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Looking for a specific transformer

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kellys_eye

Jun 25, 2010
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30V AC - sure about that?

What is the 'end voltage' going to be? If it was 12V then the circuit would have to DROP the other 18V and that could mean a lot of wasted energy (heat) and design problems.

You really need to know the full parameters of the circuit you want before making any judgement on the transformer needed.
 

Electric-T

Jun 4, 2017
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30V AC - sure about that?

What is the 'end voltage' going to be? If it was 12V then the circuit would have to DROP the other 18V and that could mean a lot of wasted energy (heat) and design problems.

You really need to know the full parameters of the circuit you want before making any judgement on the transformer needed.


I really want 15Vac. But i am having trouble finding a vendor with the transformer i need.
 
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Minder

Apr 24, 2015
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I really want 15Vac. But i am having trouble finding a vendor with the transformer i need.
Look for a suitable toroidal transformer, they are a cinch to add or remove windings, generally about 1.5/2 turns/volt.
M.
 

kellys_eye

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If you know what DC voltage you need then look for a power supply that can provide that instead. It'll give you what you want irrespective of the way it does it!
 

BobK

Jan 5, 2010
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If you really want a transformer that is 15V 30A, your best bet is to salvage a Microwave Oven transformer, remove the high voltage winding, and wind a new 15V winding. You can find lots of instructions for doing this on the net.

Bob
 
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hevans1944

Hop - AC8NS
Jun 21, 2012
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It sounds to me (so far) that @Electric-T doesn't have any design specifications. Why does he really want 15 VAC (post #22)? Battery chargers require DC of course. How can you possibly find a vendor without specifying the transformer you "need" and how did you determine what you "need"? In post #13 you state:

Im trying to design a variable voltage battery charger. Some that ive seen pull as much as 30 amps. Im not sure if i need all that current. Maybe ill look into making it so i can make amperage variable as well. If this is throwing up any red flags let me know.
Lots of red flags. You can have variable voltage OR variable current, but not BOTH at the same time. All battery chemistries have very specific charging protocols for safe and effective charging. A 12 V lead-acid battery, for example, is typically charged with about 14 VDC using a current-limited power supply. Lithium ion batteries have a more sophisticated (read: complex) charging protocol. None of the battery chargers I have seen have a variable voltage control, but you are free to "design" your own. You will need to specify the voltage range and the current that will be supplied to the load over that range before even beginning to select a transformer with appropriate secondary voltage and current capabilities. Saying you need a 1800 watt transformer, simply because that is the maximum power you can draw from a 15A convenience outlet, is no way to begin a design.

As @kellys_eye mentioned in post #19, you must work backwards from the load toward the source of power, taking into consideration all the inefficiencies along the way, to arrive at the transformer power capability you need. What if the number that results is greater than 1800 watts? Do you just sigh and say, "Oh, well, I guess it can't be done..." No, you do not. You either plan to use a circuit with a higher current capability, say 20A, or a higher primary voltage, say 240 VAC, or you reduce the load-driving capability to accommodate the power you do have available.

Many years ago I acquired a power transformer whose primary was rated 120 VAC. The transformer weighed in at over seventy pounds dead weight. It came with four 1N3085 stud-rectifier diodes, rated for 100 V and 150 A each, mounted on massive air convection-cooled heat-sinks. The diodes were wired in a full-wave bridge rectifier configuration, connected to TWO 8 VAC center-tapped secondary windings on the transformer connected in series-aiding.

I lugged this treasure home and built a support framework for the transformer and bridge rectifier using slotted angle-iron scraps held together with 1/4-20 bolts, nuts, and washers. As I aged, I became too weak to lift it, so I added four wheels to the framework and a bungee cord to pull it along. Today I plugged it into a wall outlet and measured some volages. Line voltage: 122 VAC, 60 Hz. Secondary voltage: 16.3 VAC. Unfiltered (raw) DC output voltage: 14.185. Output filtered with 16,000 μFd electrolytic capacitor: 20 VDC. Output current capability: enough to turn over the engine in my 2002 Ford Ranger pickup truck. Clearly I "lucked out" in acquiring these parts. All I wanted to do was build a car battery charger. To that end, I installed a 50-0-50 A D'arsenval meter in series with one of the power supply output terminals and a largeish Variac between the convenience outlet and the transformer primary. The Variac allowed me to adjust the output voltage and the analog meter allowed be to monitor the charging current. Good enuf, I told myself, but later I ruined the meter by trying to start the truck without remembering to remove the meter. The Variac was eventually put to use elsewhere, so I used about three feet of heavy gauge nichrome wire, arranged in an air-spaced coil with an alligator clip to select how much to use, as a current limiter for car battery charging.

