Back to the basics (painfully simple)

[zachtheterrible]

I've got a couple simple questions that i know are painfully :x simple.

1. When people say that they've got a power supply that puts out 10v @ 180 ma (just an example), how do they vary the amperage?

2. Does mah mean the same thing as ma? (milliamperes)

3. When a circuit requires say 10v and 0v, is 0v the equivalent of connecting to the negative lead of a batt?

4. How can you calculate the voltage drop across a resistor?

5. How come when I put my multimeter that is set to read amperage across a battery, I come up w/ no reading?

6. What is the difference between linear and non-linear electronics?

I'm probably gonna have more questions . . . just need some more time to think about them. Thanx :lol:

 

[udi_hakim]

1. When you say you have a power supply that can give 5V/0.5A it is the maximum rating of current you can draw from it.
The current you are drawing is depended on the load you connet to the power supply.
Lets say you connect a 1Kohm resistor to the above power supply the current that is drawn: I = U/R -> i=5/1000=0.005A=5mA

2. mA = 1/1000A

3. mah = milliampere hour, its not the same...

4. You should use ohm's law that say I(current) = U(voltage)/R(resistance)

5. A) You dont want to do it! it should finish the buttary in no time, doing that is just like shorting the + and the - of the battary.
B) You should get a read of the current that is drawn by the internal impedance of the battary and the resistance of the multimeter, maybe your fuse is defected or you have a multimeter that has another input for the current measurement.

6. You should read some info about diodes and transistors in order to get the differences.

 

[bmcculla]

An little bit more on your first question: I had the hardest time really understanding the relationship between voltage and current. A power supply is a voltage source. A voltage source will output whatever current nessesary into the circuit (within its design limits) to keep its voltage constant. Current sources do the opposite.

The 10V and 0V can be thought of as the + and - terminals of a battery. Voltage is relative so there is no true 0V the 10V and 0V of a circuit means that the 10V should be 10 volts higher than the 0V; they could be 110V and 100V measured with something else set as 0V and the Circuit would still work. This trick is used for op amps that require dual supplies: 10V,5V and 0V is the same as 5V,0V,-5V. As long as you are consistant with what 0V is eveything will work fine.

About Linear vs. nonlinear electronics: Resistors are linear; the voltage and current are linearly related (the graph is a line). V= IR the voltage and current are related by multiplying a simple constant (R) (the definition of linear is y= Ax if A is a constant then x and y are linearly related). Linear things are easy to calculate.

Non-linear things have Current voltage graphs that are not lines. Transistors and diodes are non-linear. Non-linear things are much more complicated to work with. A lot of work is put into making non-linear things linear so you can calculate solutions more easily. Feedback is the usual way to linearize nonlinear things.

Brent

 

[TillEulenspiegel]

You know , that's what I like about this board , all seem to want to help and take the time instead of criticizing. Well done gentlemen.

 

[zachtheterrible]

tilleulenspiegel is right . . . thx 4 not calling me a dimwit . It's kind of funny, I've built transmitters, oscillating circuits, ir alarms, etc., and I understand how they work, but I've never totally gotten down the fundamentals.

1. W/ the output of a pwr supply, i don't actually have a 5v .5 amp pwr supply, but what would be the maximum current of a pwr supply @ 5v, saying that there is no resistance (which is very unlikely)? 5 amps?

3. What is millampere hour?

4. How do I get the current in the first place to do the calculation?? I don't know how to measure the resistance of my entire circuit.

5. How do you measure current then, if you can't put the leads on the battery?

 

[udi_hakim]

1. Theoreticly if you will put no resistance you could draw unlimited current.
But in real life if its a battary you will draw the maximum current it can give V = U-r*I .
If you will try to do it from the wall you will cause a short and the fuse to shut you down to prevent damage.

2. A milliampere hour (mAh) is 1000th of an ampere hour (Ah). Both measures are commonly used to describe the energy charge that a battery will hold and how long a device will run before the battery needs recharging. (I copied it from somewhere...)

3. You'll have to post your circuit so i could explain how to calculate the total impedance.

4. The maximum current should be written on the battary, you should remember that the internal resistance (r) of the battary increases with its use so the current will be lower.

 

[gerty]

When you use your meter, set on current, you place it in series with a load. The meter will then display how much current that load draws. If you put it across a battery you have no load, it's the same as shorting across the battery. You will usually blow the fuse inside the meter(you might want to check yours) when doing this.If you want to check current in a circuit, set your meter higher than you think the current should be, disconnect one battery lead , place one meter lead one the battery terminal you disconnected and the other lead on the wire you disconnected from the battery. That way all the current you are drawing is going thru the meter.. If you check your fuse and it's blown...welcome to the club, we all do it from time to time. Make sure you replace it with the proper size and rating, going to a higher amperage fuse will not increase the capacity of your meter, it will only smoke it..

 

[Screech]

. Make sure you replace it with the proper size and rating, going to a higher amperage fuse will not increase the capacity of your meter, it will only smoke it..

my meter did'nt smoke, but it can't read current anymore.
It only reads resistace when it feels like.
It can still read voltage.

