They needed to connect one end of the secondary to mains earth and the core of the transformer (which for safety reasons is best earthed) makes a convenient connection point.

 

Agreed, but the OP doesn't want that if he's using a full bridge rectifier.

 

Why do you want to attach the secondary to the frame? Look at the picture I posted. The secondary is terminated with crimped lugs. Those lugs are bolted or screwed to whatever they connected.

What if your secondary is attached to the frame and a short between the primary and frame happens?

As for any "safety" reason, a short from the primary to the frame is far more likely to happen.

 

Why do you want to attach the secondary to the frame? Look at the picture I posted. The secondary is terminated with crimped lugs. Those lugs are bolted or screwed to whatever they connected.

What if your secondary is attached to the frame and a short between the primary and frame happens?

As for any "safety" reason, a short from the primary to the frame is far more

Why do you want to attach the secondary to the frame? Look at the picture I posted. The secondary is terminated with crimped lugs. Those lugs are bolted or screwed to whatever they connected.

What if your secondary is attached to the frame and a short between the primary and frame happens?

As for any "safety" reason, a short from the primary to the frame is far more likely to happen.

all three of my transformers have been different, I wasn’t sure if that changes the way they need to be setup when they are being rewound. I know now that was a dumb question.

 

The wire of the primary is deeply nicked. It looks like you used a hacksaw to cut off the secondary, and that is the fast and lazy way, and it leads to destruction, as you have seen. LEARN FROM YOUR MISTAKES. It is quite possible to cut a secondary off with wire cutters, being very careful to not even touch the primary side. But it will take longer. Avoid all of that bad advice on you-tube, do not even look at it.
You will also need to remove the magnetron heater winding, and the magnetic shunt on the transformer.
Winding a new heavy secondary will also require patience

 

The wire of the primary is deeply nicked. It looks like you used a hacksaw to cut off the secondary, and that is the fast and lazy way, and it leads to destruction, as you have seen. LEARN FROM YOUR MISTAKES. It is quite possible to cut a secondary off with wire cutters, being very careful to not even touch the primary side. But it will take longer. Avoid all of that bad advice on you-tube, do not even look at it.
You will also need to remove the magnetron heater winding, and the magnetic shunt on the transformer.
Winding a new heavy secondary will also require patience

Please read this response with a kind and courteous inner voice. I know you cannot see me and my social cues, which causes a lot of fights on forums in my experience. The most recent primary winding is in pristine condition. Any knick’s you are seeing are metal shavings from removing the secondary, or it is burnt material from the short in the secondary (why the microwave was free in the first place). I learned from my mistakes and covered the primary up with .015 in. shims and used finesse and patience.
It just occurred to me that maybe you are referring to the first pictures in this thread. Based on what everyone else has told me, as well as my own opinion, the primary was trashed. I used a variety of tools except for a hacksaw. The most recent pictures on page two are of another transformer that had a shorted secondary that I removed successfully following everyone’s advice. I measured the output and it was 53 volts.
the goal was to create a 7 volt 20-25 amp power supply.

 

Please read this response with a kind and courteous inner voice. I know you cannot see me and my social cues, which causes a lot of fights on forums in my experience. The most recent primary winding is in pristine condition. Any knick’s you are seeing are metal shavings from removing the secondary, or it is burnt material from the short in the secondary (why the microwave was free in the first place). I learned from my mistakes and covered the primary up with .015 in. shims and used finesse and patience.
It just occurred to me that maybe you are referring to the first pictures in this thread. Based on what everyone else has told me, as well as my own opinion, the primary was trashed. I used a variety of tools except for a hacksaw. The most recent pictures on page two are of another transformer that had a shorted secondary that I removed successfully following everyone’s advice. I measured the output and it was 53 volts.
the goal was to create a 7 volt 20-25 amp power supply.

Sorry this is a mistake ^^^^.. I didn’t measure the primaries output I measured the input into the primary before starting on my secondary winding and it was 53 volts. Just for a quick test I sloppily ran some ten gauge around the core, about 11.5 turns. When l I tested with my multimeter I was at 11.98 volt output.

 

Sorry this is a mistake ^^^^.. I didn’t measure the primaries output I measured the input into the primary before starting on my secondary winding and it was 53 volts. Just for a quick test I sloppily ran some ten gauge around the core, about 11.5 turns. When l I tested with my multimeter I was at 11.98 volt output.

