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I am a beginner in electronics design, currently i am stuck in a simple project, hopefully i can get some advise from you how to solve it.

I am current doing a simple project on short circuit checker. I have to check on some pins of a chip on a PCB. I need to touch on the pins at once and check out exactly which pins are shorted, for example there are 10 pins, i need to know whether pin1 and pin2, pin2 and pin3, pin3 and pin4 and so on are shorted or not ..

I am only allow to use 5VDC power supply. At first i was planning to use LEDs to design a always "ON" LEDs simple circuit, when the pins are shorted, the LED will go off, but i failed to do so.

there are internal connection among the pins, some have 200k.ohms, and some have even M.ohms

Hopefully i can get your reply ..

Thanks in advance

Regards,
Desmond

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So do I understand that you have 26 connections to poll at the same time and you are looking for one of the connections to be shorted? It would help to see the circuit, but here are some ideas:
26 of the same circuit in parallel
Add some rotary switches
Use 4066 cmos switches

Do you have a way to post a drawing of the circuit? I have noticed more people tend to get involved when there is a drawing or schematic attached. you know the old saying, a picture is worth....

MP

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hereby i attached my my circuit diagram to you, which is a failed one, because when the pin1 and 2 shorted together, the LED between pin2 and 3 will OFF too .. due to the short circuit.

besides, there are some external circuit on that PCB, among the pins, there are some resistance range from 4k to 10M ohms.

i am really in urgent this project, hopefully can get your advise

thanks.

circuit_diagram.zip

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yes, that thing is just sit at the top of the chip on the board, i dont really know how the chip internal circuit works, that's why i can only measure the resistance in between each of the chip.

when i switch it on, the LEDs supposed to be light on, whenever the pins are short, the related LED in between the pin will OFF.

but here comes a problem, as you seen in the chip circuit diagram, the pin 26(P2) and 27(U) are connected into the chip, when i switch on, the LED sits between this two pins couldnt on, just seems like a short circuit scenario. the resistance of P2 and U is very big till 500k when no voltage supplied. i suspect the resistance will reduce to a very low measure when i supply the 5 volt into it.

if in this case, how can i check the short circuit in between the P2 and U?

thanks again ..

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desmond, it seems that you are certainly going to need to get the data sheet of the chip to see what is in there. Most data sheets will give some kind of an internal diagram. Once you have this, you will be able to find a solution. As you have stated, there are resistances and shorts internal to the chip that is causing you the problems.

MP

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He said shorted together. After looking at the diagram he has posted it should be clear that he is using the term "short" to mean connected together. What is not clear is what is connected to what in the IC. When you have a schematic with an IC that performs as some type of switching mechanism, it is good to add the internal working devices in the schematic so that it is more clear to see the function. Many manufacturers like to keep their chips as a mysterious black box so that it is not so easy to duplicate the product. Hopefully, desmond does not have this problem.

MP

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MP, im aware of what is meant by pins being shorted together, what im getting at is that internally IC pins generally do not short together, a short occurs from input to power supply ground or Vcc, depending on polarity.

If by "shorted together" the OP is referring to bad soldering between pins then the LED scheme would suffice. Obviously when an LED is connected across two pins ( parallel) that have bin soldered together and power is supplied to the LED it will not light up.

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harlow MP and nettron, thanks so much for your reply.

I have found the datasheet of the chip and i have attached it here.

To be precise, the problem occurs this way. The LED which i used needs 2 volt forward voltage but i have tried 1.8 +/- is enough for it to light up.

If i connect the LED parallel with for example pin1 and pin2 .. if pin2 and pin3 (this pin is not connected to anything) shorted (soldering short), the voltage between pin1 and pin2 will drop to 0.6 volt or some others are 1.1 volt which is not enough for the LED to light up.

So is there any LED or component which can used as an indicator can be function with that little amount of voltage? or maybe how should i design the circuit for that?

Thanks.

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Nettron, I refer you to the data sheet of a 4066 cmos switch. Internally, pins are commonly shorted together by some type of reaction to the other pins. For example, without the data sheet on this chip, one might connect a test circuit which adds voltage to the control pin and causes two other pins to "short" together as "close".
Knowing that such an example is possible makes one cautious to look at the internal workings of a chip when making such a test device. When grounding various pins or adding an excitation voltage to various pins on a chip, there are cases when other pins are affected and can become "shorted" together.

MP

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MP, your missing the point, it doesnt matter what type of IC is being tested....but anyway, to overcome the problem with the 4066 or any other type of analog switch, you could use a sequencing scheme to multiplex the LEDs one by one. If two pins are "shorted" two LEDs will come on (light up).

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harlow, nettron, even if i tested each pair of pins each time .. it still have a little problem, that is if i am testing pin 6 and pin 7 (pin6 connected to 5volt, pin 7 connected to ground), when pin 7 shorted with pin 8 (this pin is not connected to anything), the LED between pin 6 and pin 7 wouldnt light up, because the voltage across is only 0.6volt.

if in this case, what can i do?

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Desmond, i see what you mean but im assumimg you will not be using the tester on an IC that has power connected to it ?
To better explain what i mean with the sequencing scheme would require a schematic. Not sure what kind of results it will give you i havent breadboarded it yet but looks feasable. I can draw up and post a schematic a day or so if you dont mind the wait.

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thanks, sure i dont mind to wait for it.

one thing that i dont understand is .. how can i test the pins without connecting any power to it? even if testing using multimeter, there is also certain voltage is it?

maybe i still cannot get what you mean, so i will wait for your schematic .. really thanks a lot .. :)

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hie, i have tested those LED which cannot light up cause by other pins shorted are range from 0.6 volt to 1.4 volt ..

0.6volt to 1.6volt is not enough to light up the LED which typical Forward voltage is around 2volt.

Do i have any method to indicate those pins are actually not shorted due to the voltage is available but just not enough to light up LED?

it has been a headache since many this pins will affect that and that will affect this .. all drops below the forward voltage of a typical LED.

Thanks a lot.

Desmond

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