Maintaing of Amperes How possible

I have step down transformer of 12v 3A but i m in need of only 12v 1A on output how i can regulate the current

A 12V, 3A transformer is the rating of the transformer. It means that it is the max current it can handle and not the current it can supply. A transformer does not supply any current on its own.
Current is drawn by the load. If the load draws only 1A current, that is the current that flows through the transformer.

IT mean i can run a fan of 1A 12v with a transformer of 100A12v
it mean only voltages should be maintained and the amperes of the transformer or the supply do not effect the amplinces???????
m i right????????????

Yes. You should ensure that the Fan is getting the correct voltage at its input/power terminals. If your fan is a 12V fan then it is OK. It will draw only 1A current which will flow through the transformer secondary. The primary will have even less current as given by the turns ratio relations.

BTW, Does this fan work out of ac voltage or dc voltage? I have not heard of a 12Vac fan.

this is not funny

Sir its DC
Sir make it clear plz either more ampere transformer will effect the whole circuit compositely or not?????????
i would like to tell that both transformer and the whole circuit are of same voltages

A transformer will step up or step down only an AC Voltage. A transformer cannot be used for DC.

If you have an AC source this will be converted by the transformer to 12V AC signal. You will have to convert this signal to DC using a rectifier circuit and bring it to 12V DC before giving it to you fan.

If you have a DC source you cannot use a transformer, it will most likely burn.

LM 317 can work on 1.5-35v DC and only on 1A
if i connect it with 12v 3A transformer using bridge between IC and Transformer either it wil work or wil burn????????????
if it will burn so here ampere regulator is needed to regulate 3A to 1A how it will be possible????????

What is the ratio of the transformer??

Is is 220V - 12V transformer?

Yes 220-12v transformer

You need to make the following Ckt.

On the secondary of the transformer (12V side), you should make a rectifier with capacitor filter. This will give a DC voltage of 12*√2 = 17V. This should be followed by the the regulator IC LM317 or LM7812 which will give DC 12V.

The regulator IC cannot take more than 1A, so you cannot load the IC with more than that load. Even for less than 1A you might need to have a heat sink.

What you are not understanding is that it is not the transformer or the regulator that determines how much current goes through it. The 3A for the transformer and the 1A for the regulator is the MAXIMUM limit. It can handle ANY current BELOW this limit. The amount of current is determined by the load.

Lets say that after the circuit I described, you put a 12 Ohm resistor across the 12Volts output of the regulator, it will draw 1A (I = V / R). This current is the one which will flow through the regulator and the transformer.
If you put 24 Ohms instead, it will draw only 0.5A.
If you put 6 Ohms, it will draw 2A. This will damage the regulator since its limit is 1A.

You have to see what is the current drawn by your fan. It will have this specification on it either as Current itself or as Power and Voltage where you will have to calculate the current drawn by it. (Power = Voltage * Current).
If the current drawn by your fan is more than 1A, you can not use this arrangement since it will damage the regulator.
There are more efficient methods available but they are more advanced.

smhn72@gmail.com said:

LM 317 can work on 1.5-35v DC and only on 1A
if i connect it with 12v 3A transformer using bridge between IC and Transformer either it wil work or wil burn????????????
if it will burn so here ampere regulator is needed to regulate 3A to 1A how it will be possible????????

Click to expand...

The current rating is the maximum it can supply.
If your transformer is rated for 3A, that means that it can supply up to 3A. How much it actually supplies depends on the load. If you have a fan rated at 12VDC 1A, it can be run from a power supply rated at 1A, 2A, 3A, 5A, 100A and so on, but not if rated at 0.5A.

Similarly LM317 is rated to provide 1A (1.5A I think). So the load can draw up to 1A / 1.5A. If you attempt to draw 3A, you have a problem.

So, you don't need to regulate current. You just need to make sure that the load doesn't draw more than that rated for the transformer and the components.

Hope this helps.
Tahmid.

If you put 24 Ohms instead, it will draw only 0.5A.
If you put 6 Ohms, it will draw 2A. This will damage the regulator since its limit is 1A.
it is fine example
but tel me of how many watts i should use the risistor
quater watt or 1w or 2w or more
how i can determine it

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Dear Tahmid
is there any relatioship awailbale that can provide me the exact risistance to draw a current of my will
that also tell of how many watts the risistance should be used????????

The relationship is the simple Ohm's Law. V = IR. Thus, I = V / R
P = VI = (V * V) / R = (I * I) / R

So, just use I = V / R
P = (I * I) / R

Hope this helps.
Tahmid