), and the filter cap looks more like another diode than a capacitor. I'm only just starting to do SMD work and I'm not yet familiar with the many different guises components can take, but there should be a capacitor to ground on the output, and from the traces it appears to be this black ceramic cylinder with a single red (or pink/purple) band?

[Lomax]

Ok, after another hour's searching, I think I've found the diode - it is indeed a Schottky barrier diode, PRLL5817 (20V Vr, 1A If, 450mV Vf @ 1A). But what about that thing that is supposed to be a capacitor but doesn't look like one?

Edit: Mouser didn't have the LL5817 so I ordered the Rectron SM5817-W instead. I also ordered a couple MAX856, in case the converter is also gone.

[MrAureliusR]

That thing you have labelled as a capacitor is likely a MELF package resistor. I don't think I've ever seen caps in that package, only resistors. I would be VERY surprised if anything other than mechanical shock would cause an inductor to fail. What kind of reading are you getting across it?

EDIT: The only cap I see there is the tantalum one.

[Falcon69]

I think it is a MELF(LL-41) Diode. Here is a picture of one, though not the correct size as you have.

http://diode.en.hisupplier.com/product-51019-SMD-Diodes.html


EDIT: Having trouble finding what the Violet Band means. Vishay lists these, but only list a red, green, and blue single band. To have a violet in the band, would mean a second band is present (violet would be 2nd band). This could be either a General Purpose Diode, or a Zener.

[Lomax]

I think it is a MELF(LL-41) Diode. [snip] This could be either a General Purpose Diode, or a Zener.

I agree, a Zener seems likely - if so they've clamped the output to something below the 856's 5V or 3.3V, which I guess would make sense if it's powering the BL LEDs. But what voltage? It may be that it's a red band, and only looks purple in the photo...

That thing you have labelled as a capacitor is likely a MELF package resistor. I don't think I've ever seen caps in that package, only resistors. I would be VERY surprised if anything other than mechanical shock would cause an inductor to fail. What kind of reading are you getting across it?

But a resistor to ground on the output of an 856 doesn't make any sense - the adjustable converters use a voltage divider on the output to set the voltage, but then there would be two resistors, and they would not be a power type. And the inductor is badly burnt; not badly enough to go open, but definitely badly enough to short.

[Falcon69]

If it is a RED band, according to this here:

http://www.marsport.org.uk/smd/sod80.htm

it is a:

http://www.mouser.com/ProductDetail/Vishay-Semiconductors/BA682-GS08/?qs=RwahMLkpD2uXZBoZ3aLdPg%3D%3D

[Lomax]

If it is a RED band [snip] it is a [PIN diode]

Ok, thanks - but why would you put a reverse biased PIN diode to ground on the output from a DC-DC converter? I am not familiar with this type of diode, but after reading a little about them it appears their main application is in RF switching? I have looked at some example designs using the MAX856, and compared to the (visible) traces it seems to me the only things that would make sense in that location is either a polar cap (filter) or a Zener (shunt)?

[MrAureliusR]

A reverse-biased diode to ground is the hallmark of an inverting DC-DC converter.

[wraper]

If it is a RED band [snip] it is a [PIN diode]

Ok, thanks - but why would you put a reverse biased PIN diode to ground on the output from a DC-DC converter? I am not familiar with this type of diode, but after reading a little about them it appears their main application is in RF switching? I have looked at some example designs using the MAX856, and compared to the (visible) traces it seems to me the only things that would make sense in that location is either a polar cap (filter) or a Zener (shunt)?

This could be TVS diode for overvoltage protection.

[Lomax]

Thanks everyone! I'm still not convinced though; I mean it's clearly a diode of some sort, both appearance (black DO-213AB / MELF with single band) and measurements indicate this (in circuit it measures 885 Ohm in the forward direction, and open in reverse), but the PIN / TVS suggestion seems unlikely to be correct? This is a low voltage device (I have double checked that the backlight consists of 2x3 LEDs and not an EL film) which is powered by two AA batteries - why would it need a MHz or HV spec diode on a DC-DC converter? MrAureliusR might be on to something, though I haven't been able to find any examples of the MAX856 used in this configuration (typical example circuit attached). Being somewhat stubborn, I've continued the search today, and found something in the Vishay packaging codes for MELF diodes (also attached): lo and behold, they do a MELF packaged Zener with a single red band, ZGL41-100A thru ZGL41-200A - but again the specifications don't seem to make any sense, since these are 100-200V Zeners. Hmmm...

Edit: Oh and the inductor measures 0.3 Ohms, again in circuit.

[EPTech]

Hi there,

The inductor looks fried indeed and that may be caused by the FET in the switcher IC to be shorted out. Check whether there is a short between 8 and 7 of the Max 856. The diode with the 6 or 9 is most likely a fast schottky diode. It looks to me like the capacitor above the inductor is the input capacitor and the output goes under the 7333 (low drop-out 3v3 regulator) to the output capacitor under it.

Looking at the traces I suspect the component with the pink stripe to be between the output voltage and the out-pin (6) of the MAX856. The 856 is a selectable 5V or 3V3 step-up converter. If the device only uses 3V3 it may be that the component with the pink stripe is a diode or a zener diode to increase the output voltage of the MAX856 as much to accomodate the drop-out of the 7333. Do not ask me why they did it. I think it unlikely that the component with the pink stripe has broken. or the schottky. I think the FET in the MAX856 has shorted out.