So bottom line is, if you need a "charger" with variable zero to twenty-volt output capable of delivering 150 amperes, a transformer like mine is what you need. They are available. Just add some "smoothing" filter capacitors, and some electronics to control the output voltage and current, and you are done. Or, as the British say, "And Bob's your uncle!" See photos for possible construction tips.
Front View Output Terminals.JPG
Front View with cable.JPG
Rear View Rectifiers.JPG
Rear View with cable.JPG
Secondary ACV.JPG
 

Electric-T

Jun 4, 2017
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If you really want a transformer that is 15V 30A, your best bet is to salvage a Microwave Oven transformer, remove the high voltage winding, and wind a new 15V winding. You can find lots of instructions for doing this on the net.

Bob

Thanks. That sounds like what its coming down to
 

Electric-T

Jun 4, 2017
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It sounds to me (so far) that @Electric-T doesn't have any design specifications. Why does he really want 15 VAC (post #22)? Battery chargers require DC of course. How can you possibly find a vendor without specifying the transformer you "need" and how did you determine what you "need"? In post #13 you state:


Lots of red flags. You can have variable voltage OR variable current, but not BOTH at the same time. All battery chemistries have very specific charging protocols for safe and effective charging. A 12 V lead-acid battery, for example, is typically charged with about 14 VDC using a current-limited power supply. Lithium ion batteries have a more sophisticated (read: complex) charging protocol. None of the battery chargers I have seen have a variable voltage control, but you are free to "design" your own. You will need to specify the voltage range and the current that will be supplied to the load over that range before even beginning to select a transformer with appropriate secondary voltage and current capabilities. Saying you need a 1800 watt transformer, simply because that is the maximum power you can draw from a 15A convenience outlet, is no way to begin a design.

As @kellys_eye mentioned in post #19, you must work backwards from the load toward the source of power, taking into consideration all the inefficiencies along the way, to arrive at the transformer power capability you need. What if the number that results is greater than 1800 watts? Do you just sigh and say, "Oh, well, I guess it can't be done..." No, you do not. You either plan to use a circuit with a higher current capability, say 20A, or a higher primary voltage, say 240 VAC, or you reduce the load-driving capability to accommodate the power you do have available.

Many years ago I acquired a power transformer whose primary was rated 120 VAC. The transformer weighed in at over seventy pounds dead weight. It came with four 1N3085 stud-rectifier diodes, rated for 100 V and 150 A each, mounted on massive air convection-cooled heat-sinks. The diodes were wired in a full-wave bridge rectifier configuration, connected to TWO 8 VAC center-tapped secondary windings on the transformer connected in series-aiding.

I lugged this treasure home and built a support framework for the transformer and bridge rectifier using slotted angle-iron scraps held together with 1/4-20 bolts, nuts, and washers. As I aged, I became too weak to lift it, so I added four wheels to the framework and a bungee cord to pull it along. Today I plugged it into a wall outlet and measured some volages. Line voltage: 122 VAC, 60 Hz. Secondary voltage: 16.3 VAC. Unfiltered (raw) DC output voltage: 14.185. Output filtered with 16,000 μFd electrolytic capacitor: 20 VDC. Output current capability: enough to turn over the engine in my 2002 Ford Ranger pickup truck. Clearly I "lucked out" in acquiring these parts. All I wanted to do was build a car battery charger. To that end, I installed a 50-0-50 A D'arsenval meter in series with one of the power supply output terminals and a largeish Variac between the convenience outlet and the transformer primary. The Variac allowed me to adjust the output voltage and the analog meter allowed be to monitor the charging current. Good enuf, I told myself, but later I ruined the meter by trying to start the truck without remembering to remove the meter. The Variac was eventually put to use elsewhere, so I used about three feet of heavy gauge nichrome wire, arranged in an air-spaced coil with an alligator clip to select how much to use, as a current limiter for car battery charging.

