Electrical guys

You're digging into my tech school days (over 20 years ago), but best as I can remember:

Double the primary voltage, you'll get double the secondary. There may be some current and/or voltage limitations due to core saturation. Also, don't stray from the original designed frequency too far, the core laminations are made for the frequency.

IIRC

-- Bill Browne Excalibur

Claims, Taft-Hartley Fund, & Pension Administration Software

Not with any ordinary off-the-shelf transformer.. read a recent thread on this board:

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

"Tim Williams" wrote: Not with any ordinary off-the-shelf transformer.. read a recent thread on this board:

incorrect. Saturation in a transformer is related to the number of "ampere-turns." Since Howard Stine seems to be aware that the current flow is controlled by the voltage/impedance relationship of the secondary, it must be assumed that he knows to keep this within the limits the transformer is designed to handle. As long as the I^2R losses don't overheat the transformer, the current doesn't saturate it, and the voltage does not break down the insulation, it ought to work.

If you apply twice as much voltage to the primary, you'll get twice the voltage on the secondary. You can even go vice versa, feeding voltage to the secondary and taking voltage from the primary. In other words a transformer can be used as either a step up or step down device simply by turning it around in the circuit.

(Note that there may be a small efficiency decrease when using a transformer "backwards". That's because of the way the windings are commonly placed on the core. In other words, the nominal primary is usually wound closest to the core while the secondary is commonly wound on top of the secondary. This lowers the magnetization current requirement of the nominal primary by a small amount.)

Power, however, must remain the same in any case to avoid core saturation problems. In other words, since P = I*E, if you step up the voltage, you have to reduce the current drawn by the same amount, and vice versa. For common transformers, max power before saturation is approximately 42 watts per cubic inch of core.

Now all this is premised on the insulation of the windings being sufficient to withstand higher voltage. The minimum insulation value used on common transformers is 600 volts, so if you stay below that, you shouldn't get into trouble.

Gary

Twice the rated voltage will try to produce twice the magnetic flux in the core. A typical steel transformer core will saturate long before twice the rated flux is reached. Since magnetizing current goes up very fast as the core saturates, the transformer will likely overheat and burn up just from magnetizing current.

Mill

IIRC from when I used to design transformers about 40 years ago, the short answer is NO. The long answer is that a transformer with no secondary load connected equivalent circuit is mostly an inductor across the input. There is a Voltage times Time figure for the transformer that determines how saturated the core gets. So you can double the input voltage as long as you halve the time ( or double the frequency ).

Dan

Correct. (Note this is the inductive magnetizing current, *not* load current, which by convienent magnetic quirks is seperate.) I=V/R and R (yeah, Xl, so what) = 2piFL, so current depends on inductance and frequency. So we can factor out amperage in favor of voltage and frequency.

Dangerous advice Leo, have you actually tried this?

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

Thanks a million guys.

To tell the truth this all came about as a result of my wishing to use an old 12 volt digital car radio in my shop. I have plenty of 110 volt AC to 6 volt DC converters but none to 12 volts DC. Just thought that since the radio takes very little current I might get away with pushing 220 V to the 110 volt converter and get about 12 volts out of it. I thought I would be wise to ask first.

H

Dan Caster wrote:

When one designs a transformer, the number of primary turns is figured to keep the core from saturating given core material, X-sec'n area, frequency and intended primary voltage. The wire size is then chosen so the primary fills about half the window area. This determines the maximum current. Doubling the primary voltage will almost certainly saturate the core causing *very* large currents to flow even with no load. This will let all the smoke out.

If efficiency is not critical, you can go the other way - run with half the design input voltage.

Ted

Not even close. Start with

d phi V = N ------ d t

Using core geometry, saturation flux density, Bmax and a bit of calculus, you quickly discover that input voltage, frequency and waveform are what determine core saturation.

Ted

Howard Stine wrote in news: snipped-for-privacy@home.com:

Assuming your existing converters are rated for the current draw of the radio, you can just plug two of them into the wall, put the outputs in series, and you have 12 volts.

