Analog meters output more current to measure resistance at low currents? Vacuum Tube

[chemelec]

Using a DUAL TRACE SCOPE, You would put One into channel-1 and the other into channal-2
This will allow you to see the Phases, Relative to each other.

 

[Billy Mayo]

No, i need to use the oscilloscopes ext. input and put an AC in phase

The circuit inverts AC to 180 degrees, you can't see the polarity flipped because the ext input needs a reference to see the AC at 180 degrees

 

[cowboybob]

No, i need to use the oscilloscopes ext. input and put an AC in phase

The circuit inverts AC to 180 degrees, you can't see the polarity flipped because the ext input needs a reference to see the AC at 180 degrees

I'm getting the impression that you think a positive going AC signal is "in phase" and that same AC signal going neagative is "out of phase".

Is that a correct interpretation of your use of the concept of "Phase"?

 

[Reloadron]

You want to measure phase using an oscilloscope? Check out this link. I believe it will make phase relationship very clear.

Ron

 

[Reloadron]

In other breaking news as to the current a Simpson 260 will typically deliver using the RX1 range? The Simpson is a 260 Series 6 XLP set on the RX1 range. The current out is measured in a loop using a Fluke 87 DMM.

With the 260 reading about 1.5 Ohms the loop current is 184.0 mA. Actually more than the 100 mA I expected to see.

Ron

 

[rumpfy]

The original post was about the output current from an analogue voltmeter compared to a digital model. The Simpson gets mentioned along with the AVO.
With an analogue ohmmeter, the device has an internal power supply generally of 3 volt (2 carbon zinc cells in series). The AVO model 8 has a single cell and is a 1.5 volt system.
Regardless of the actual design, the principle of the thing is that the battery supply sends a current out to the meter terminals through a series resistance, of a value, which is set by the specific ohms range that has been selected. This value can be IMMEDIATELY found by looking at the meter scale at the 50% of full scale value for volts or current. For the Simpson, according to a picture I found, this value is 12.5 ohms for the 'ohms x 1 range. For any other range, the resistor value is 12.5 times the range multiplier. So for the ohms range of 'Ohms x 100', the value of the series resistor inserted by the switching design is 12.5 x 100' or 1250 ohms.
For ALL ohms ranges, there is a VOLTMETER circuit which measures the voltage at the ohm-meter terminals. When the terminal probes are short ciruited (for setting the zero ohms), the current which flows in the leads is equal to the battery voltage divided by the series resistance which has been selected for that ohms range. In the case of the Simpson, the current flowing on the Ohms range is 240 milli amp.
If the ohms x 100 range is selected, the current flowing at the zero set step, is 2.4 milliamp, an so on.
For any range, when the resistor under test has a value equal to the internal resistance selected for that range, the current is ONE HALF of the current value at the zero ohms setup. This is why the mid scale ohms calibrated value is equal to the internal resistance for that range.
One thing the Simpson does not have is a reverse Voltage scale with the battery value at the meter zero current point and at the full scale current point, the voltmeter scale is zero. By having such a scale, it is possible to plot the forward characteristic of semiconductors. The ohm scale is selected to set the maximum test current which is calculable for each ohm range. So at the meter reading obtained, we can read the test voltage and approximate the test current. This can be done for several ohm ranges to get an idea of how the 'diode' behaves at various forward current values. It is also important to notice that the simpson, and many other ohmmeters an damage low current devices such as small LED's by testing them for continuity on an ohm range that cooks them. Many small T1 led's have a maximum current of about 25 or so milliamp.
With digital ohmmeters, there is no such fixed relationship with measuring currents and ohm ranges. I have a small cheap thing which outputs about 10 mA on the 200 ohm range but it is useless for diode checking because if the 'resistance' measured is out of the digital range it goes blank.
Also, I have had to test tunnel diodes, and again the analogue meter is wonderful. It just gives the required readings without all the delays and confusion that one gets with a DVM. testing of SCR's too is easy when you have two analogues meters; one on the gate to check the minimum trigger current, and also for the anode circuit to check the holding current. A digital meter is fairly useless for all this stuff; BUT, it has its place.
Hope this all helps.

 

[MrAl]

Hello again,

According to the schematic posted earlier in this thread, the 260 would put out 130ma with the leads shorted together. Apparently there are other "versions" of this meter though.

In that schematic they show a 1.5v battery as the power source for the resistance tests, but if they use two batteries in series for other versions of the meter then it could easily put out twice that much which would be more like 260ma. Of course if they lower the internal shunt resistance that could also be the cause of a higher current. As i was saying before, my Simpson 160 has an internal 30 ohm resistor and 1.5v battery for the low ohms test setting, so the max current for this meter is only 50ma.

