0

I have searched for weeks on the internet but it seems SMD devices have no fixed rules- anyone can name any chip any number/letter! No result searching this forum either.

I am trying to identify a few SMD components to draw a schematic and repair a circuit that drives a VFD (vacuum Fluorescent Display).

32P is an SOT-23 (I think) 3-pin device. I assumed it was a transistor but now I wonder.

s67 is a 3-pin plus tab device that I believe is a transistor (or Mosfet?). I think it looks like a SOT-223 but the tab doesn't seem to bend down; it looks like it is flat, so maybe SOT-89.

E5L, H4K and 1 9RL are 2 contact devices (look like SOD-123 and I think zener diodes).

The E5L has 6.6V at the cathode and it never varies, so it does seem it's a zener (has a resistor from positive rail to the cathode then the cathode leads to two pins of a VFD (Vacuum fluorescent display). The H4K is arranged the same way and has 10.3V at most, even when I increase the input voltage to the circuit, so again seem to be a zener.

The 1 9RL (it looks like a space between 1 and 9, but maybe that's not significant) seems to be a zener too and arranged the same way. When the circuit powers on there is no voltage on the cathode, so the cpu it connects to must be pulling it down. I removed it from the PCB and tested it on a PSU with a limiting resistor. It topped out at 5.6V so seems to be a 5.6V zener?

So far the rough schematic I drew doesn't make full sense and I haven't spotted any problem that would solve the fault. Can anyone help? It's driving me mad.

Cluster VFD supply Schematic

photo of PCB

Note: In the dark, the VFD has a dim pink glow on the front cathode wires while turned on. As the display turns off, some of the characters blink in normal green brightness for a fraction of a second. It would seem the voltage biases fall through the correct levels for a split second.

Aug 23rd: Thanks Jens. I just saw your diagram. This makes sense now of what is on the PCB and what I was looking for. Yes, I was wondering if the 32P devices were forming a rectifier, but could not find any components on the internet named 32P. Clearly you have seen this type of circuit before :) and know how the transformer windings should be. So the components I was wondering about would seem to be Ok, from what you have say. I have redrawn my schematic to incorporate your insights. enter image description here

I suppose that answers my original question and so I cannot get any other help on this then (?).

Oh well, thank you very much for showing me what the schematic should be and how it works. I appreciate this. I will soldier on and try to think of what I can try next. Maybe there is a more suitable forum area for VFD problems. My next thought is why doesn't the 37V appear on at least one of the VFD pins. Or is it normal for the 37V to go to the driver ICS (as I have already checked) but maybe the driver ICS have failed? Also, I need to figure out why the 5.6V at the zener is held down to zero and if that is by design or fault. I have nothing else to go on except that the VFD did operate dimly when I got this instrument panel and then stopped showing anything.

Thank you again for moving me a stage further.

Chris
  • 11
  • 3
  • 3
    This question would be greatly enhanced by photos of the items in question and your schematic – Bryan Aug 21 '22 at 04:52
  • 1
    Related: https://electronics.stackexchange.com/questions/334128/how-do-i-identify-smd-components-or-how-do-i-identify-any-component/334129#334129 – Voltage Spike Aug 21 '22 at 06:05
  • Thanks for links- I did check most of those sites but can't locate these. The outline page is very useful though- I think my ID of the packages is correct now. I put my schematic into PDF so you can zoom in and keep detail, but this site doesn't seem to allow attaching it, so I had to paste images. This is how I believe it to be; all connections shown are correct but I'm unsure about those SMD components. There may be other connections that I haven't traced yet, like the one I found that goes to the cpu, but this is the essential part of the power supplies to the VFD, I believe. – Chris Aug 21 '22 at 18:30
  • I have an older PCB with very different arrangement that has four diodes in series and parallel pairs, seemingly where those 32P devices are. On that one, there is 40V at the top of the pairs and 20V in the middle of the pairs. So I wonder if those 32P devices are being used either as rectification or multiplier circuitry, with the transformer, to get the (37V on the new PCB). I cannot determine the windings of the transformer, unfortunately, and because both boards do not light up the VFD, I don't know if any of these voltages is what they're supposed to be, although some make sense. – Chris Aug 21 '22 at 18:38
  • Your circuit is working correct, all given voltages are typical. The display has another problem. – Jens Aug 22 '22 at 01:05
  • Thank you very much Jens. I just saw your extremely helpful diagram and explanation. See my original post where I have updated it and responded to your help :) – Chris Aug 24 '22 at 00:04

1 Answers1

1

The basic idea of your circuit is a sinus converter using this topology: Sinus Converter Fragemnt T2 and T3 are conducting for half of the period and during off time there is a sinus half wave visible at the collector.

C3 together with the transformer primary winding dominate the resonance frequency.

These SOT32 output components are dual diodes in serien configuration and form a bridge rectifier.

L1 allows the sinus form of the oscillation. Without it the circuit would be a push pull square wave forward converter.

R2 defines the allowed current in the transistors.

Jens
  • 5,598
  • 2
  • 7
  • 28