How to check out resistance from diode in series with resistor

[bittware]

Hello experts,
My device under measurement is a diode in series with a resistor. I want to figure out the linear portion of the circuit, i.e. the resistance. The junction between the diode and the resistor can not be approached. But the test current or voltage can be applied to the two terminals of this series circuit. The diode type is uncertain.
Do you have any good idea to check out the resistance from this circuit?
Thanks.

 

[pjmelect]

Put two different currents through the diode and resistor and measure the volt drop across the diode and resistor at each current and use Ohms law to calculate the resistance value. This will also measure the diodes resistance, but there is no getting around this.You should of course chose a reasonable current where the diode will be linear.

 

[Analog Ground]

I can think of two ways. First, measure the IV cure and do a curve fit to a model of the diode in series with a resistance. Solve for the resistance. The IV behavior for a silicon diode is very predictable and the equation readily available. Second, I suggest a measurement approach. Measure the IV curve and find a spot far enough up the curve where the response is linear. The slope of this linear area is the resistance. Here is a graph of the idea. Curve A is only a diode. Curve B is a diode with a series resistance. A lot has to do with how big is the resistor. If small, it may be difficult to tell it from the response of the diode.

 

[bittware]

Put two different currents through the diode and resistor and measure the volt drop across the diode and resistor at each current and use Ohms law to calculate the resistance value. This will also measure the diodes resistance, but there is no getting around this.You should of course chose a reasonable current where the diode will be linear.

Sorry, I did not get it. You can only measure the diode in series with resistor as a whole. How to extract the voltage drop across the resistor from the whole voltage drop?

- - - Updated - - -

I can think of two ways. First, measure the IV cure and do a curve fit to a model of the diode in series with a resistance. Solve for the resistance. The IV behavior for a silicon diode is very predictable and the equation readily available. Second, I suggest a measurement approach. Measure the IV curve and find a spot far enough up the curve where the response is linear. The slope of this linear area is the resistance. Here is a graph of the idea. Curve A is only a diode. Curve B is a diode with a series resistance. A lot has to do with how big is the resistor. If small, it may be difficult to tell it from the response of the diode.

Thanks for the idea.
Second approach sounds good. However, the resistance only shows up at "a spot far enough up the curve". My application prevents me from doing so as the current may be too high. I need measure that takes place at tiny level, say less than 1mA. Any further thought?

 

[Analog Ground]

OK. One more long shot. At zero bias, the diode looks like a very small capacitor in parallel with a large resistance.. There may be some type of measurement at zero bias and a small high frequency signal and a "vector impedance meter" of some sort which can give you the series resistive component. I have no idea how this would be done. Theoretically it seems possible but I don't know how to do the measurement.

 

[Tunelabguy]

However, the resistance only shows up at "a spot far enough up the curve". My application prevents me from doing so as the current may be too high. I need measure that takes place at tiny level, say less than 1mA. Any further thought?

The first methods described by Analog Ground will still work, even at 1mA. You will just get less accuracy. But no measurement is perfectly accurate. So you have to specify how much accuracy you need. In general, the accuracy of the Analog Ground method depends on the applied voltage. The more that voltage exceeds the nominal 0.7 v. diode drop, the better the accuracy. Whether that can be achieved with only 1 mA depends on the range of resistors that could be in series with the diode. If the resistor is around 20K, for example, then 1mA of current will generate 20v across the resistor, which is much more than the 0.7v across the diode. Therefore we can say we have effectively taken the diode out of the consideration, to within about 5%. So you could measure the voltage at 1 mA and the voltage at 0.5 mA, divide the difference between these two voltages by 0.5 mA and get a fairly good approximation to the unknown resistor. On the other hand, if the resistor is in the range of 10 Ohms, then 1 mA will only produce about 10 mV, which is swamped by the 700 mV across the diode. So it will be very difficult to make any kind of estimate of the resistor if the resistor were in that range.

 

[Analog Ground]

The first methods described by Analog Ground will still work, even at 1mA. You will just get less accuracy. But no measurement is perfectly accurate. So you have to specify how much accuracy you need. In general, the accuracy of the Analog Ground method depends on the applied voltage. The more that voltage exceeds the nominal 0.7 v. diode drop, the better the accuracy. Whether that can be achieved with only 1 mA depends on the range of resistors that could be in series with the diode. If the resistor is around 20K, for example, then 1mA of current will generate 20v across the resistor, which is much more than the 0.7v across the diode. Therefore we can say we have effectively taken the diode out of the consideration, to within about 5%. So you could measure the voltage at 1 mA and the voltage at 0.5 mA, divide the difference between these two voltages by 0.5 mA and get a fairly good approximation to the unknown resistor. On the other hand, if the resistor is in the range of 10 Ohms, then 1 mA will only produce about 10 mV, which is swamped by the 700 mV across the diode. So it will be very difficult to make any kind of estimate of the resistor if the resistor were in that range.

This is right on and expands on my original post where I said "A lot has to do with how big is the resistor. If small, it may be difficult to tell it from the response of the diode.". If the application is to measure the intrinsic series resistance of the diode (small R), this is a very complicated topic. I suggest doing a net search on "measuring diode series resistance". Many methods are found. Some use very sophisticated curve fitting, some measurements at two currents, etc. Some are complex research papers, some are industry app notes. In any case, more information is needed about the application (big R vs small R?). We may have gone about as far as possible with a forum discussion. Interesting topic!