Experiment using a potentiometer

[Taniaz]

The voltmeter is working alright. The minute you disconnect the the multimeter it works. It was the same problem with the analogue ammeter. It shows some voltage as you move the wiper but I'm not sure whether that's correct because when the wiper is at the center of the rheostat, the voltage should be 3 V (half of what the power supply is set at).

 

[Taniaz]

SO when I connect the multimeter as such, I get readings.
BUT when I force the probes into the voltmeter real tight, the voltage goes up by a lot and the multimeter stops working so I presume I should just let them mildly touch. I hope this is correct. I'm only getting readings of upto 2 V maximum even though the power supply is at 6 V.

 

[Electron Spin]

I still say both meters are hooked up in parallel...IF this is the case, well...I've said it too many times here already...

If they are, according to the photo, You are chasing the wrong dogs like internal resistances of a perfectly good meter, load etc, as some peeps have suggested...In over 20 years of lab work I have never seen a "bad meter". unless on of the cells went internally weak inside the meter, but that is rare..If it can max out via adjustment, in the resistance mode, it is good...

Would be nice if I could verify the connections to the variable resistor with a different picture angle..I cannot see clearly if there is a wire connected to the top slider of the variable resistor...Are you must be aware that a "Pot" can be hooked up as a rheostat (current divider) or a Potentiometer, (voltage divider)?
.

 

[Taniaz]

I still say both meters are hooked up in parallel...IF this is the case, well...I've said it too many times here already...

You are chasing the wrong dogs like internal resistances of a perfectly good meter, load etc...

Would be nice if I could verify the connections to the variable resistor with a different picture angle..

See that's the thing, with a series connection as in the picture in post 25 registers nothing at all. The variable resistor is being used as a potential divider so two connections are made to the power supply and one connection from the wiper to the voltmeter.

 

[Electron Spin]

This test schematic is correct...This one right here.

See that's the thing, with a series connection as in the picture in post 25 registers nothing at all. The variable resistor is being used as a potential divider so two connections are made to the power supply and one connection from the wiper to the voltmeter.

I cannot see the connection on the other side of the slider...so how can I comment?

Have you done point to point voltage checks whilst moving the slider to different positions? I have seen a lot of large pots have many dead spots due to arcing but that was always in very high power applications as in RF. :+50KV.

Again, it looks like the ammeter is hooked up in parallel... But without seeing it in a better light..I would be assuming...

Concerning your color leads, right now you have two that are swapped, which does makes it a bit more confusing when troubleshooting at a 2 dimensional picture.

Again, With that picture angle I cannot verify the slider connection.

 

[Taniaz]

Yes and this is what I have as per the schematic but it's not working!
Even if I replace the ammeter with a multimeter with the same connections i.e. in series, absolutely nothing happens! Our objective is to get the current through just the voltmeter.

What if this shows up in the A'level practical exam and this is a part of the question, I can't just ignore it and move on.

 

[Electron Spin]

Yes and this is what I have as per the schematic but it's not working!
Even if I replace the ammeter with a multimeter with the same connections i.e. in series, absolutely nothing happens! Our objective is to get the current through just the voltmeter.

What if this shows up in the A'level practical exam and this is a part of the question, I can't just ignore it and move on.

No fear little buddy..No fear...lemme look at it a bit later this afternoon...I must go teach now..

A third VM would be great to check Volt measurements...It would really help a lot iffa' ya' had an extra.

When is you exam? Whats our drop dead time?

 

[Electron Spin]

See that's the thing, with a series connection as in the picture in post 25 registers nothing at all. The variable resistor is being used as a potential divider so two connections are made to the power supply and one connection from the wiper to the voltmeter.

Edit...I see what I missed..sorry...

 

[Taniaz]

I do not see a connection coming from the wiper? I'm I going blind?

The wiper is connected to one terminal of the ammeter and the other terminal of the ammeter is connected to the voltmeter.

 

[Electron Spin]

I do not see a load of any kind.

I see P/S, two meters, a variable pot, and some connect wires...what are you using for a load?

The round symbol in the left of the diagram..

We must have a load or this thing will not work...Ohhh Noooo!

 

[Taniaz]

The first part of the experiment said to use a bulb then they asked us to remove the bulb and just measure the current through the voltmeter so that we could subtract it from the reading we got for the bulb for accuracy.

 

[Electron Spin]

The wiper is connected to one terminal of the ammeter and the other terminal of the ammeter is connected to the voltmeter.

And, both meters must be connected to a load...The ammeter in series and the Voltmeter in parallel.

 

[Taniaz]

And, both meters must be connected to a load...The ammeter in series and the Voltmeter in parallel.

Yes they are in the first part of the experiment, then they ask us to remove the load and just measure the current through the voltmeter.

 

[Electron Spin]

The first part of the experiment said to use a bulb then they asked us to remove the bulb and just measure the current through the voltmeter so that we could subtract it from the reading we got for the bulb for accuracy.

I'm sorry I missed that... That current would be so small as not to be relevant...sounds like a bad procedure...Your instructor is going to know that the internal resistance of a voltmeter is so high as to draw just about zero amps...Its called loading effect, btw...I know what this experiment is trying to show..Ohm's law relations and maybe loading effect of different meters..

The meter "loading affect" ,I think is beyond the scope of this level...

 

[Taniaz]

This experiment was on the characteristic of a filament lamp, and the other one similar to this was the characteristic of a semiconducting diode.

Just one question, we are using an ammeter in both experiments, would a milli-Ammeter be better?