(my electronics supplier didn't have the right size fuse in stock).

 

[bmcculla]

1) Power supplies have an output resistance. if you connect a 0 Ohm load to a 5V supply the current will depend on the output resistance of the supply (and probably blow the fuse in short order). The output is a product of how the supply is built but is always non-zero in reality. The better the supply the lower the resistance. An ideal supply would have zero output resistance and be able to supply infinite current. Similarly Batteries have an internal resistance. If you hook a wire across the battery terminals the battery will get hot -thats the heating due to the current through the internal resistance.

3) if you circuit is simple resistors you can just measure the whole thing as if it were a single resistor. If it has capacitors inductors its resistance isn't fixed (it depends on frequency) and so can't be measured with a meter. Transistors also make things complicated. gerty is right about how to measure current.

Brent

 

[TillEulenspiegel]

. Make sure you replace it with the proper size and rating, going to a higher amperage fuse will not increase the capacity of your meter, it will only smoke it..

my meter didn't smoke, but it can't read current anymore.
It only reads resistance when it feels like.
It can still read voltage.

(my electronics supplier didn't have the right size fuse in stock).

Yah, the meters usually have two fuses 1 @ <.5 A or below quick blo for V AC-DC-mA, the other at ~10A or so to read Amps, thats why on most meters you have to move the V+ lead to another socket on the meter. The way current is measured is to put a "shunt load " ( a big hefty resistor at 1 Ohm or less) and calculate the A using Ohm's law. or on older analog meters to design the full deflection current of the meter to equal the peak A@rating. I do not use ceramic fuses in my Flukes as they will give a greater over current over time then AG slo or fast fuses. I would rather replace a pesky 10 A quick blo 2ce a year then have to pay fluke $175.00 because i did something dumb.

 

[fat-tony]

I do not use ceramic fuses in my Flukes as they will give a greater over current over time then AG slo or fast fuses. I would rather replace a pesky 10 A quick blo 2ce a year then have to pay fluke $175.00 because i did something dumb.

From what I understand, the ceramic fuses are used because they don't violently explode when "you do something dumb", although it could have been the Health & Safety instructor trying to scare us a bit. He told us all to throw out our $5 meters because in a fault condition, the fuses could arc across and blow the meter up in our faces . I believe him, but that's assuming I'm working with some big-ass voltage sources. A normal bench supply ISN'T going to do this.

 

[Nigel Goodwin]

From what I understand, the ceramic fuses are used because they don't violently explode when "you do something dumb",

That's what I'ver heard about the reason for ceramic fuses, but, strangely enough, I've seen a lot more ceramic fuses than glass ones which have violently exploded.

The ceramic fuses also seem to have a habit of failing for no apparent reason.

 

[Phasor]

my meter did'nt smoke, but it can't read current anymore.
It only reads resistace when it feels like.
It can still read voltage.

Have you taken your meter apart, and had a look at the circuit board?

I accidently blew up the current function on my meter one day (put it across 240V while still in Amps range) - made a big mess of the fuse, but it also blew a large track clear off the circuit board. Everything else still worked fine. After bridging the gap with a piece of wire (and soldering in place), it works again.

You might want to have a look at it, maybe you did the same thing as me.

From what I understand, the ceramic fuses are used because they don't violently explode when "you do something dumb",

Some of the ceramic fuses, are what is known as "HRC" fuses, or High Rupturing Capacity. I believe the Fluke fuses are HRC types (though don't quote me on it!). HRC fuses typically have multiple weak spots along the length of the fuse, so when it blows, it blows at several points, to minimise the size of the arc. Additionally, the heat of the arc melts the filling into a glass-like substance, which helps to quench the arc quicker.

Pretty much all fuses above about 100A are HRC types.

 

[zachtheterrible]

Sorry 4 the long reply chums, but ive been gone campin'.

When you use your meter, set on current, you place it in series with a load. The meter will then display how much current that load draws.

So if I get how much current a circuit draws, how can I calculate how long that circuit will stay on w/ a certain battery?

 

[gerty]

First you have to know the capacity of the battery. Let's say 4 ah for example, that's 4 amp hours. Then if you know how much your circuit draws, let's say 1 amp, you can do the math. theoretically.4/1 =4 hours.. This is all dependant on several factors, age of batt, charge of batt, wether or not current draw fluctuates, and others..This is a general rule of thumb as you can see..

 

[Screech]

Phasor's

Have you taken your meter apart, and had a look at the circuit board?

I accidently blew up the current function on my meter one day (put it across 240V while still in Amps range) - made a big mess of the fuse, but it also blew a large track clear off the circuit board. Everything else still worked fine. After bridging the gap with a piece of wire (and soldering in place), it works again.

You might want to have a look at it, maybe you did the same thing as me.

I didn't have a very good look, but now will.
Thanks.

 

[zachtheterrible]

Thanx!!