When measuring output transformer secondary resistances, I have simply used an LM317 configured as a current source with a known current. DMMs typically do fine with measuring small voltages, so you don't really need much current to get a good reading.
A pseudo-current source can be easily constructed using a resistance in series with the resistance under measurement. The additional series resistance has to be bigger by a factor of at least 10. The additional series resistance is connected to a known voltage and the current measured. The unknown resistance is then connected in series and the voltage accross noted. The value of the unknown resistance is then found using Ohm's Law.
Note:
The basis assumption is the test resistance will not appreciably affect the previously measured current.
This setup avoids using a current source, a Wheatstone Bridge or its op-amp equivalent.
Note:
The basis assumption is the test resistance will not appreciably affect the previously measured current.
This setup avoids using a current source, a Wheatstone Bridge or its op-amp equivalent.
With the advent of cheapo small DMMs, I nearly always end up having two of those meters on the bench and connected in some way. Two such meters will give you a resistance measurement if you have a calculator handy as well (V/I).
I have two AN8009 that have survived 2 years of use so far. They use a 10mohm shunt with their 1 and 10A FS autorange. Set the meters up before energising, and turn off before disconnecting. An in-situ measurement may provide better confidence that the dropper resistor is measuring as it is marked, and a calculator can confirm the power dissipation is acceptable (V.I).
Or set them up to measure the heater winding voltage, and the valve terminal voltage, to confirm your heater supply and wiring are all good, and without having to probe around in live circuitry
I have two AN8009 that have survived 2 years of use so far. They use a 10mohm shunt with their 1 and 10A FS autorange. Set the meters up before energising, and turn off before disconnecting. An in-situ measurement may provide better confidence that the dropper resistor is measuring as it is marked, and a calculator can confirm the power dissipation is acceptable (V.I).
Or set them up to measure the heater winding voltage, and the valve terminal voltage, to confirm your heater supply and wiring are all good, and without having to probe around in live circuitry
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So, it looks like we're all 'agreed'. Use commonly available el-cheapo-or-better digital multimeter. If the meter coöperates, then a 10 Ω resistor in series with the DUT (device under test) along with a 1.5 volt battery sets up a pseudo-current source.
So if
The problem I was originally attempting to solve thru a 10× or 100× op-amp DC 'precision' amplifier was to cover for multimeters that have a hard time measuring low-millivolt voltages. Gotten some flack from EdBarx for it, but still … it isn't a hard option. The wire-lengths and milliammeter nulls … is much harder for the average bench jockey.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
So if
B = battery voltage (measured on DMM)
V = midpoint voltage
R = 'upper' series resistor
thenV = midpoint voltage
R = 'upper' series resistor
DUT = RV / (B-V) in Ω
as in my 0.146 Ω resistor and a 1.44 measured battery (which I just did):B = 1.44 V;
V = 0.021 V (21 mV on 300 mV scale);
R = 9.81 Ω, measured;
DUT = 9.81 × 0.021 / ( 1.44 - 0.021 )
DUT = 0.145 Ω
Which is close enough. Lucky for me, my other DMM has a 300 millivolt scale. This wasn't the $9.99 Radio Shack el-cheapo, but my $29.99 Chinese Fluke knock-off model. Its been good to me. V = 0.021 V (21 mV on 300 mV scale);
R = 9.81 Ω, measured;
DUT = 9.81 × 0.021 / ( 1.44 - 0.021 )
DUT = 0.145 Ω
The problem I was originally attempting to solve thru a 10× or 100× op-amp DC 'precision' amplifier was to cover for multimeters that have a hard time measuring low-millivolt voltages. Gotten some flack from EdBarx for it, but still … it isn't a hard option. The wire-lengths and milliammeter nulls … is much harder for the average bench jockey.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
That solution is OK if measuring transformer wdg resistance is all we need to do. But useless for the forward characteristics of a semi device, for example LEDs, Si rectifiers, LV Zeners & on & on. Not everyone needs to do that but a better solution would be a purpose built CC source of at least 10V. And 100 mA. These daze much easier to do than in 1969.🙂
LM317 now does well & opens up the possibility to check forward characteristics of odd devices like tungsten & carbon filament light bulbs. And toob heaters & filaments.😀.