Also I would recommend a nylon washer under the top screw if that is attached to the chassis. pin 6 should not be shorted to the chassis.

[Lomax]

The inductor looks fried indeed and that may be caused by the FET in the switcher IC to be shorted out. Check whether there is a short between 8 and 7 of the Max 856.

Thank you, this is exactly the kind of info I was hoping to get! When measuring pins 7 & 8 (in circuit) I get 763 Ohms, so not a short. I have already ordered replacement inductors and a few MAX856 chips, so I was planning to replace it in any case.

The diode with the 6 or 9 is most likely a fast schottky diode.

Yeah, that one has been identified as an LL5817, which is indeed a fast Schottky (and it's a "9", not a "6"). Pretty standard stuff for a DC-DC converter.

Looking at the traces I suspect the component with the pink stripe to be between the output voltage and the out-pin (6) of the MAX856.

I can't actually see what the out pin (6) is connected to, without removing the chip; I would have guessed it goes straight to the LX pin (8) - this is how most MAX856 sample circuits I've looked at are connected - but curiously I measure 266 Ohms between them. The pink/red stripe diode is most definitely connected to GND - and so are pins 5 (Low battery detect) and 7 (GND) - see attached photo with the screw removed.

The 856 is a selectable 5V or 3V3 step-up converter. If the device only uses 3V3 it may be that the component with the pink stripe is a diode or a zener diode to increase the output voltage of the MAX856 as much to accomodate the drop-out of the 7333.

I don't think the 7333 is part of the same circuit - there are basically two boards in the device, the one we're looking at appears to be a dedicated power supply board, and by counting the converters/regulators it looks like it provides (at least) four different supply voltages from the AA batteries (I think they are: logic, backlight, laser, sensor). This certainly fits with the symptoms in that all other functions of the device are operating normally: LCD display works, laser works, logic works, sensor works (and appears to measure accurately). It's worth noting that the positive battery terminal goes straight to the burnt out inductor (red wire in photos), and the traces in this area are thicker than elsewhere on the board.

[EPTech]

So the screw it actually connected to ground. Then it makes sense that there is not isolator there.

If the top side of the pink striped component is connected to ground, then it is most likely a TVS diode preventing the output from ever going way above the intended output voltage. In either case it should not conduct to ground unless you go above the intended 5V or 3V3 (depending on the set output voltage via pin 2)

Did you measure between pins 7 and 8 in both directions because the 763 ohms may be the inverse diode accross the FET. Anyway 763 ohms is still way to much to burn out the inductor. That does not mean however that the IC is not broken. The logic inside the regulator may still turn the fet on permanently.

Good luck.

[wraper]

two AA batteries - why would it need a MHz or HV spec diode on a DC-DC converter?

Do you understand what the TVS diode is? Why do you call it is a HV spec diode? Clamping voltage can be as low as a few volts.

[Lomax]

Why do you call it is a HV spec diode?

Because only two diodes that match the packaging and marking have been found; one is a PIN diode (which I believe are mainly used in VHF switching, like MHz range), and  the other is a 100-200 V Zener, which at least in a device powered by two AA batteries might be considered "high voltage" - especially when hooked to the output of a 3.3/5 V DC-DC converter... Yes, I think I understand the purpose of a transient suppression diode; when voltages above the breakdown voltage are present the diode, erm, breaks down, and dumps it to ground, right?

I'm really just trying to establish what the specifications are for the diode with a red/pink/violet band - knowing its function would obviously be a big help here. If it is indeed for transient protection, why use a 100 V part? I would have thought pretty much every single component in there would die at voltages far below that.  I suppose it could be that it's there to protect from transients which may come via the DC input connection; it does have a jack for an external DC power supply (7.5 V), though this is all the way down at the bottom of the pistol grip, so I haven't traced it.

[Lomax]

Good luck.

Thank you! What I'll do, once I get the components I've ordered, is to replace the obviously burnt inductor, as well as the MAX856, and the output Schottky diode - these parts have been identified so the worst it will do is replace working with working (and waste £2). A bit of a gamble, but easy enough to do - in fact it will probably take more time to remove/reinstall the PCB. 

[wraper]

'm really just trying to establish what the specifications are for the diode with a red/pink/violet band

Unsolder and measure the clamping voltage. Just like with zener. Transil basically is a zener, just made so it can withstand high peak power. Do you really think you can find every color coded part by googling? And BTW according to the datasheets, every transil in the MELF package which is made by Vishay, seem have the same color band, regardless of voltage rating.

[jmsc_02]

Dear,

If you see the MAX856 datasheet, you will find an application circuit descrobed and seems like yours PCB.

https://datasheets.maximintegrated.com/en/ds/MAX856-MAX859.pdf

Regards

Edit -> Just I saw that you've searched it.

[Lomax]

Thanks again to everyone - I have now replaced the inductor and the MAX856; it turned out the replacement Schottky diode was too large (the original one is a Mini MELF, apparenty), but the old one measured ok so I put it back. Not too shabby for a first attempt at SMD repair - but sadly it didn't fix the problem Ah well, that inductor definitely needed replacing anyway, and the replacement doesn't seem to get hot during usage. I'm now looking at the LCD display assembly, specifically three transistors on its PCB , which seem to operate as switches for the backlight (third photo). When peeling back the flat cable I found two ants, who had at some point become stuck in the adhesive - maybe they are the culprits