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Nelson
07-17-2003 23:52:51
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Re: Re: Re: TO-20 ammeter in reply to James, UK, 07-17-2003 04:43:16
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I'll try. I am at work & don't have any reference with me, so I hope I'll get it right from memory. The analog (needle movement) meter has a magnet inside & also a needle (moving part) attached to a copper coil winding. When current flows through this coil it creates a magnetic field & reacts (attracts/oppose) with the magnet to cause the needle to move. This type of meters is rated by how much current it requires to cause a full scale movement- the needle swings all the way to the oppsition end, usually the right. The less current it requires to do it the more 'sensitive' the meter is. Now lets look at a simple circut, a 2-cell battery flashlight. If you want to measure its current, you must connect this meter in series with it. Lets say we put the first battery on your table with (+) side up, then put the second one on top of it the same way. Now connect one lead of the meter to the bottom of the first battery, then the other lead to the body part of the light bulb. If you put the bulb on top of the second (the top one) battery & touch the (+) of the battery with bulb's contact (the bottom part), you will now have completed circut. The current (electrons) flows from the bottom (-) of first battery, through our simple current meter, through the light bulb to the top of the second battery (+). The trouble is this meter cannot handle this much current, so a by-pass (shunt resistor) is needed. now lets take out our simple meter & install a resistor in its place. Now the current flows through the resistor instead of our meter. By Ohms Law, if an current flow through a resistor it creates a voltage drop across the resistor. Ohms Law also said E(volt) = I(amp)*R(resistant), so if the resistor stay fixed (Amp meter's shunt resistor is built-in, non-adjustable) then the more current flows the more voltage drop across the resistor will be. Now if we connect our simple meter across the resistor in our simple flash light circut, it will measure the voltage drop instead of current. When enginners designed the amp meter the sensitivity of the meter will match the shunt resistor. So the meter itself actually measures voltage (drop across the shunt resistor) & the resultant needle movment will represent current (on the face plate). I hope it clears up a little, got to go for now before I end up confused myself....
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