# Is there a rule to calculate the correct voltage for a light bulb?

Say I have an unknown led light, independently several person know it could be 12v or 24v or 110v even 220v

Say It's rated for 9v (and you dont know), let's say you start applying fewer volts until it start turning on and stronger, but how can you know the PROPER and CORRECT voltage?

Does it apply for a small motor? Rated say at 6v but you think it's rated at 12v or 9v.

What's the rule resistance? watts? amps?

• It I knew it was an LED light, I'd know that it's likely not to be hurt by $100\:\mu\text{A}$. So I might start out using a constant current power supply setting for that current level and then measure the voltage across the LED light. That would tell me something, already. I might also see if I could notice its illuminance in a darkened room at that level. It's then very easy to adjust a constant current source to increase the current (assuming things aren't looking disasterous) and do so until the lamp appears to be operating at a level I might expect from it. Then, measure the voltage.
– jonk
Mar 5, 2020 at 7:49
• The thing is, why would you bother to worry about an unknown component be it lamp or motor unless the perceived value was such that the perceived benefit in performing extensive testing was worth it given the chances that any investigation might mis conclude. Where is there any sense in this? Mar 5, 2020 at 8:24
• @Andyaka It's pretty common in toy projects and especially when using salvaged parts. You wouldn't design something professionally using salvaged parts, but not every design is professional. Mar 5, 2020 at 11:55
• Salvaging cheap parts is a waste of time. Mar 5, 2020 at 11:57

https://www.micromo.com/technical-library/dc-motor-tutorials/motor-calculations

And to be considered: For a DC motor which run below its nominal voltage has slower no-load speed and a lower stall torque. This not cause any problems with the motor. If anything, the motor will last longer because it won’t be subject to the same level of stress, heat, friction and general wear as it is when running at the full nominal voltage.

And if it runs above its nominal voltage, it is going to magnetically saturate the steel in the motor’s core. This increases the current drawn causing the motor to run hotter and shorten the motor’s life.

Rated voltage is chosen or defined based on material and/or component properties and also on endurance tests of multiple samples.
Therefore, without knowing what's in the "black box" you cannot know the rated voltage.
Without having a number of samples, you cannot know test/deduce what the rated voltage is.

E.g. different LED's with the same voltage rating (can) have different intensities. If you apply the rated voltage to a ultra bright LED and apply an overvoltage to a 'normal' LED, they may have the same intensity. But the normal LED may get damaged due to the overvoltage.
So, checking the intensity of LED's cannot be used to trace back what the rated voltage would have been.

The current/voltage chart of the LED is somewhat J shaped: LED conducts much more (and emits light) above the certain threshold voltage. This voltage is about 1 V for infra red, 1-2 V for the visible light and can be even higher for UV LED. The operating current of the LED should normally be above that threshold while it is not obvious how much can you drive it up. You can take a current vs voltage chart with a suitable milliamperemeter (0 - 50 mA range should work for the most LEDs) and regulated 0 - 4 voltage supply, start from the 0 voltage and do not overdrive. You can do this safer if you have a second voltmeter: put a 100 ohm resistor between power supply and LED and measure the voltage directly on the LED.

It is more tricky if the thing has multiple LEDs and additional electronics inside. But it this case it is a device, not a component. It should have some name, or number printed on it that may tell more if googled on the web.