How to find the hot resistance of an incandescent bulb

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RayRay1132

Member

• Jun 14, 2021

• #1

How would I measure the resistance of an incandescent bulb while it's lit up?
Thanks in advance, Ray -KD2JID

 

MaxHeadRoom78

Well-Known Member

Most Helpful Member

• Jun 14, 2021

• #2

Measure current and voltage.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jun 14, 2021

• #3

(and apply ohms law!)

 

Dick Cappels

Active Member

• Jun 18, 2021

• #4

OR...measure the power and the voltage, which might be safer if you have a reliable wattmeter on hand.

 

unclejed613

Well-Known Member

Most Helpful Member

• Jun 18, 2021

• #5

using current and voltage is a more flexible method, for instance measuring the resistance of a forward biased semiconductor junction at a given current, since the junction resistance is nonlinear...

 

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KeepItSimpleStupid

Well-Known Member

Most Helpful Member

• Jun 19, 2021

• #6

Just remember,the voltage across and the current through. Remember to not use the same meter. i.e do not meaure current with your DVM and then remove it to measure voltage.

 

crutschow

Well-Known Member

Most Helpful Member

• Jun 19, 2021

• #7

KeepItSimpleStupid said:

Just remember,the voltage across and the current through. Remember to not use the same meter. i.e do not meaure current with your DVM and then remove it to measure voltage.

Click to expand...

The error from doing that should be small.

 

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KeepItSimpleStupid

Well-Known Member

Most Helpful Member

• Jun 19, 2021

• #8

Fluke 77. 10A range. 50mV/Amp.

See https://xdevs.com/doc/Fluke/FLUKE 77, 75, 73, 70, 23, 21 Series II Service.pdf

So, 1A at 120V is 120W or a 50mV drop. So, it would probably work. 50mV is tiny part of 120V.
So, the voltage would be off by 0.042% in my case.

 

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RayRay1132

Member

• Jun 20, 2021

• #9

I'm planning to use a couple of sets of these incandescent bulbs in my homemade high voltage power supply made of a rectified microwave oven transformer. I figure I could install a couple of sockets in series with the primary and secondary's and swap out the bulbs as needed for current limiting.

 

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KeepItSimpleStupid

Well-Known Member

Most Helpful Member

• Jun 20, 2021

• #10

A 60W bulb would limit to basically 60W. Usually, you don't want the light on.