Question about Single pole single throw

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Old 07-27-17, 01:51 PM
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Question about Single pole single throw

Hey all, quick question. Say I have a resistor connected in series with an open-contact single pole single throw switch across a 24 volt DC source. If the switch is open, how much voltage would be measured across the switch and how much voltage across the resistor? And vice versa how much voltage across switch / resistor if it were closed?
 
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Old 07-27-17, 04:01 PM
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Short answer. With the switch on (contacts closed) the resistance of the closed contacts is negligible and you will measure zero volts across the two switch terminals and 24 volts across the resistor.
With the switch off (contacts open) the resistance of the air gap between the contacts is infinite, no current will flow, and you wil measure 24 volts, the supply voltage between the switch contacts and zero volts across the resistor.

Comments: It really depends on the value of the resistor and the internal resistance of the voltmeter or multimeter. If the value of the resistor is very high, like several megohms, that is likely to be of the same order of magnitude as the resistance of the meter and the voltage measured across the switch terminals with the switch off will be somewhat lower than 24 volts since some of the 24 volts will be dropped across the meter innards and the rest of the 24 volts will be dropped across the resistor.. The measured voltage across the switch terminals with the switch off will be equal to the resistance of the meter divided by the sum of resistance of the meter plus the value of the resistor, all times the supply voltage (24).

Normally, measuring the voltage across the two switch terminals gives no useful information to the do it yourselfer. You would measure from one switch terminal to neutral (or in some cases with a grain of salt measure from a switch terminal to ground).
 

Last edited by AllanJ; 07-27-17 at 04:18 PM.
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Old 07-27-17, 08:19 PM
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Welcome to the forums.

This sounds like a school homework problem.
 
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