So again, it depends on the meter being used. The only way to tell for sure is to measure it with another meter being sure to take the internal shunt resistance of the second meter into account also when doing the measurement.

Also, many meters can not test LED's because their internal source voltage used for testing on the Ohms scale is too low and will not be able to forward bias some LED's. A 3v LED for example will not bias very well with only 1.5v open circuit test voltage. With any luck you might see the LED's First Light which would be very very very dim.

 

[Billy Mayo]

By having such a scale, it is possible to plot the forward characteristic of semiconductors.

How does the analog scale plot the forward characteristics of semiconductors?

The ohm scale is selected to set the maximum test current which is calculable for each ohm range. So at the meter reading obtained, we can read the test voltage and approximate the test current. This can be done for several ohm ranges to get an idea of how the 'diode' behaves at various forward current values.

How do you find out the test voltages and test currents for each ohm range? short the leads out for each ohm range and write them down?

But the Diode under test on all the ohm ranges will be measuring the semijunction in Ohms right? at various current values?

testing of SCR's too is easy when you have two analogues meters; one on the gate to check the minimum trigger current, and also for the anode circuit to check the holding current.

Can you explain more about this? do you put the meter in series with the gate and another meter in series with the anode ? when you put the meter in series is there a resistor with it?

 

[Reloadron]

Hi Ya MrAl, yes, the good old Simpson 260 line goes back to the 1940s with no shortage of design changes including the battery supplies. That was why I spelled out Series 6 XLP. I believe on the RX1 range it only uses the 1.5 volt D cell. It uses a 9 volt battery for the higher resistance ranges. This one also has what is called a LP (Low Power) ohms function.

Ron

 

[MrAl]

Hi Ron and Billy,

Ron:
Oh yes i thought that one must be different. Can you get a picture of the meter face by any chance? That would help me understand even better what that meter is doing. I would need a picture of the whole face where the numbers are clearly legible to read off the Ohms scale grads.
My Simpson 160 (kinda old and beat up like me now ha ha) uses 1.5v for lower ranges and, believe it or not, a 22.5v battery for the higher Ohms scale! Yes that is twenty two and a half volts. Im not even sure if they make those batteries anymore but if they do they are probably expensive i bet.

Billy:
You have to realize that the meter internal resistance is fixed, and when you place a resistance (or diode) across the leads to test it the total resistance in the circuit goes up. That means the current changes from what it is with a short on the leads. Shorting the leads tells you the MAXIMUM current it will put out,but when you place a resistor to test across the leads the current goes down and it could go down by a whole lot. For a 12 ohm resistor across the leads it would go down by half for the meter we talked about (the 260 at least that one version), and for a larger resistor it would do down even more. A 1500 ohm resistor for example would reduce the current to only about 1ma. That's much less than the computed 130ma or the measured 240ma we see elsewhere in this thread.

If you know the internal resistance (which you can determine) you can then determine the current with a given resistor value. If the short circuit current is 200ma (for example only) and it has a 10 ohm internal resistor, then with another 10 ohm across the leads the current would be 200ma/2=100ma. With a 30 ohm resistor across the leads it would be 200ma/4=50ma. But it is probably best to measure this current with another meter if you really want to know what it is for sure because the internal battery will load differently for different ranges and external resistances under test.

 

[rumpfy]

Taking the subjects in turn;
I got my info by googling 'simpson' and got info for Simpson 260, series 3A and 7M. The pic shows two 1.5 volt batteries. It is possible that the battery voltage changes between ranges, BUT, the principle of the ohm-meter is the internal series resistance used for each range and REGARDLESS of the internal supply voltage, the resistance is ALWAYS equal to the calibration scale at 50 % of the full scale deflection. The internal voltage will affect the test current. My statement that the internal resistance of the Simpson was 12.5 ohms came after looking at a photo of the scale.
If mr Al's meter is based on 30 ohm, then 30 will be at the half scale point.
Mr Al's simpson is like my AVO model 8. 1.5 volt and 15 volt for the ohm x 10,000 range. Of course, the AVO is kept for 'special'; but the jap meters have two 1.5 volt cells so they can do LED's. The newer of the jap meters has a second 9 volt battery for an ohms x 10,000. When testing led segments on 7 segment displays and the like, it is critical that the tests be done on the ohm x 10 range where the maximum current is only 15 mAmp.

Billy,
if you take a photo of a meter you can use for experiments, then post it and I will explain in detail. I will 'watch' this thread for you.