So bottom line is, if you need a "charger" with variable zero to twenty-volt output capable of delivering 150 amperes, a transformer like mine is what you need. They are available. Just add some "smoothing" filter capacitors, and some electronics to control the output voltage and current, and you are done. Or, as the British say, "And Bob's your uncle!" See photos for possible construction tips.
View attachment 35156
View attachment 35157
View attachment 35158
View attachment 35159
View attachment 35160

Wow thank you! This is a lot of information. I will definitely look it all over. It seems pretty close to what im thinking of
 

Electric-T

Jun 4, 2017
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So just to give some closure to this topic ive done the math and studied more on the theory of what im doing. Ive worked from the load backwards and came to the conclusion: the transformer im looking for is more like a 120vac to 30vac rated for 10A. (300W)
I really appreciate all the input and help from everyone. Im going to keep studying theory and continue to learn. I think im getting somewhere with this design and hope i can post a finished product when im done. The first build is exciting.
Thanks again guys!
 

Audioguru

Sep 24, 2016
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30VAC? A battery is charged with DC, not AC. A fullwave bridge rectifier makes DC pulses with a peak voltage of (30VAC x 1.414V) - 2V for the diodes= 40.4VDC which is much too high to charge a 12V battery that is normally charged with 14VDC. Since the peak voltage is much higher than 30VAC then the power rating of the transformer must be increased by the same amount. So for 40VDC at 10A the transformer must be rated at at least 400W.
 

BobK

Jan 5, 2010
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Can you show us how you came up with 30VAC?

Bob
 

Electric-T

Jun 4, 2017
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Can you show us how you came up with 30VAC?
Bob
30 vac is just a healthy voltage above what i need. Its a much more common transformer as far as ive seen. Of course i understand i have to rectify the voltage to DC first. Power will go through a potentiometer before it is feed into any battery. That is the goal of the design. A variable voltage charger.
 

Electric-T

Jun 4, 2017
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30VAC? A battery is charged with DC, not AC. A fullwave bridge rectifier makes DC pulses with a peak voltage of (30VAC x 1.414V) - 2V for the diodes= 40.4VDC which is much too high to charge a 12V battery that is normally charged with 14VDC. Since the peak voltage is much higher than 30VAC then the power rating of the transformer must be increased by the same amount. So for 40VDC at 10A the transformer must be rated at at least 400W.
Yes i realize the voltage has to be rectified. Thats not the part of the circuit in question. Ive put some thought into it. If i want to charge a twelve volt battery i need around 15V. But finding that transformer has proved nearly impossible. 120v to 30v has been much more common. Seeing im going through a potentiometer anyway i didnt see a big deal with a little extra voltage when i can just dial it down to what i need.
 

Audioguru

Sep 24, 2016
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Hundreds of transformer voltages are available. A common transformer that is 12VAC produces rectified and filtered 15VDC.
One hundred years ago a light dimmer (potentiometer) for a theater stage was huge, weighed a few hundred pounds and cost a fortune. Today we use electronics in a battery charger to replace it.
 

Electric-T

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Hundreds of transformer voltages are available. A common transformer that is 12VAC produces rectified and filtered 15VDC.
One hundred years ago a light dimmer (potentiometer) for a theater stage was huge, weighed a few hundred pounds and cost a fortune. Today we use electronics in a battery charger to replace it.
I know what im making probably already exists but i enjoy the build and learning about electronics. Plus it would be a nice thing to have around
 

kellys_eye

Jun 25, 2010
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Some suggested research that you should consider reading up on.

1. Rectification - halfwave, fullwave (and its effect on the DC output and current delivered)
2. Battery chemistry - whatever type you're considering charging (the voltage/current/time and temperature requirements - they ALL make a significant difference)
3. Voltage regulation - the various methods available
4. Current limiting - the various methods available

You appear to be in the dark about many of these basic subjects as indicated by your statement that you can use a 'potentiometer to dial it down'..... and your misunderstanding of AC/DC conversions and why the DC voltage used is important (in the circuit ability to dissipate the 'waste' power etc).

If you can even come back here and list the end-product specifications we can deliver on potential designs for you to consider.

As it stands (and appears) you don't have even the most basic understanding of what is required - or what you actually want!
 

davenn

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Seeing im going through a potentiometer anyway i didnt see a big deal with a little extra voltage when i can just dial it down to what i need.

you cannot put 10 - 0V at 10A through a potentiometer it will smoke instantly
you will need to use a DC-DC buck converter
 

Audioguru

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A battery charger uses the correct transformer voltage, not a buck converter to reduce a voltage that is too high.
 
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