Joule Thief

poynt99 · January 07, 2011, 08:35:41 PM

Quote from: dasimpson on January 07, 2011, 08:28:47 PM
i know how to use a meter but you still dont understand what i am asking

ok here it is i have a battery then a resister does the risiter limit the current the circuit can use example say the battery can give 1000ma but i only wont 1 ma would a resister do this?

You claim you know Ohm's law, but by this question, clearly it is not yet solid in your thinking

Yes, of course a resistor will limit the current in a circuit, this is what resistors do

So if you have a 1V battery, capable of much higher current than you require, you can neglect the limiting effects (internal resistance) of the battery itself, and just use Ohm's law to figure out your required resistance.

Thus, if you want 1mA maximum and your battery voltage is 1V, then 1V/1mA = 1k

.99

dasimpson · January 07, 2011, 08:42:26 PM

Quote from: poynt99 on January 07, 2011, 08:35:41 PM
You claim you know Ohm's law, but by this question, clearly it is not yet solid in your thinking

Yes, of course a resistor will limit the current in a circuit, this is what resistors do So if you have a 1V battery, capable of much higher current than you require, you can neglect the limiting effects (internal resistance) of the battery itself, and just use Ohm's law to figure out your required resistance.

Thus, if you want 1mA and your battery voltage is 1V, then 1V/1mA = 1k

.99

ty i know ohms law but putting it into practice like you just have i have to come to grips with i have only used it to measure current flow
what id also like to ask is what resistor would be best to use in a circuit for using ohms law to find current use with out affecting the current avaliable to much

dasimpson · January 07, 2011, 08:44:24 PM

i have read that the internal resistance effects the freqancy the jt works out how would you go about finding the internal resistance of a battery

poynt99 · January 07, 2011, 08:47:10 PM

Quote from: dasimpson on January 07, 2011, 08:42:26 PM
ty i know ohms law but putting it into practice like you just have i have to come to grips with i have only used it to measure current flow
what id also like to ask is what resistor would be best to use in a circuit for using ohms law to find current use with out affecting the current avaliable to much

Often, a current sensing resistor (CSR) is used for this purpose. In this case, you measure the voltage across this resistor to calculate the current. But if you can use one of your meters in series with the circuit, and the current is steady DC, you will have an accurate measurement without affecting the maximum current available from the battery, especially if you use the 10A meter setting and jacks (lower value CSR).

.99

dasimpson · January 07, 2011, 08:50:07 PM

Quote from: poynt99 on January 07, 2011, 08:47:10 PM
Often, a current sensing resistor (CSR) is used for this purpose. In this case, you measure the voltage across this resistor to calculate the current. But if you can use one of your meters in series with the circuit, and the current is steady DC, you will have an accurate measurement without affecting the maximum current available from the battery, especially if you use the 10A meter setting and jacks (lower value CSR).

.99

yeah i normaly use the ampage side of my meter the resion i ask is i have an mp3 player that runs off 1 aaa and i wonted to know the current draw but it wont turn on properly if i try and use the ampage reading on my meter