Any electronic component experts? SMD identification.

[ProDave]

Trying to do a repair and stuck with identification of a surface mount thransistor.

 

What I know:

 

It's a SOT23-3 package, and it's a PNP bipolar transistor (has b-c short but identified that from still working b-e junction)

 

The writing on it says A7 in large letter then in smaller text either s or 5

And then perpendicular to that in smaller letters it says 64

 

The best I have come up with with a lot of searching is this ebay listing https://www.ebay.co.uk/itm/234621418101?hash=item36a086ba75:g:Cr4AAOSwpG9htJOf

Select the 4th down on the part number box.  That tells me it's a BAV99 but googling that tells me it's 2 series switching diodes.  Now it may be that, if so one of the diodes is short circuit.

 

So before I order some random component that may or may not be what I need, anyone got any idea how to identify from the limited information what I have?

 

[ProDave]

I have managed to get a photograph of the device I am trying to identify

 

 

[Radian]

Without looking it up, think it's a dual silicon diode. BAV99

That's how sad I am.

[MJNewton]

1 hour ago, ProDave said:

The writing on it says A7 in large letter then in smaller text either s or 5. And then perpendicular to that in smaller letters it says 64.

 

It'll be an S.

 

A7 = Type = BAV99

S= Manufacturer = Infineon

64 = Date Code (YM) = 2006 April

 

Here's the datasheet.

 

Edited December 14, 2022 by MJNewton

[RobLe]

😬
 

Have you desoldered it?  If not, are you sure it’s bust?  A bav99 series diode might be placed with one part used, the other shorted out.  That’s because the part might be used elsewhere, and it’s cheaper to reuse than introduce another diode type.  Can you label the diagram (1,2,3, and which is shorted or a diode).

Desoldering them is tricky without busting them mind.  Why is it bust (diodes don’t usually blow by themselves…)?

[ProDave]

20 minutes ago, MJNewton said:

 

It'll be an S.

 

A7 = Type = BAV99

S= Manufacturer = Infineon

64 = Date Code (YM) = 2006 April

 

Here's the datasheet.

 

Thanks. you can rely on buildhub to solve a problem.

[ProDave]

20 minutes ago, RobLe said:

😬
 

Have you desoldered it?  If not, are you sure it’s bust?  A bav99 series diode might be placed with one part used, the other shorted out.  That’s because the part might be used elsewhere, and it’s cheaper to reuse than introduce another diode type.  Can you label the diagram (1,2,3, and which is shorted or a diode).

Desoldering them is tricky without busting them mind.  Why is it bust (diodes don’t usually blow by themselves…)?

Not de soldered it, but referring to the data sheet linked above, pins 1 and 3 are shorted,  so that is clearly one diode short.

 

The photo is not the failed part, i could not get the lighting right to photograph it, it is a photo of an identical part in a different part of the board that was easier to get to.

 

the failed one has a corner blown off, which is what alerted me to it on a visual inspection.

[Radian]

I've used these on just about every product I can think of. Quite often used as a clamp to keep a signal input voltage within the supply rails. In this configuration pin 1 is connected to 0V, pin 2 to +V supply and pin 3 is the signal being clamped between the rails. Having aid that there would usually be a current limiting resistor in series with pin 3 to avoid blowing the diode! Of course if you shove enough voltage up it you'll still blow it up (and the resistor).

[ProDave]

If you want some context, I am trying to rebuild the power supply part of the controller in the industrial sized GSHP at our village hall.

 

 

The tally of failed components found so far, 5 failed capacitors, 2 failed tab power transistors and this diode.  the failed diode is just below the left hand blue capacitor in this photo

 

The scorched pcb area just below the transistor with a heatsink (one of the failed ones) is not as bad as it looks.  Those 2 smd resistors are all in fact okay, the scorching is the pcb track underneath them did the job of a fuse when the PSU failed.

 

I don't rate the chance of it just working when I fit the new components as very high, but I said I would give it a go.  If I can't fix it, it;s £1400 for a new controller. No pressure there then.

 

And of course I can't find any service information.

[Radian]

What's the voltage rating on the two big capacaitors?

