Instructables

how to reduce current with out affecting the voltage?

i have a 12V transformer and i want to hook it up to my NST but it has a current rating of 8A but my transformer is to powerful and the lowest voltage is 12V.

kamalraghav2 months ago

There are 2 ways of doing it and both are heat producing, either way the excess energy is lost out.

1. add a small value resistor in parallel to your power source and it can be calculated as below. Plus an additional zener for over voltage protection.

say your source current rating is Ir and your resistor draws I1 and your sink or target is Is (eye sink).

Now Ir = I1 + Is, imply I1 = Ir - Is (i.e. Rating current - Sink/target current)

Since the parallel resistor maintains the potential needed by your device we calculate the resistance value = 12volts/ I1 (i.e. standard 12 volts divide by Current through resistor). It will be a high wattage wire coiled resistor in most likely case and would require cooling so that you dont fry anything unintentionally.

2. using an inductor to save up energy build and a fast switching RC circuit which discharges the inductor pretty quick before reaching the specified charge. Need to do the calculation as i have become rusty.

3. use a high capacity regulator chip like 78H12A. Need to check if it suffices for 12V and 5Amps output which it gives out.

Regards,

seandogue3 years ago
A current limiter is one way of limiting current delivery to a load. However, Kirchoff's law can't be over-ridden. 

For purely resistive loads

Iload will always equal  Vload/Rload
ejp10873 years ago
I'm not sure I completely understand your question, but I'll give it a try. If you have a 12V power supply that is rated at 8A, that means that it will be able to source a maximum of 8A. If whatever you are hooking it up to draws less current than that, then there is no problem. Since current is directly related to voltage (V=I*R) your current will be set by the resistance of your circuit, not by the 8A rating of your power supply (unless your circuit draws more than 8A). If you are hooking this up to charge a battery, then there might be a problem. If the battery charges too fast, it could get kind of hot. If you are charging a battery, you could just add a resistor in between the power supply and the battery, and that should limit the current without effecting the end voltage that the battery will charge to (at least I don't think it will).

Let me know if this helps, or if I misunderstood your question.
Jimmy Proton (author)  ejp10873 years ago
sorry about the confusion, i meant that the NST has a current rating of 8A, the 12V power supply has more than 8A but i cant go lower than 12V
That's okay. The power supply "rating" means that it will break (overheat, explode, what have you) if you try to pull more than 8A out of it. If your load only needs 6A, for example, that is all that the power supply will put out. Does that make sense?
Jimmy Proton (author)  kelseymh3 years ago
ok, i took a transformer from a car battery charger and i want to use it for my NST, the output is 12V and i dont know the current but its to high, my NST needs an input of at least 12V and a current or at most 8A, i need to lower the current of the charger transformer without affecting the voltage of it
Sorry, that was the point of my post. You should NOT need to "lower the current" of the transformer. Your load (the NST) will attempt to pull as much current through the transformer as it needs to do its job. If the transformer can handle that much current (or more), then you're just fine.
Jimmy Proton (author)  kelseymh3 years ago
tha transformer has an 8A fuse, its my only one so i dont want to break it and i dont want to take it out and risk breaking my NST (but it think i already have)
Ah! That's more information :-) That does give you the "rating" for the transformer, but you'll need a bit more. The fuse is there to limit the current, as you know, but the question is, on which side? Is the fuse on the primary (where you plug into the wall), or on the secondary (the 12VAC output)?

Transformers, like everything else in the universe, conserve energy. In this case, that means Vin×Iin = Vout×Iout. With Vin=120 VAC (U.S. mains) and Vout = 12 VAC, the transformer can put out ten times as much current as it draws from the wall.

If the fuse is on the primary (which is more likely), that means it can only handle 8A from the wall, but it could put out as much as 80A on the 12V secondary. If the 8A fuse is on the secondary side, then your load can only pull 8A before blowing the fuse, and the transformer is only going to pull up to 0.8 A from your wall plug.

Does all that make sense? To me, the arithmetic is simple, but I know that many people can get confused by all the subscripts and ratios and whatnot. Work through it, and let me know if you've got any questions.
Jimmy Proton (author)  kelseymh3 years ago
the NST does not plug into the wall, it is a 12-15V DC 8A max transformer, i am using the other transformer to lower the voltage but the current of of the step down transformer is to high for it
Err. You want to feed your NST with 12-15 V DC ?? From a transformer ???
No way.

A PC power supply should put out 8A @12V quite nicely.

Steve
Jimmy Proton (author)  steveastrouk3 years ago
i wantes something permanent
DC ?
Jimmy Proton (author)  steveastrouk3 years ago
indeed
Jimmy Proton (author)  Jimmy Proton3 years ago
i think i broke both of my transformers and i made a new one with the core but i got a question about it to
Then BUY a PC power supply, or find one in a scrap PC.

Job Done.

Steve
I had to put spectacles on halfway through your answer. Are you whispering? Maybe I need a hearing aid. :-D Oh wait a minute, my speakers are switched off.
Yeah. That's because I missed out a slash closing one of the subscript tags. And you can't go back and edit a comment with the software I'bles uses.
. stoopid noobs

The fuse is there to limit the current, as you know, but the question is,

Actually, the fuse doesn't limit current, it fuses (blows)  if too much current is passed through it for "long enough". The transient current in a short circuit can be several thousand amps.

Steve
In one of your replies you say, you want to feed your NST with 12-15 V DC ?? From a transformer ???
No way.

A PC power supply should put out 8A @12V quite nicely.

Steve
It should be fine, the transformer rating is just the maximum it can supply.

Though if my 12kv obt draws less than an amp from Maine then you might be able to find a wall wart for powering it.
rickharris3 years ago
Voltage and current are connected but not the same. Your NST will need the correct voltage (in general) BUT it will only draw (pull) as much current as it needs.

However your power supply will need to be able to supply as much current as is required as said elsewhere it may give up the ghost.

NOTE Your NST may expect a reasonable DC supply which your former battery charger may not give.

A wall wart is perhaps your best option or purchase a replacement power supply.


I also forgot to say that getting the polarity right (+ and -) is important or things may die on you.