Electrical voltage and wattage question
 

I light I have uses a 200W 19.7 volt halogen. The bulb that was in it was blown and I wanted to see if the lamp worked at all. I put a 20W 12v bulb in it and it worked fine. I saw that I had a 50w 12v bulb and figured that 50 is closer to 200 than 20 so that would be better. I put it in and the bulb blew. Figured may be it was a bad bulb so I tried another one and it blew. Now I am some what scared of putting in the 200W 19.7 volt bulb that I have on order.

Can someone here give me peace of mind and explain how this wattage/voltage thing works? Why does a 20W work fine but not a 50w?

I'll give it a shot. We are talking about voltage (V), current (I) and wattage (W); the governing equation is:

I * V = W

Therefore, to determine the current used by a 50W, 12V bulb:

I = W/V
I = 50/12
I = 4.2 amps

The resistance through that bulb is:

R = V/I
R = 12/4.2
R = 2.9 ohms.

Now if we put that bulb in a 19.7 V lamp:

I = V/R
I = 19.7/2.9
I = 6.8 amps

So, you are putting 6.8 amps through a bulb that was designed for 4.2 amps; and it blew. There is no reason to think the correct bulb will blow.

Does that help?

That part did. The rest of it .... not so much. Did what you type explain why 20w worked and 50 did not?:o

Not bad for a guy that's not an EE............;)

I dabble. ;)

If they are both rated for 12V service and being used in a 19.7V lamp, they are both seeing more current than they were designed for. My best guess is that the 20W bulb has enough "margin" in its design to work with higher current, but the 50W bulb did not. I would not expect the 20W bulb to last very long in that service.

voltage is E not V lol :D

This is the statement that doesn't make sense. If you put a 12V bulb in a circuit that is going to provide 19.7V, the bulb is not going to survive very long. The fact that it didn't immediately blow is a tribute to the manufacturer of the bulb.

You cannot substitute lower voltage bulbs and expect them to survive more than a few minutes.

Think about the vehicle. If the voltage regulator decides to go AWOL and offer up 17V, all the bulbs in the vehicle shine brightly, but their life is decidedly limited.

Same deal with your 12V bulb.

Not in my world. We use numbers like 380kV and 1000MVA. ;)

Oh.......no.......you cannot use 380kv...........you must use 380 KE

:eek:

I already take enough grief from my sparky friends for being a "flange-head."

I'd imagine you are two are at polar opposites in a power plant.........never shall the electrical generation portion be involved with the actual work to get them those electrons.............

And to think they used to teach Ohms law in seventh grade Science!

http://www.aoce.com/images/EV_EFUNm0...wFormulas2.jpg

Great Chart!
 

BB,

Thanks.

Mayhaps, we should affix it as a tag to all Electrical consumers WW ?

Actually, I work pretty closely with some electrical guys; we are part of the same little consulting group. There are lots of interfaces that we have to keep straight, so we each know just enough about the other guy's area to be dangerous. All kidding aside, most sparkies are pretty bright; I learn a lot from them.

Recently, two friends of mine named Stan (one electrical, one mechanical) were working together on a consulting gig for several weeks. They called themselves "sparkystan" and "mechanicstan" to avoid confusion (you would have to meet these guys to fully appreciate it).