I appreciate that I started this discussion, but you have all completely lost me. This sounds more complicated than the amp that I'm working with.
jhstewart₉;6128351 said:
I can see that, actually. 100 mA is a bit over what a 9 V battery itself likes to source. I suppose one could pack a few 4-cell AA battery plastic el-cheapo holders in a project box. 3 of them. The only reason I like cells is that they're ready to rock when you pick up the box (after not too many years). On the other hand, 250 mA 12 volt wall-warts are next-to-free these days. I'd probably cut off the connector, and permanently attach the thing to the project box. Tune up the 100 mA (or maybe have a ON-OFF-ON switch to deliver 3 different well trimmed amperages … 1, 10, 100 mA. Use a good-sized capacitor to make it reliably filtered DC even at 200 mA. Gets up to 10 V pretty easily even with the 317 drop-out.
Still … for cobbling something together NOW, which takes but a resistor, a 1.5 V cell, and a breadboard … to test a known-low-Ω resistor, the previous bit certainly embodies the 60 second solution. Which also has its place, I've come to appreciate.
When I was younger, I was perpetually making little purpose-built boxes and such to do special testing. Maybe I had hopes of matriculating to a metrologist-in-practice; never went that way tho'. Of late, I just quick-breadboard ideas, try them out, and then pull em right apart again, and stuff the little bits back in my tidy bunch of repurposed 1960s screw-and-nut trays.
I guess its like why I love doing the math every time instead of looking up some darn formula (that may well be wrong). But I'm old. What can I say.
⋅-=≡ GoatGuy ✓ ≡=-⋅
You're welcome.
In a nutshell … there are 4 practical ways to measure such low resistances. One, obviously, is to put a bunch of them in series, and measure them all. Then divide by the count. This is what you did.
The second, which I talked up, was to make a DC exact-multiplier amplifier. Just take the little tiny voltage drop and multiply it by 10 or 25 or 100 times, to get it into the range of your meter. This also works. I breadboarded it.
The third, championed above, is to make a 'constant current source', which when fed thru an unknown resistor follows E = I R …Ohm's Law. Measure (if your meter is up to it) the millivolts, and you can quickly convert that into Ω.
The fourth, summarized well by EdBarx … was to implement a really really cool ancient circuit, called a Whetstone Bridge. It uses 3 resistors in addition to your unknown resistor. One is 'variable', usually a 10-turn instrumentation trim-pot. With a sensitive amp-meter, you can 'see' whether there is current going into the 'unknown resistor leg' … or away from it. By fiddling with the knob, you can turn that into a ZERO. Once its at zero, the power is turned off, and the measure of the 10-turn pot is made. It, in proportion to 'its leg's other resistor', is the same ratio as the unknown resistor to its 'other' resistor.
OF course, there is the 'fifth' method, which is to purchase a nice $29 to $79 meter with 4½ digits … and a 10 Ω full scale. It can easily measure mΩ.
⋅-=≡ GoatGuy ✓ ≡=-⋅
Today I went to measure everything, and in the process I managed to smoke one of the .27ohm resistors. I think what happened is that I put it on the variac and in the process the probes touched when I used alligator wires. No other harm seems to have been done. I'll try again tomorrow since I have extra resistors.
Need some kind of current limiting. But using a Variac on raw AC is not a great idea. Better run thru something like a heater transformer (5V or 6.3V) from your Variac, driving your 0.27R resister thru something like 50R/10W to limit the current.
Then measure E & I with your DMM at the resistor in test, apply Ohms law.🙂
The birds will sing & the sun will shine, Success!!
Then measure E & I with your DMM at the resistor in test, apply Ohms law.🙂
The birds will sing & the sun will shine, Success!!