[SOLVED]

I am still learning a lot with circuits and I have come along way in the past year. But this has got me stumped. The part circled is what I am trying to figure out what it is. Is it a transistor? Looking at the schematic it doesn't seem so.

The diodes are on the back side and I doubled checked the polarity to make sure the schematic is correct. These are glass but not zener diodes. They have no markings but I removed both and tested both up to 31V 10mA and neither one broken down and conducted current. They do have a forward voltage drop of 0.6V.

The micro puts out +5V and current flows through the unknown part. The voltage reads the same on both sides of the unknown part when LED is on and I see no where else 5V can come from but the micro. The only purpose I can see for the unknown part is some kind of protection for the micro. I don't understand the what the diodes are used for and figured they must have something to do with the unknown part.

Any clues or help would be appreciated,

Rob

 

Circled? Which one?

 

hi,
I would say that is a general purpose NPN surface mounted transistor, shown circled on the PCB image.
E

 

They look like filters.

 

Circled? Which one?

First photo has red circled part. Schematic is is the part top left connected to micro.

 

hi,
I would say that is a general purpose NPN surface mounted transistor, shown circled on the PCB image.
E

That is what I originally thought, but it doesn't seem to fit the schematic. A PNP would be a better fit but still wouldn't make sense here.

 

The look like filters.

I will try and look filters up on Digikey and such. Any idea what type of filter?

 

looks like jumpers to me, placed on a grid to choose what signal goes where.

 

looks like jumpers to me, placed on a grid to choose what signal goes where.

They could have just left off the resistor. I have another board with all like spots populated the same. If a jumper then why the center tap to ground?

 

A filter seems to be the most probable. Looks very similar to this high frequency LC filter http://www.digikey.com/product-detail/en/NFL21SP106X1C3D/490-4016-2-ND/1016175 The filter circuit on that datasheet seems to support what I can test on the part.

I still wonder what the diodes are used for. This board does have 2 mosfets switching a single 12 volt pump motor to vary the flow. It does have a schottky rectifier for flyback diodes. Maybe just for extra protection?

I will consider this solved. Thanks DickCappels for the clue.

 

hi.
On rechecking the rest of the PCB image.

The 0V line is common to all the SMD transistors
A +12V track is marked

Adjacent to edge of the PCB is Marked RED , [next to it is GRN] which suggests to me an LED.
On one pin of the transistor is 1 K resistor which is linked adjacent to the RED marking.

I would say that the ringed in RED is a possibly a MOSFET transistor LED driver via the 1K, this would power an LED with approx 10mA.

The Gateof the transistor is connected directly to a pin on what appears to be a MCU device.

Why do you think the RED ringed device is a filter.?

 

The schematic is correct. I worked at it for a long time to make sure it is.

On the part testing resistance on micro side and resistor side I get nearly 0 ohms. Testing between either side and the middle I get an open circuit. Using the diode testing function on my meter I do not see any PN junctions and the part is in good working order. This matches the circuit diagram on the Digikey linked parts datasheet the best so far.

The 12V trace is for external equipment via the missing screw terminals. The RED and GREEN are LED's as shown in the schematic I drew. The horn is a piezo electric buzzer. The other populated parts are connected to a type of pulse speed sensor on the pump motor.

 

hi,
What is the Micro type and the pin number that you show on your diagram as 'micro +5v'

 

Parallax SX28AC/SS-G
http://media.digikey.com/pdf/Data Sheets/Parallax PDFs/SX20AC,SX28AC.pdf

Pin 9 RB0 Bidirectional I/O Pin; comparator output; MIWU/Interrupt input

Flexible I/O
• All port pins individually programmable as I/O
• Inputs are TTL or CMOS level selectable
• All pins have selectable internal pull-ups
• Selectable Schmitt Trigger inputs on Ports B and C
• All output pins capable of sourcing/sinking 30 mA
• Port A outputs have symmetrical drive
• Analog comparator support on Port B (RB0 OUT, RB1 IN-, RB2 IN+)
• Selectable I/O operation synchronous to the oscillator clock

 

The diodes are almost certainly for protection against electrostatic discharge.

The filters are probably to keep radio frequency noise on the I/O pins from leaving the printed circuit assembly.