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Old 13th Mar 2009, 6:09 pm   #1
Pete_kaye
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Default Strange dropper Resistor.

I have removed the 30 Watt 725 Ohm dropper from the Ever Ready 5017 as it appeared to be o/c/.
It has asbestos tube and base insulator.When I tested it again ,it gave a 500K reading on one section and a voltage reading after,dropping down lke a capacitor to zero.I had cleaned the copper contacts.
If I ever restore it ,I will have to use a diode and resistor.Before I throw away this dropper,i would like to know why it does this.
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Last edited by Pete_kaye; 13th Mar 2009 at 6:14 pm.
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Old 13th Mar 2009, 6:26 pm   #2
Steve_P
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Default Re: Strange dropper Resistor.

Did you measure it on a Digital or Analogue meter. Digitals can do this sort of thing, especlally with the body or any moisture in the item being measured.

Cheers,

Steve P.
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Old 13th Mar 2009, 6:27 pm   #3
AlanBeckett
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Default Re: Strange dropper Resistor.

Pete,
If you put mains on it looking like that you're a better man than I am Gunga Din!
Alan
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Old 13th Mar 2009, 6:55 pm   #4
Nickthedentist
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Default Re: Strange dropper Resistor.

As Steve says, DMMs can do funny things when presented with inductive resistances. My newish Fluke gives funny readings when measuring transformers' resistances a lot of the time.

Nick.
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Old 13th Mar 2009, 9:27 pm   #5
Mr Moose
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Default Re: Strange dropper Resistor.

Hello,
Looking at the circuit this set has filter capacitors accross the mains. When you put the meter across the broken section you could charge these capacitors with the test voltage from the test meter, via the resistor and the valve heaters. Because the resistor is open circuit the capacitors can't discharge back through the resistor so the capacitors will hold the voltage till they self discharge or you discharge them with your meter.
Yours, Richard
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Old 13th Mar 2009, 10:22 pm   #6
kalee20
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Default Re: Strange dropper Resistor.

Quote:
Originally Posted by Nickthedentist View Post
My newish Fluke gives funny readings when measuring transformers' resistances a lot of the time.
If you are measuring the resistance of a transformer primary, short-circuit the secondary. It won't affect the resistance, but will kill the inductance (which confuses DMM's autoranging systems).

However, I'd be really surprised if the inductance of a wirewound resistor was enough to upset a DMM. Presumably when you say, it drops to zero, you mean the current drawn drops to zero (ie resistance reads infinity)? In this case, I'd suggest moisture in the thing, as Steve P says.
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Old 13th Mar 2009, 10:31 pm   #7
ultra-linear
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Default Re: Strange dropper Resistor.

And if you're using an autoranging Fluke, switch it out of autorange mode.
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