Variable Transformer project

[3rRoR]

Hey guys,
I'm trying to put together a variac with DC output.
I have a 5 amp variable transformer (TYPE 21) I'm going to use to adjust the output, so "high" output voltage isn't a problem. I have a transformer (center tap?) I robbed out of an old battery charger I never use along with a half wave rectifier and 10A circuit breaker.

I want to build this up in a nice project box of some sort and use it for testing and battery charging mostly, but I want a few options with it. I would like a AC outlet regulated and one non-regulated witch is pretty easy to do. I would also like a DC output w/ground. I have a wiring schematic I've put together, but it was before I got this transformer I plan to use now. The schematic uses a regulator transformer and not a center tap, so I'm a little confused as I"m no electronics expert.

**broken link removed**


I do have a few questions about the wiring of the transformer and rectifier, however.
The battery charger i took apart had a switch on the input to select 6V 6A, 12V 2A, and 12V 6A.

The primary side of the transformer has a black, yellow, brown and red wire. and 3 outputs on the secondary side. If I put 120V AC on the black/red, I have around 28v AC on the secondary side (each end) black/yellow around 18V and black/brown has around 24V with the rectifier out of the circuit. Here's where I get confused. If I add the half wave rectifier that was on the transformer originally, I then get DC voltage between the middle and end connector on the secondary of the transformer.

I'm having trouble understanding this. I would also like to replace the half wave with a full wave rectifier that I have and planned on using in the beginning. Will the full wave effect the amperage of the circuit?

I would like to wire this up with the most possible voltage/amps DC possible on the secondary side as I can use the variac to adjust when needed. Does anyone know where I can get a schematic of this transformer? It has MS496 00 on it, but I can't seem to find anything related to it.

If you have any questions, or I've forgot to mention anything, please let me know.
Thanks guys/gals,
Eric

 

[MrAl]

Hello,

You say you want to regulate the AC output to the outlet too? Regulating an AC voltage is not that easy to do. Regulating the DC output is fairly easy to do though.

 

[magnetomachine]

Me is confused, can't the AC output be regulated by a variable resistor?

 

[3rRoR]

Hello,

You say you want to regulate the AC output to the outlet too? Regulating an AC voltage is not that easy to do. Regulating the DC output is fairly easy to do though.

I guess I didn't post that correctly. What I meant was, I want to have a 120vac outlet that's not effected by the variac and one that I can adjust the voltage with the variac. I've got a few things I have to do with my son this morning, but when I get back, I'll throw a schematic together and try to be more clear.

By problem is I'm not 100% sure about the wiring on the primary side of the transformer, and the center tap part of the transformer.

Thanks guys for your replies.

Eric

Edit
Here's an updated schematic
https://i1114.photobucket.com/albums/k523/3rror1/variac1.jpg

 

[Reloadron]

I believe this line places a little confusion into things:

I would like a AC outlet regulated and one non-regulated witch is pretty easy to do.

If I look at the output voltage of a variac or even a fixed transformer the output voltage will be a function of the input voltage. For example if I have a transformer with a primary voltage of 120 VAC and a secondary voltage of 12 VAC I can pretty much figure that I have a 10 to 1 turns ratio. With an input voltage of 120 VAC the output will be 12 VAC. However if that input voltage drops (sags) to 110 or 100 VAC the output will change proportionally. If I have 100 VAC in I'll have 10 VAC out. The same is true if the line voltage in increases. There is no regulation. Now there could be regulation but as Mr. Al points out that gets into a complicated and more expensive affair. Transformers designed to work this way can be explained this way:

Regulate power dips and surges to protect sensitive equipment. Power conditioners take AC power input and provide a constant output power level. All have NEMA 5-15 outlets and a 6-ft. long cord with a NEMA 5-15 three-prong plug (unless noted).
Choose a power output that is equal to or greater than the power your equipment requires. You can determine your volt-amp requirement by multiplying the amp draw of your equipment by 120 VAC.
Note: General purpose models handle input voltages from 87 to 140 VAC and provide a constant output voltage of 117 VAC (±8%). UL and C-UL listed, unless noted.