 

[Reloadron]

Hi Al & Rumpfy

Yes, the old and reliable Simpson 260 series went through many changes. Interesting the meter has survived since around 1940 making it over 70 years in existence. Over the years the design changed a few times as it evolved. Al, I also have a 270 version with a mirrored back movement and I have a 269 which like your 169 uses a 1.5 volt D cell and one of those 22.5 volt batteries. The latter 22.5 volt batteries are getting hard to find. Anyway, here is a picture of the face of the meter I used. This has been my beater working meter for about 30+ years.

The Blain above OHMS top left is where I inscribed my name on it years ago.

Ron

 

[Billy Mayo]

Rumpfy you got the meter pic , can you please explain

By having such a scale, it is possible to plot the forward characteristic of semiconductors.

How does the analog scale plot the forward characteristics of semiconductors?

The ohm scale is selected to set the maximum test current which is calculable for each ohm range. So at the meter reading obtained, we can read the test voltage and approximate the test current. This can be done for several ohm ranges to get an idea of how the 'diode' behaves at various forward current values.

How do you find out the test voltages and test currents for each ohm range? short the leads out for each ohm range and write them down?

But the Diode under test on all the ohm ranges will be measuring the semijunction in Ohms right? at various current values?

testing of SCR's too is easy when you have two analogues meters; one on the gate to check the minimum trigger current, and also for the anode circuit to check the holding current.

Can you explain more about this? do you put the meter in series with the gate and another meter in series with the anode ? when you put the meter in series is there a resistor with it?

 

[Billy Mayo]

My manger said that my fluke 87 continuity tester mode outputs 9 volts at 1mA of current, so when testing Diodes in Continuity mode , there is a polarity on the probes because it will forward bias a diode in continuity mode

 

[rumpfy]

This is getting a bit hard!!!
I got the manual from post #15 and that says Simpson Model 260. But it has a circuit diagram. Billy, I get the pic sometimes then it disappears; but I got that the ohms value at 50% of full scale was 6.
Can I make my comments to the manual from post 15.
Firstly, there is some 'fiddling' goes on with analogue meters due to the inherent uncontrollable series resistances. In the Simpson, I said in post 66, that the series resistance on the ohms range is 12.5 ohm. BUT, the circuit diagram says there is a 11.5 ohm resistor on the Rx1 range. The simplified diagram is shown on page 17 at Fig 5. However, if we look at the Rx100 range on page 18 at Fig 6, we see something a bit different. Fig 6 shows the 11.5 and 1138 ohm resistors in series, and in parallel with these two is the metering circuit consisting of the meter itself at 2000 ohm, plus the 'Ohms Adjust' pot. If this is at say half setting, then the shunt resistance across the 1149.5 ohm pair is about 28850 ohm. The effective resistance of this network is close to 1105.5 ohm. Then, the series resistance through the test leads is 1105.5 PLUS the 110 ohm down to the negative terminal and this makes the total series resistance equal to 1215.5 ohm. So what is NOT SHOWN for fig 5 is the internal resistance of the meter and this is probably taken to be about 0.6 ohm so the total series resistance on the Rx1 range is about 12.1 ohm.
When I look at the arrangement for Rx10000, we see the shunt resistance of say 7000 ohm in parallel with a total of 23000 ohm and this gives an effective value of 5366.7 ohm. The total series resistance through the measuring circuit is then 5366.7 plus 117700 to give 123066.6 ohm. So again, the total series resistance is close to 12.5 x 10,000.
The purpose of the above, is to show how the resistance calibration value at 50% of the full scale will ALWAYS give the actual series resistance in the measuring circuit. When you know the series resistance you can the calculate the test current from the battery voltage. In the case of the Simpson, this is 1.5 volt so the test current is either 1.5/12.1 = 125 mA, or 1.25 mA. For the Rx10000 range there is a problem with the circuit. Fig 7 shows the 6 volt battery connected with opposite polarity to the 1.5 volt cell. The circuit diagram has them shown in series and this is the more likely connection. I reckon the test Voltage on Rx10000 is 7.5 volt. But Fig 10a,b on page 22,23, shows something else which doesnt add up to the battery arrangement at Fig 7 or Fig 9. Given the battery voltage is 7.5, the test current on Rx10000 is 7.5/123 kohm = 0.06 mA.
Thats probably enough of the ohm range stuff. The unfortunate aspect of the Simpson is there are only two useful ohm ranges for testing diodes. My jap meters have a 20 ohm series resistance with 3 volt battery and four ohms ranges. The currents are; Rx1=150 mA; Rx10 = 15 mA; Rx100= 1.5 mA; Rx1000=0.15 mA, and the Rx10000 is 9 V/200000 = 45 microamp, but is not used much.
I'll post this and log off to get photos of my meter, and then continue.