[ProDave]

2 minutes ago, Radian said:

What's the voltage rating on the two big capacaitors?

Both 1000uF 50V.  There are two 470uF 50V on their side under that sub panel, and a 220uF 25V in an even harder to reach place.

 

This unit is fed with 24V ac from a transformer and derives it's internal dc voltages from that via this blown PSU section.

[Radian]

So at least it's nothing fancy like an inverter power drive. Although you'd think it was something exotic from the exorbitant replacement cost.

Can you identify what voltage rails are used? The 50V caps probably filter the 24VAC after rectification giving 33V. The inductors don't look like SMPS buck components. Possibly line filters for the AC in. Looks like there may be a solitary SM inductor hiding behind one though. I'd expect a SMPS regulator IC around there somehwere.

 

You probably have as much chance as me in getting this going  again but I think I'd try identifying the supply rails in use and do two things with that info:

1. Cut the track for each voltage as close to it's origin as possible
2. Use current limited bench psu's to provide those voltages to the rest of the PCB (and slowly bring up the current to see if sensible)
3. If all seems OK, try and repair the on-board PSU

To do the repair probably means identifying and replacing all the blown parts in that area - which would be aided by tracing out the schematic which will probably look similar to any app notes for the SMPS IC if you can identify it. Then just power it up with the rest of the PCB still disconnected - and use current limiting on the input. This could be from a DC bench supply as the 24VAC probably gets rectified almost immediately.

 

Of course it might not be that simple if its a multilayer PCB with internal power planes. Still, it looks like a pretty basic 2-sided board to me.

[ProDave]

This board is not exactly cutting edge stuff, there is a lot of 74HCT logic so we can assume the main output voltage will be 5V There may be other outputs I have not tried tracing the circuit yet.  As far as I can tell only a 2 layer board so that should be possible no hidden tracks or blind vias.

 

If I get no luck with fixing the on board PSU then powering it from an off board PSU is plan B

 

The unit is made by Carel.  If you buy one direct it will not be programmed.  It is Dimplex that will sell you one pre programmed for the heat pump.  But the parameters won't be set up.  If we can get this one working we are hoping it's program and parameters are intact, but of course a failing PSU could have damaged other stuff on the board.  The shopping list for parts so far is only about £10 so worth a try.

[Radian]

My guess would be +5V and +12V for relay drivers. But what's that daughter board with the PCB edge connectors and shiny bracket on top for?

[ProDave]

I am pretty sure the 24V is used for all the I/O there are certainly 24V relays in the box it connects to.

 

The daughter board is probably the microcontroller, mostly enclosed in a metal can, and what you can see is a lot of chips with an awful lot of pins.  I might have to uncouple that board, it would make replacing some of the capacitors easier, but it is soldered to the main board with a "link" in one corner.  I am nervous of unsoldering that, I think it may have been put there specifically to stop you unplugging it.  My fear would be the CR2032 battery is on the main board, and uncoupling that could lose it's backup and you might promptly clear it's memory.  Not worth the risk without knowing for sure.

[Radian]

4 hours ago, ProDave said:

5 failed capacitors

Possibly the root cause of SMPS failure if they were in that area. I wouldn't risk losing battery power - although makes you wonder if this is another one of those 'deliberately fails out of warranty' scams. The high cost of a replacement board is suspicious enough.

[ProDave]

Making progress on this.

 

Initially the rebuilt PSU did not power up, and the TIP32 got very hot.  So  left it for a while.

 

I had a another look today and found another short circuit SMD diode, a power device this time.  Rather than look it up and order one, I surface mounted an ordinary axial diode.

 

Now we are at least partly in business.  The PSU powers up, it appears to have 2 outputs, one that stabilises at 5.3V and the other at 21V.  That would tie with a logic supply and the 21V is for driving the dozen or so 24V coil relays.

 

On power on the backlight to the little LCD display come on, the lights behind the pushbuttons flash once on power up, as do 3 LED's on the main board, but no text on the display and no apparent reaction to pressing any of the buttons.

 

I guess the next thing is go and try it back in it's box and see if there are more signs of life with everything connected.