DC transformer question

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Old 11-03-15, 08:13 PM
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DC transformer question

Lets say you have a step-up transformer. If you connect it to the circuit backwards/in reverse, will it become a step-down transformer? (Using the secondary coil as the input instead of the output). Since a transformer is just 2 coils and some soft iron, its not like it has any official polarity, correct?
 
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Old 11-03-15, 08:26 PM
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Yes.... you can use the transformer in either direction.
 
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Old 11-03-15, 08:26 PM
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Transformers do not work with Direct Current. They need the alternating current because they work with a constantly changing magnetic flux.

Theoretically a step-down transformer can be used as a step-up transformer but there are subtle differences in the way the windings are made that make it either a step-down or a step-up.
 
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Old 11-03-15, 08:32 PM
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Furd, isnt there such a thing as a DC transformer? I definitely could be wrong
 
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Old 11-03-15, 08:35 PM
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I didn't see your title. No..... a transformer is an AC only device.

Transformers work via induction of electrical forces by changes in magnetic fields, so the constat fields produced by dc currents won't work at all.
 
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Old 11-03-15, 08:39 PM
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Dang... Oh well. Is there any substitute for stepping up DC voltage?
 
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Old 11-03-15, 08:43 PM
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Furd, isnt there such a thing as a DC transformer?
IF you use a VERY broad definition the answer is yes. An old fashioned automobile ignition coil is a type of transformer and it does work with DC but ONLY when the dc is pulsed on and off. Photographic strobe lights are DC powered and they often (not always) will use a type of transformer but again, it uses pulsing CC.

Perhaps you are thinking of a model train "power pack" or a "wall-wart" power supply that outputs direct current. In both of these the transformer steps down the AC line voltage to a lower level and then sends that lower AC voltage to a rectifier module to convert it to direct current. You absolutely cannot feed a dc voltage into the output of such a "transformer" and then get line voltage alternating current out of it.

(Added) Yes, you most certainly CAN step up a dc voltage but it takes quite a bit more equipment than a simple transformer. In most cases there is a hefty price to pay in lost efficiency as well.
 
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Old 11-03-15, 08:45 PM
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Thank you for all the info
 
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Old 11-03-15, 09:45 PM
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What is it you are trying to do ? What are you powering ?
Maybe there are alternatives.
 
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Old 11-05-15, 12:46 AM
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Two caveats on using a transformer backwards:

1. The secondary coil, now being used as a primary, must be able to handle the voltage and current it would draw from whatever is supplying it. The voltage rating of each coil is in the specs of the transformer.

2. A coil designed as a primary usually has more resistance than the other coils to correctly load the source voltage. Using a different coil as the primary could load down your power source.

What you need to change one DC voltage to another is an inverter, sized to match the voltage you have, the voltage you need, and the current you need to draw.

If you want to power a 120 volt AC device from a 12 volt DC car, you can buy inverters made for the job. They are usually in the form of a box with a plug to fit the cigarette lighter socket and an AC outlet. I bought one for about $35. You can also use the inverter to power an AC adaptor that runs a DC device at another voltage (e.g. plug your 24 V electric wheelchair charger into the inverter, plugged into your cigarette lighter.

Watch the current drawn from the inverter. You can easily run down your car battery with the engine off and the inverter on. Remember that the draw from the car battery is 10 times the draw from the 120 volt outlet.
 
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Old 11-05-15, 05:06 AM
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1. The secondary coil, now being used as a primary, must be able to handle the voltage and current it would draw from whatever is supplying it. The voltage rating of each coil is in the specs of the transformer.
As a starting point you could try putting 120 volts into the secondary of a 240 to 120 volt transformer to get out 240 volts but not put 240 volts into the secondary of a 240 t0 120 volt step down transformer to get out 480 volts. (There are some uses for 480 volts particularly when feeding distant outbuildings but that is off topic for here.)

There may be a large no-load current draw if a transformer is connected backwards. This must be taken into account to prevent overload when figuring out what loads may be connected to the transformer (secondary) at the same time. Also, that represents wasted energy.

For international users, it is generally safe to feed a 50 Hz transformer with 60 Hz but caution and current measuring equipment must be used if feeding a 60 Hz transformer with 50 Hz.
 
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Old 11-06-15, 11:30 AM
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PJ, im trying to charge a capacitor bank to 225 volts.
 
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Old 11-06-15, 12:59 PM
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Yes, you can use the step down transformer as step up or vice versa.
Let's say you have a 220V (primary) to 12V (secondary) transformer rated for 1A output.
When you apply 220V to the primary, you get around 12V at the output. You can get max 1A at the output.
Now you can use the same transformer as step up. If you have a 12VAC source, connect it to the 12V winding (the secondary, which is now to be used as primary). Then at the output, you'll get approximately 220V. Now, the transformer was rated for 12VA power (12V 1A output). So, make sure that the load at the output is less than 12VA.
 
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Old 11-06-15, 06:41 PM
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im trying to charge a capacitor bank to 225 volts.
Ok...... from what..... batteries, AC line, ?
 
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Old 11-08-15, 10:29 PM
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You can use an AC source with a voltage multiplier made of capacitors and rectifiers to get a higher DC voltage.
 
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Old 11-11-15, 01:50 PM
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im trying to charge a capacitor bank to 225 volts.
The solution therefore is a capacitive voltage divider. It functions like a resistive voltage divider apart from the impedance falls with frequency. If your frequency is known and controlled then a capacitive voltage divider is fairly suitable. Adjust the capacitors in order that the peak output voltage will be the 220V you wish to charge the output cap to and place a diode to the output cap just like you would do from any other 220V AC source.

One thing to be cautious about though. The power line will sometimes have large voltage surges. These can make it thru the capacitive divider proportionally similar to the intended voltage. This might overcharge the output cap on rare occasions. It may be best if you put some type of clamp circuit across the cap that activates once the voltage is a bit over the normal value. This can save several random failures after installation.
 

Last edited by PJmax; 11-11-15 at 05:50 PM. Reason: removed advertising? link
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