Hi,
i am designing Low Resistance Measurement,my problem here is i need power supply that provide current source 5A and very low voltage almost zero(if it possible).
the resistance i would like to measure is in uOhm,,
1)i was planning using ic regulator for current www.micrel.com/_PDF/mic69502.pdf ,, anyone have experienced using it?and wwhat is the problem?
2)i was planning using the dischargin of capacitor,so when caps is full then we have microseconds time at that discharging of the caps for measurement,ive tried a simple experiment with it and it could provide very low voltage and because of very low resistance in caps it could provide high current too?but my problem here is in measuring the voltage & ampere at that microseconds using microcontroller?have anyone tried the same thing?
3)what is the side effect if we have high drop out voltage?because my output would be at least 0,1volt?
4)is there any other/simpler way to get my specification current source?
It is possible to use MIC69502 as a current regulator as when you look at its internal diagram it consists of 3 blocks: a 0.5V reference, an Opamp (as error amplifier) and a PNP power transistor. The selection of the current sensing resistor will be similar to the LM3157 – see picture below – ref.=0.5V.
If you don’t have MIC69502 you can replicate it just by using those three blocks mentioned above …
Be aware though that all these options will require a supply voltage, which in the case of MIC6905 is around 5-6Vdc. Without it no sensible regulation can be achieved ..
I don’t think you can relay on a charge of a capacitor to precisely measure resistance. I’d rather add a dc switch to the circuit mentioned above, turn the circuit on, allow it to stabilise, take a measurement (read voltage at constant current) and turn the switch off ..
:wink:
IanP
Sure it's possible, but just keep in mind that the "efficiency" of the supply is going to be extremely low with extremely low resistance loads. It can be done both with linear regulators or SMPS (SMPS will be much less lossy than linear regulators, but more complicated).
The capacitor idea may work in theory for measuring resistance, but in reality it won't work for uOhm loads because the variation in the ESR and capacitance (with temperature and aging) will completely swamp out the effect of the actual load resistance.
Maybe you could employ a different measurement strategy? Maybe it would be possible to create some resonant L/R/C circuit with the measured resistance. When excited with ac signal the changing resistance would effect in changing complex impedance of the circuit. Maybe you could measure the resistance by locating resonant peak? I don't know if this is feasible (component-value-wise) but it's a different idea.
for the supply voltage 5-6 Vdc,do i need battery or another power supply for it?is there any effect of regulating regulated voltage?but if i use batteries then there'll be lot of batteries or charging the batteries since the high ampere produced,am i right??
In any case you do need a power source, a battery or a ps ..
If your circuit is "smart" and takes high-current measurement over short period of time, say, 1ms-5ms, then a battery should last for some time ..
:wink:
IanP
you said that on previous post about the replicate regulator that i could make it by myself,so how i pick the right transistor for what my requirement?
I'd consider p-channel MOSFET, eg.
www.irf.com/product-info/datasheets/data/irfh5053pbf.pdf
and increase the supply voltage to >=9V (Vgson) ..
:wink:
IanP
what is exactly ESR?i didnt find more information on that,can you explain it more?and what is the correlation with it
yes ive been doing research for this ESR and i didnt found ESR exmple that suitable with my specification,i need an accurate microOhm measurementThis may be worth looking into, to help with what you're doing.
ESR stands for Equivalent Series Resistance. It has to do with a capacitor's internal resistance when charging and discharging. ESR is not the same as leakage resistance.
Even a very small resistance in a cap (a tenth of an ohm) can reduce its performance.
There are ESR meters available, and there are credible messages of praise from technicians who claim the meter lets them quickly diagnosis bad caps that they would ordinarily have called good, and they can repair equipment that would previously defy their best efforts.
An ESR meter works by injecting a pulsed waveform under 300 mV amplitude. This is coming through a low ohm resistor (maybe 5 ohms).
Because the applied voltage is so low, it does not turn on any diodes in neighboring wires.
The probes are applied to the terminals of the cap. A good capacitor draws away the entire signal because it has very low ESR. An oscilloscope reads just about zero.
A capacitor which has any amount of ESR causes the oscilloscope to show a reading.
Example, 0.1 ohm ESR will cause a reading of 300mV times .1/5, equals 6 mV.
That's the concept for how to make a poor man's ESR meter. It is not far from being a low ohms meter as you plan to make.
To refer to the problem specification in the original post, are there any reasons to do a µs & high current measurement? This would be the case when measuring non-linear components, e.g. transistor or diode saturation voltages. For linear resistors, you get most likely a more accurate measurement with less circuit effort by performing a moderate time duration (10 ms up to s) measurement at a more convenient current current level.
Yes you told about microOhm before. But there's a trade-off between measurement duration and voltage measurement accuracy, due to different mechanisms: e.g. amplifier noise, current source settling time, circuit inductance.i was going to measure resistance in a contact,and the ordo its on microOhm
Yes you told about microOhm before. But there's a trade-off between measurement duration and voltage measurement accuracy, due to different mechanisms: e.g. amplifier noise, current source settling time, circuit inductance.
At the end of the day, it may be easier to achieve the intended resistance resolution with lower current and longer measurement time. Looking at the key parameters of industry standard µOhm meters can give insights.
As I already mentioned, a pulse measurement can't be avoided if a non-linear resistance is involved, e.g. with power semiconductors. As far as I'm aware of, this isn't the case with a contact resistance measurement.
When measuring very low resistance in the past, admittedly in the low mOhm rather than uOhm range, the technique I used was four wire differential measurement. The current was passed through a known very low value resistor before the part under test, the voltage across that resistor was then compared with the voltage across the measurement points. Screened wires were used to carry the voltages to a remote amplifier. Using Ohms law the difference in voltages could be used to calculate the unknown resistance regardless of the current as long as it was high enough to provide a reasonable measurable PD. It also eliminated cable resistance which was many times that of the devices I was measuring. Because the current isn't as important, switching it on and off is much easier.
Brian.
because i need to generate a constant current to make the calculation ohms law easy , or i have misconcept again?Perhaps I'm missing the point but why does the current have to be so well regulated?
I'm still not sure about the constant current. I agree it makes the math simple but you can still do the calculation the way I suggested and it works at ANY current, it doesn't even have to be constant. The only requirement is that is should be high enough to allow reasonable voltage to be measured. You are correct that the current at source is the same as at the measurement point, ignoring input currents to the measurement amplifiers.
Four wire measurement is essential, the voltage drop in the wiring would otherwise be magnitudes higher than that across the measurement point. There may be advantage in placing the amplifier(s) close to the measurement location to further reduce noise in the system.
Brian.
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