12 volt digital car radio in

volts DC. Just thought that

to the 110 volt converter and

Two ways...

1... Go to Radio Shack and look in their catalog for the cheapest 110v to 12V "Battery Eliminator" you can find A 12 Volt, 1 Amp unit is now about $8.00. Then find out which wire is Positive and hook to the old radio input via a 2 amp fuse in a Radio shack in-line fuse holder ($1.19). Hook the Negative wire from the power supply to the case of the radio or its ground wire (some are grounded via mounting stud or bolt on the rear. Stick a 3 foot piece of hook-up wire into the antenna jack (strip the first 1/4" ) and angle it up and straight. Hook up CHEAP conputer speakers and enjoy.

For a bit better reception, go to an auto parts store and get the CHEEPEST antenna they offer (~$7.00).

2... Hook your 2 6V supplies in series to make 12 volts. Their chassis may be grounded, so make sure they do NOT touch each other or the radio. Imagine 2 flashlight batteries in series in a flashlight; positive on first cell hooks to negative on the second. Positive on the second goes to the radio Positive, Negative of the first goes to radio Ground.

3... (Bonus!!) I just picked up a CD 'Boom Box' at a yard sale for $5.00. Screw the car radio....

Jeff Thompson

Mill

This is absolutely correct. A transformer has voltage limits due to the size of wire that it is wound with. I know this can be figured with all them fancy terms and math equations but the simple answer is if you hook up a transformer that is not rated for what you are doing all the smoke WILL come out of it. I have seen it done many times, mostly from some goof hooking up a transformer wrong.

tim

Gary's comments are mostly correct, but two cautions, if you please:

1. core saturation can be a problem leading to destruction of the transformer - this happens when you run a transformer "backwards" with too high an input voltage.

1. there are other non-linearities, including the hysterisis of the core magnetism and some freuqency dependencies that can surprise you.

If you want to just change 110 to 220 or visa versa, then these effects are usually not worth worry> >Thinking about the operation of a transformer.

I hear what you're saying. The thing of it is that I have spare radios and lots of small change to do these sorts of things, but what I was after, was to see if my thinking was out to lunch or not.

Thanks again a million. I was very heartened by all the help and especially to find out that it could work.

Jeff Thomps> Howard St> > Thanks a million guys.

old 12 volt digital car radio in

volts DC. Just thought that

V to the 110 volt converter and

12 volt digital car radio in

volts DC. Just thought that

to the 110 volt converter and

You've had plenty of mostly helpful replies to this but some interpretations of the replies could be dangerously misleading. While it's perfectly true that in the right set of circumstances you can play ducks and drakes with the applied voltage and power loading of a transformer this does NOT apply to the voltage step up that you are trying to achieve.

Core saturation is the problem. 110v 60Hz DC converters typically run their cores at roughly 2/3 saturation level. If the the applied voltage is doubled the core would be driven so far into saturation that it will no longer behave as linear transformer. There will be an enormous increase in primary current which is now only limited by the primary resistance. If you are cautious and lucky the fuse will blow. If there is no fuse it will burn up within minutes.

Note that this has nothing to do the power you are trying to take out of the transformer - the funeral pyre is controlled by primary voltage and will still occur even with no load connected.

On a possibly more helpful note I see you have plenty of

110v to 6v converters - what is wrong with using two 6v converters and connecting the outputs in series?

jim

Not correct. Magnetizing current is proportional to H. B is proportional to the time integral of voltage. They are related by mu (permeability) if the magnetic material is linear. Transformers are not designed that way. Chokes generally are.

You apply a voltage to the primary, usually from a stiff source such as the mains. You can calculate B from the voltage-time integral and core material and geometry. From B and the material B/H curves you can determine H and thus amp-turns.

Trust me. I have designed and built many power transformers at frequencies from 60Hz to 100KHz. They worked as predicted.

Ted

Current limit is due to wire size. Voltage limit is due to number of turns.

Ted