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# how to reduce current with out affecting the voltage? Answered

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.

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## 32 Replies

SachinthyaR (author)2018-02-11

I have a transformer 230v ac to 28v and 6A, so I want to convert it power supply 0-25v dc voltage and 1A Current ,how I done this task ?

GilbertV9 (author)2016-06-14

I'm sure you've found the answer to your problem already but just in case anyone else out there is having a similar issue.

Just use a computer power supply the 12V(Yellow = +12V & Black is Ground) line will be fine, because though an ATX power supply can supply(1000W) 83A on the 12V rail its a Voltage constant supply so it will only supply the current necessary to operate your device. That's why you can plug a fan in at 40mA and a HDD that uses 3.5-4A on the same rail. Hope it helps! :)

YouNeverKnowItUntil (author)2015-06-23

connecting it to three-phase electricity will reduce the current.

narain_s (author)2014-12-04

solve it,

i have machine of fabric raising. the machine manufacturer has installed the dc mechanical brake on Ac main motor. the winding coil fit in dc mechanical brake. it was burned. after repairing it has burned again and again. despite same dc card and voltage. now, coil repairer says for reducing the dc voltage. now coming out dc voltage 230. but, we required 170 volt dc. reply.

kel.moto.3 (author)2015-05-13

Obviously the manufacturer made a miscalculation somewhere, I would have the brake rectifier checked for correct voltage first if its the same one you say you are using ,if that checks out good, then the problem may be in the new part, using a coil with lower voltage wiring for proper resistance .an easy fix is to add something that draws 60 volts dc before the brake voltage ,a variac or an adjustable type pot. a voltage regulater ,60v transformer reducer added to the rectifier .. theres a few options.

Austinthemighty (author)2015-01-14

so i have a pc power supply and i am going to use a pci cable to draw power from for a led lighting system and the led light system can only take up to 12 v at 2 a will that destroy the led light system or will it be fine? (also i cant open up the box that controls the light system to fix the blown fuse)

kel.moto.3 (author)2015-05-13

Use a 2amp fuse between the power supply and the LED lights,but the most important of all is before you try any power, LEDs use both series and parrallel if you connect the power wrong the LED will burn out easily ,if you run the LEDs in a string ,depending on what your doing and need more lights 110 v ac in series with 6 amp fuse will work too.so for example if LED is each 4 volts to a max of 78-82 volts a string 15 leds on 2 strings each 30 total in series. try this neat trick 2 AA batteries on 1 led make sure pos and neg are correct or could burn it out. LED lasted continuosly on for over a month.

ejp1087 (author)2010-09-30

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)2010-09-30

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

kelseymh (author)2010-09-30

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)2010-09-30

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

kel.moto.3 (author)2015-05-13

If your trying to use a battery charger as a 12volt power supply ,A 3
stage floating charger should work with a bridge rectifier there are amp
settings already on the charger and it should not damage your NST .If
your using just the transformer out of an old battery charger then i
would recommend a Magnetic Amplifier or a Ferroresonant Magnetic
Coupler both use magnetic shunts to adjust the voltage/current on
primmary or secondarys of the transformer ,and can be easily built, but
should add a heat sink and fan if using continous..third use a 110v to
12vDC inverter. i dont recommend a computer power supply it wont
produce the current needed to run a NST

kelseymh (author)2010-09-30

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)2010-09-30

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)

kelseymh (author)2010-09-30

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)2010-10-01

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

steveastrouk (author)2010-10-01

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)2010-10-01

i wantes something permanent

steveastrouk (author)2010-10-01

DC ?

Jimmy Proton (author)2010-10-01

indeed

Jimmy Proton (author)2010-10-01

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

steveastrouk (author)2010-10-01

Then BUY a PC power supply, or find one in a scrap PC.

Job Done.

Steve

caarntedd (author)2010-10-01

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.

kelseymh (author)2010-10-01

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.

NachoMahma (author)2010-10-01

. stoopid noobs

steveastrouk (author)2010-10-01

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

kamalraghav (author)2014-04-29

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,

seandogue (author)2010-10-01

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

Iloadwill always equal  Vload/Rload

steveastrouk (author)2010-10-01

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

killerjackalope (author)2010-10-01

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.

rickharris (author)2010-10-01

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.

rickharris (author)2010-10-01

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