• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Need some theory help

Status
Not open for further replies.

B.man58

New Member
hi folks,
I am doing a distance education course in Electronics and am at the elementary stage. I spoke to a distance education tutor that left me worse off than I was before calling. Hopefully someone will be kind enough to assist me.
Here is the question that left me baffled: If a 300 ohm resistor is used on the 10 volt range of a voltmeter, a _____ ohm resistor would be used on the 100 volt range.
Can simeone show the workthrough to solve this problem? Thanks in advance.
 

MaxHeadRoom78

Well-Known Member
A moving coil meter is essentially a current meter, so to measure voltage, a series resistor has to be provided that will allow a full scale reading when the required range is used, in this case a 300ohm is used for the 10v F.S. range, calculate the current required to deflect the meter to F.S.
Take it from there!
Max.
 

JimB

Super Moderator
Most Helpful Member
OK, lets try this.

First, I assume that this is a simple moving coil meter which is used for this voltmeter.
Second, I assume that we are neglecting the resistance of the coil in the meter. (Because this is very basic).

With a 300 Ohm resistor in series with our meter, it gives full scale deflection when 10 volts is applied.
Using Ohms law we can calculate that the current through the meter is I = V/R = 10/300 = 0.0333 Amps.

To make the meter move to full scale when 100volts is applied, we need 0.0333 Amps to flow through the meter again, so using Ohms law again R = V/I = 100/0.0333 = 3000 Ohms.

Does that make sense?

JimB
 

JimB

Super Moderator
Most Helpful Member
I thought the idea was he was supposed to work through it himself?;)
No one learns by given the answer.
Max.
I agree, as a general rule.
But in this instance I decided that it was appropriate to run through the whole problem, showing all the working.

As you are perturbed by my complete answer to this problem, I will set a problem myself for B.man58 to solve on his own:

What resistance would be used to make the meter read 50 volts full scale?

JimB
 

spec

Well-Known Member
Most Helpful Member
hi folks,
I am doing a distance education course in Electronics and am at the elementary stage. I spoke to a distance education tutor that left me worse off than I was before calling. Hopefully someone will be kind enough to assist me.
Here is the question that left me baffled: If a 300 ohm resistor is used on the 10 volt range of a voltmeter, a _____ ohm resistor would be used on the 100 volt range.
Can simeone show the workthrough to solve this problem? Thanks in advance.
This is a very sloppy question- like many you see from academia. No wonder you are confused. Jim has the answer that they are after though.

I fundamentally disagree that a student cannot learn by having an answer worked out for him- that is one of the best ways to teach someone so long as you show the method, especially with a question as badly worded as this.

spec
 

JimB

Super Moderator
Most Helpful Member
This is a very sloppy question- like many you see from academia.
I agree.
What first struck me was the high current FSD of the meter, I don't think that I have ever seen such I high current.
Such a meter (I guess) would be like something out of an old power station or Frankensteins lab!

JimB
 

spec

Well-Known Member
Most Helpful Member
I agree.
What first struck me was the high current FSD of the meter, I don't think that I have ever seen such I high current.
Such a meter (I guess) would be like something out of an old power station or Frankensteins lab!

JimB
:hilarious:

I have seen quite a few exam questions, especially recently, and I would say that in a about 15% of the cases the questions have gross errors or are so loosely worded that, unless you had the experience to know what answer was required, you would have no chance of satisfying the examiner. I once talked to some one who had interviewed and rejected a student. The interviewer laughed because the interviewee has said that the Q factor of a tuned circuit indicated its quality. I didn't comment because the interviewer was my boss.

There is a saying, which I just made up: ignorance begets ignorance. The worst case of this seems to be in the east- no countries specified- where some of the questions posted on ETO beggar belief. By the way, some of the cleverest engineers come from the east.

The very worst case I saw on ETO recently was where a student had slavishly applied a simulator to a circuit that was grossly and fundamentally incorrect. This is understandable for an amateur newbee, but not for a student at any level. Just the other day I posted an alternator block diagram that was based on some training notes that had been around for years. I didn't spot the error, but one of the ETO members pointed out that three field diodes were the wrong way around. That tells volumes about the standard of teaching- that the students simply didn't understand the very basics of electronics.

We used to suffer from this at work where many grads simply did not understand electronics and were totally incapable of doing a practical design. On the other hand, others were brilliant, but in most of those cases they were also hobbyists and self taught, in addition to their tuition.

spec
 
Last edited:

Ratchit

Well-Known Member
hi folks,
I am doing a distance education course in Electronics and am at the elementary stage. I spoke to a distance education tutor that left me worse off than I was before calling. Hopefully someone will be kind enough to assist me.
Here is the question that left me baffled: If a 300 ohm resistor is used on the 10 volt range of a voltmeter, a _____ ohm resistor would be used on the 100 volt range.
Can simeone show the workthrough to solve this problem? Thanks in advance.
The meter current for full scale deflection (FSD) has to be the same for each current range. Therefore, we designate x = resistance of meter, and y = resistance of voltage range resistor. The equations to satisfy the conditions are 10/(x+300) = 100/(x+y) = FSD . Since we don't know the FSD, we might assume that the resistance x is close to zero, giving y a value of 3000. If we assume a 1 ma FSD, then we have two equations with two unknowns and can easily calculate x to be 9700 ohms and y to be 90300 ohms.

Ratch
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top