need to find transformer amp rating

[rajesh1987]

hi

if one 12v/5w bulb need 0.416 amp means, if i need to connect 23 bulbs parallely means i need to multiply this amp with 23(no of bulbs) , if so i need to to get 12v/9amp transformer for 23 bulbs to connect parallely, is it correct?

or i need to get higher rating of amp..

regards

 

[kripacharya]

its correct. 9.583amps actually

 

[rajesh1987]

1. so i need to buy 9amps or higher amps

2. whether if i connect 23 bulbs parallely with 12v/ 9amp transformer means the brightness of bulb will get diminish ?

regards

 

[Audioguru]

An incandescent light bulb draws 10 times or more current when it is turned on then the current drops to its rating when the filament reaches 2000 degrees C.
So a 9A transformer with a 9A incandescent load might quickly burn out or the lights might never reach full brightness.

A cheap 12V/9A transformer might produce a much higher voltage (18V or more?) when it powers only one or a few light bulbs which makes them VERY bright. Then the voltage drops to 12V when 23 light bulbs are powered and they will have normal brightness.
A high quality 12V/9A transformer might produce 13V when one or a few light bulbs are powered and they will have almost normal brightness.

 

[rajudp]

if you need no isolation you can go for a step up transformer it will be less since the transformed volt amp will be below 20W

 

[FvM]

So a 9A transformer with a 9A incandescent load might quickly burn out or the lights might never reach full brightness.

It's quite normal to operate low voltage lamps with 12 or 24 V transformers up to their rated power, they surely don't burn out. Simply because the transformer's thermal time constant is at least in a minutes range. A fast 9A fuse would most likely blow, however.

The transformer's short circuit current must be sufficient high to overcome the cold filament inrush, but that's surely the case for a regular transformer.

if you need no isolation you can go for a step up transformer it will be less since the transformed volt amp will be below 20W

Are you suggesting a lamp series circuit? Not a good idea, I think.
- high risk of unequal voltage share and fast lamp burnout
- no light at all if one lamps fails

 

[kripacharya]

I think OP has the answer to his assignment already. Need to close this thread.

 

[ElectroEnthusiast]

Why would he need a transformer to glow a bulb? He can do it just by connecting the leads.