 

[Billy Mayo]

What is a DVM fluke 87 continuity tester output voltage and output current? or other DVM meters in general

 

[ChrisP58]

What is a DVM fluke 87 continuity tester output voltage and output current? or other DVM meters in general

Generally speaking, a continuity tester applies a voltage, and beeps if there is current flow.

 

[Billy Mayo]

a continuity tester applies a voltage, and beeps if there is current flow.

Applies what voltage? 9 volts ?

So the continuity tester is a current sensor?

My manager uses the continuity tester to test Diodes , and measures them in both probe polarities

 

[rumpfy]

I'm going to stick with the Simpson 260 as shown in the post #15. The 260 xlp seems different and may confuse/contradict things.

This is a pic of my jap special.
The bottom scale is the scale which allows the testing of the forward characteristic of diodes.
This is the 'reverse voltage' scale and shows 0 volt at full scale, and 3 volt at zero scale. When an 'OHMS' reading is taken, the first step is to 'zero' the ohm range by shorting the test leads and adjusting the 'ohms adjust' to give full scale reading. As resistance is added to the test leads, the meter reading drops back to the left hand side so that when an infinite resistance(open circuit) is presented across the test leads, the reading is at the left hand side. If you measure the voltage across the test leads it will be 3 volt when there is an open circuit, and zero volts when there is a short circuit. So the sum of the voltage across the test leads, added to the voltage internal to the meter circuit, will always add up to 3 volt. The reverse voltage scale shows the actual test voltage across the resistor (or diode) under test. The ohms range in use will determine the test current flowing in the test leads. The actual test current starts at the maximum at full scale and reduces linearly back to the left hand side.
If you take a diode and test it with the meter on Rx1, a typical value might be 0.85 volt and the current will be about 0.72 of 150 mA = 108 mA. If you change to the Rx10 ohm range, the forward voltage might be say 0.65 volt and the current will be say 0.78 of 15 mA = say 11.7 mA.
On the Rx100 range, the voltage might be 0.55 volt and the current will be about 0.81 of 1.5 = 1.2 mA.
Looking at the scale of the simpson in post #72, if you measure a diode on the Rx1 range and you get a reading of say 175 volt on the DC volt scale, this is 0.7 of the scale range and if the battery is 1.5 volt, then there is 1-0.7 = 0.3 of the battery voltage across the diode, and the current is 0.7 of 1.5/12.3 = .7 of about 125 mA = 87 mA. Now using the Rx100 range, if the reading is at say 190 volt, then this is 0.76 of scale and the diode voltage is say .36 volt and the current is 0.76 x 1.5/1230 = 1.8 mAmp.
So it looks like the Simpson isnt that great for testing forward characteristics, because of the few ohm ranges available and the low test voltage.
There might be some models with a 3 volt battery supply for ohms, but an ohms range selection of Rx1, R x 10, Rx 100 is a good selection.
Billy, hope this helps to understand the analog ohm meter and answers your questions. The battery voltage is measured by using another voltmeter and testing the voltage across the test leads. For current just use another mA meter(post 65). Then you can go back and calculate and see that it is a simple matter to determine the typical characteristics of each meters ohm range characteristics, just by looking at it.
In post 65 and 72, the ohm scale at 50% is 6 ohm. If the battery voltage is 1.5, then the maximum measuring current is 250 mAmp. Since the meter reads about 200 volt on the DC volt scale, then the current is 200/250 of full scale = 0.8 of full scale. On Rx1, the full scale current is 250 mA, so the current in the test leads is 0.8 x 250 =200 mAmp. The actual reading on the DVM is 184 so we are close. I've been able to calculate all this from Oz on the other side of the world. So it sort of proves the point I'm making that forward characteristics can be got from the ohm range on an analog ohmmeter.
As well, calculating back the other way, the meter reads 1.5 ohm, so the total resistance under test is 6 + 1.5 = 7.5. The current should be 1.5/7.5 = 200. The fact that it is showing 184 mA suggests the battery voltage is less than 1.5 and I will say that it actually is 1.38 volt under load. I wonder if Reloadr would check that for us.

 

[Billy Mayo]

my manager said that the simpson can measure 10 amps of current , dvm can only measure 2 or 3 amps. also the simpson has a faster response time than a fluke 8842 when measuring time in circuits