 

[Electron Spin]

Yes they are in the first part of the experiment, then they ask us to remove the load and just measure the current through the voltmeter.

OK! Well if that's the case..A a good meter (current) ,you will read very little to nothing! I can tell you that..

The point of the experiment is to have you realize that current meters have very low internal resistances so the reading will be as accurate as possible..and that volt meters have very, very high internal resistances as not to load down the circuit..That I can promise you.

In the ans to the experiment just mention when you tried measuring the current w/o a load it was near zero, if not zero..That's what they want u to realize...

 

[Electron Spin]

This experiment was on the characteristic of a filament lamp, and the other one similar to this was the characteristic of a semiconducting diode.

For diodes going into conduction they call it conduction or avalance (very non-linear)..They avalance into conduction where's a "filament" lamp will have a more linear conduction rate..but will have a degree of nonlinearity depending on the composition of the filament heating up.

There are good avalanches and there are bad avalanches..

Just one question, we are using an ammeter in both experiments, would a milli-Ammeter be better?

Generally, a stand alone meter, with a single designed function is better in all respects than an all- purpose multimeter, except versatility...

 

[Taniaz]

For diodes going into conduction they call it avalance (very non-linear)..They avalance into conduction where's a "filament" lamp will have a more linear conduction rate..but will have a degree of nonlinearity depending on the composition of the filament heating up.Generally, a stand alone meter is better in all respects, except versatility of a multimeter.

We don't have a milli-Ammeter and the diode currents are really small aren't they? Uptil 0.7 V at least?

 

[Electron Spin]

We don't have a milli-Ammeter and the diode currents are really small aren't they? Uptil 0.7 V at least?

The term mult-ammeter is confusing..a multi-Ammeter to me is a stand alone ammeter that has adjustable ranges/sensitives but only measure current or electron flow..

Generally there are two types of diodes..The silicon diode which tends to conduct when about 0.7 volts of forward bias in applied and the Germanium diode which takes about 0.3 volts of forward bias to conduct.

Silicon's are use in P/S for rectification and such, whereas Germanium are used in small signal applications, generally speaking that is..

Also, if you forward bias a diode into conduction, you better have a series current limiting resistor or you will burn up the diode..

An example...All LED readouts have current limiting resistors in series w/ each diode somewhere in the circuit...

 

[Electron Spin]

OK I just noticed the light buld is the load..a neon gas filled nixie bulb? ipid me! The schematic is showing a gas filled nixie tube bulb..are you using a filament bub

I do not see a load of any kind.

I see P/S, two meters, a variable pot, and some connect wires...what are you using for a load?

The round symbol in the left of the diagram..

We must have a load or this thing will not work...Ohhh Noooo!

SORRY! the "mixer" or round symbol is the load..got it...

A bit slow today...

 

[Electron Spin]

This experiment was on the characteristic of a filament lamp, and the other one similar to this was the characteristic of a semiconducting diode.

Just one question, we are using an ammeter in both experiments, would a milli-Ammeter be better?

A milli--ammeter is going to be 1000 times more sensitive than a meter with a full deflection of 1 ampere ..
But that is is not relevant here..I'm sure hundreds of students have used this same stuff
many times...I think the instructor is just trying to make u guys realize that a current meter has a very low resistance and a voltmeter has a very high resistance as not to upset the over all accuracy of the readings through or across the load.

 

[Taniaz]

Ok thank you :) I'll try again tomorrow and see how I get on.

 

[Electron Spin]

.....redundant post

 

[Electron Spin]

Ok thank you :) I'll try again tomorrow and see how I get on.

I'll be standing by...Gud luck!

 

[Electron Spin]

......

 

[andrevdh]

A closeup photo of just the blue CE multimeter will help.
That way we can instruct you how to use it to measure the current.
Anyway you should connect it just like the analogue one like I showed you.
Its black wire should be in the COM socket and it should be switched for DC (milli) amps.

 

[Taniaz]

Hello, so for the part with the bulb I disconnected the analogue ammeter and voltmeter and replaced them with digital multimeter.

The multimeter measuring the current was connected in series and the one measuring the voltage was connected in parallel.

I then disconnected the connections to the bulb and just tried to measure the current through the multimeter which was acting as the voltmeter.
The leads to the multimeter acting as the ammeter were connected to the 10 A connection and the ground.
The leads to the multimeter acting as the voltmeter were connected to the VΩmA connection and the ground. (I don't have a picture to go with this at the moment).

When I'd connect both meters to the 10 A connection, I'd get a reading on the multimeter acting as an ammeter and when I'd connect the multimeter acting as a voltmeter to VΩmA, it would give me a voltage but the multimeter acting as the ammeter would stop working.

 

[andrevdh]

The voltmeter has a very large resistance, so when you insert it the current would go way down creating the impression that the ammeter stopped working, but in fact the reading is way smaller so that you need to wire and put it on a much smaller scale setting. It is in fact dangerous to insert just the ammeter since it is creating a short circuit so that a part of the pentiometer is bypassed and much more current is drawn from the power supply, although it seems that the power supply has a reset button which would pop if you draw to much current from it.

 

[Taniaz]

We tried setting it to mA and micro-amps as well but it still showed 0. Yes the power supply does have a reset button, it usually resets when the wiper is allowing maximum current to go through.

 

[andrevdh]

It is set for direct current and not alternating current - assuming you are operating DC?
What make/model is is?
Also in this photo of yours you have to swop the red and black wires plugged into the ammeter around.