The above example was taken from here.

That being a simple example where the input voltage can vary but the output voltage remains pretty constant. There are more versions of regulating AC that get more expensive and complex but when we say regulation we pretty much mean to maintain a constant output voltage and current when the input varies.

As to the original battery charger circuit. Many chargers like this (for automotive batteries) use two diodes but it is actually full wave rectification as they use the transformer center tap. A good example of this technique can be found here.

As drawn the original posted circuit would work for a variable DC output. The pilot lamp on the primary of the second transformer would not work well but overall the circuit would work. The first transformer being a variac. There would be no regulation though.

<EDIT> The revised circuit would also work just fine. The original poster returned while I was getting a post together. Again, the second pilot lamp will see a varying voltage and will not work well. </EDIT>

Ron

 

[3rRoR]

Thanks Ron. I see what your talking about, but really don't have any idea how to correct it? I was thinking about it when I woke up (get things on my mind and can't sleep) but really have no idea? What do you suggest?

Thanks,
Eric

 

[Reloadron]

Hi Eric

As to the AC regulation I wouldn't worry too much about it (or lose any sleep over it ). Here in the US the AC mains are pretty well regulated. My 240 VAC mains into the panel remains right around 242 VAC @ 60 Hz. My 120 volts is generally 121 VAC +/- a volt. As to the second pilot lamp? I would eliminate it. Maybe add an inexpensive DC voltmeter across your DC output and that would pretty much take care of things. If you want a regulated DC out then you could think about using a LM317 regulator (adjustable) configured as in this link to handle higher currents than a solo LM317 could handle. Should you take that route using a regulator you could eliminate the variac. Run the mains into your second transformer, full wave rectify and filter, then place the LM317 circuit in there. Pretty much your call as to what you want.

Ron

 

[3rRoR]

Thanks Ron for all you help! I think I'm pretty clear on it now with the exception of the amperage. Since the battery charger was using the center tap (12v) at around 8 amps for charging, does this mean the amperage will drop if I rectify and cap the 24v of the secondary side of the transformer? Thanks again.

Eric

 

[Reloadron]

No, using the transformer sans the center tap the transformer will deliver 24 VAC at a max current of 8 amps (whatever it is rated for) to your full wave bridge. Now something to note is we run 24 VAC to the bridge. The diodes in the bridge will have a forward voltage drop of about .7 volt. There will be two diodes conducting at any given time (alternate half cycles of the AC input) so the bridge for an exact 24 VAC input would output about 22.2 VDC. When we add the large filter capacitor that cap will want to charge to the peak value of the RMS rectified value so you have 22.2 times 1.414 or about 31 volts if you measure across the cap. That would be normal and expected so if you see that, it is normal.

The circuit you drew will work fine as drawn. You will have an adjustable DC supply good for most applications. I just won't be a tightly regulated supply.

Ron

 

[3rRoR]

Thanks Ron for all your help on the matter. I may change the circuit eventually, but for now I think this will do. I really don't need a regulated DC supply right now, but it's nice to know how to achieve it. I'll probably add a DC meter along with a DC amp gauge just for the sake of doing so. Do you know where I might find a decent enclosure to mount this stuff in? All the ones I've found online are either too costly or just too small.

Thanks again,
Eric

 

[Reloadron]

Yeah, a new case or chassis can be an expensive proposition for projects. My best suggestion would be to look around in thrift stores or wherever for something that resembles what you need or want then begin case modding 101.

Chassis and enclosures are expensive when bought new, even what should be cheap ones. Thus, when I see a scrap case I try to gobble them up for future project boxes. As for panel meters I suggested a few old analog types, they work fine for projects like this and current analog meters with a built in shunt can be had fairly cheap. All my projects as a kid used analog meters as digital wasn't around yet.

Ron