Hi,


I am troubleshooting board. I found out there is a short between Vdd and Gnd input side of MCP1827. When I was measuring the resistance between Vin and Gnd, it was showing 249 ohm resistance. I thought it was the component failure. Then I replaced the MCP182 but still I find short between Vdd and Gnd. Even I replaced the 4.7 uF capacitor that is connected input side of Vin. When I measured resistance between Vin and Gnd, it was showing 120 ohm resistance. I dont understand why still there is a short between in Vdd and Gnd.

 

When a multi-meter measures resistance, it basically uses an internal battery to form a circuit with the resistor and measures the current. In your case, it works as if the device is powered on. The resistances measured are not unreasonable. In addition, there may be active components like diode and transistor on the input side within the device. Try measuring the resistance in both directions (probing black/red and red/black), and you may get different resistance readings as well.

 

When a multi-meter measures resistance, it basically uses an internal battery to form a circuit with the resistor and measures the current. In your case, it works as if the device is powered on. The resistances measured are not unreasonable. In addition, there may be active components like diode and transistor on the input side within the device. Try measuring the resistance in both directions (probing black/red and red/black), and you may get different resistance readings as well.

So there is no short even though multimeter reading lower resistance between input and ground.

 

So there is no short even though multimeter reading lower resistance between input and ground.

No. The MM resistance reading assumes a passive circuit to be correct. The MCP device is an active component where the actual resistance to the power source varies in non-ohmic ways with changes of the input and output voltage and load parameters.

 

When I measured resistance between Vin and Gnd, it was showing 120 ohm

So there is no short even though multimeter reading lower resistance between input and ground.

I wouldn't call 120Ω a short. Less than 1Ω - yeah, I'd call that a short in MOST cases. However, there are cases where 1 ohm may be used as part of the design. In the case of a regulator - 1Ω would be bad.

 

So there is no short even though multimeter reading lower resistance between input and ground.

Not a short but does the circuit work?
Was that measured with Vdd connected to the input voltage source?

 

Missing info.
What is value of VDD (part has a max input of 6V)?
Is it under load or open?
What is the construction/build look like (i.e. picture)?
Remember the "tab" is GND, are you allowing it touch VIN or VOUT.

 

First of all, we do not see what is connected beyond that arrow "VDD", and second, ALL electrolytic capacitors will register as a shorted circuit until they charge up and stop drawing current from the meter.
Next, completely aside from that, checking resistance of a power supply line is seldom a useful trouble shooting action. It is much more effective to have the power present and check what the connected circuit elements are doing.