I am a beginner so please bare with me. I understand I need a rectifier, or I could use diodes to convert to dc, but how does ac voltage compare to dc. If my secondary outputs 11.98 volts right now, when I rectify the ac current, does it convert to 11.98 volts dc, or a higher/lower dc current? How do I get the 20-24 amps?

 

Please read this response with a kind and courteous inner voice. I know you cannot see me and my social cues, which causes a lot of fights on forums in my experience. The most recent primary winding is in pristine condition. Any knick’s you are seeing are metal shavings from removing the secondary, or it is burnt material from the short in the secondary (why the microwave was free in the first place). I learned from my mistakes and covered the primary up with .015 in. shims and used finesse and patience.
It just occurred to me that maybe you are referring to the first pictures in this thread. Based on what everyone else has told me, as well as my own opinion, the primary was trashed. I used a variety of tools except for a hacksaw. The most recent pictures on page two are of another transformer that had a shorted secondary that I removed successfully following everyone’s advice. I measured the output and it was 53 volts.
the goal was to create a 7 volt 20-25 amp power supply.

I did indeed only see the first pictures, and that was the basis for my comment.
There are a lot of things to be careful about when working on transformers.

 

@Hutch2793
Calculating your DC output from an AC voltage will depend mainly depending on how the AC voltage was measured and type of rectification used. Load and filtering also affect it. Moreover, a decent power supply will also have some method of voltage regulation.

 

You mentioned in post #1 you wanted a pi output filter. What is your reasoning for this?

 

With a "PI" type filter the input capacitor can charge toward the peak of the AC input voltage, which will result in a higher starting voltage. The following series inductance usually tends to keep current flowing into the second capacitor after the peak, which results in less ripple voltage at the second capacitor. The exception is if the inductance and capacitance resonate at the supply frequency or a harmonic. At that condition there are problems and high voltages. Resonance is seldom a problem, but it can happen.
The second capacitor serves to hold the output voltage closer to the intended value.

 

@MisterBill2 Thanks, I know , I was interested in the OPs reasoning given his admitted inexperience.

At 25A output current (for which we do not yet know the reason why such low DC volts but high current) a pi filter has some challenges.

At mentioned above, we also don't know what the final output requirements are. Is that 7v going to be regulated to 5v for example?

I suspect, once we know the full picture, there are likely to be better ways to do this.

 

I did indeed only see the first pictures, and that was the basis for my comment.
There are a lot of things to be careful about when working on transformers.

I am terrified of them if that’s what you mean. I know it can/will kill me if I give it the chance.

 

@Hutch2793
Calculating your DC output from an AC voltage will depend mainly depending on how the AC voltage was measured and type of rectification used. Load and filtering also affect it. Moreover, a decent power supply will also have some method of voltage regulation.

The voltage regulation is why I’m using the mot. With a filter it can provide hours and hours of very low ripple dc voltage after being rectified, or using diodes. If I ever need a different voltage I just rewind, or build another. The amperage will depend on rectification, load, and filtering? That leads me to my next point, a pi filter can have rather large capacitors used, right? I thought amperage would be a direct result of that. I know I asked how to get high amps with only 7 volts, but I think I just answered my own question.

 

The voltage regulation is why I’m using the mot. With a filter it can provide hours and hours of very low ripple dc voltage after being rectified, or using diodes. If I ever need a different voltage I just rewind, or build another. The amperage will depend on rectification, load, and filtering? That leads me to my next point, a pi filter can have rather large capacitors used, right? I thought amperage would be a direct result of that. I know I asked how to get high amps with only 7 volts, but I think I just answered my own question.
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@MisterBill2 Thanks, I know , I was interested in the OPs reasoning given his admitted inexperience.

At 25A output current (for which we do not yet know the reason why such low DC volts but high current) a pi filter has some challenges.

At mentioned above, we also don't know what the final output requirements are. Is that 7v going to be regulated to 5v for example?

I suspect, once we know the full picture, there are likely to be better ways to do this.

no it will not be regulated to five. It is an electrolysis power supply.

 

I didn’t want anyone else talking about electrolysis. If I didn’t mention what it was for I knew the creativity and conversation would flow much easier. If their is a specific objective in mind everyone will just give me the exact way it’s supposed to be done and I don’t get to learn from you guys about the hardcore electrical engineering side of it(I know we haven’t discussed anything hardcore yet)!

 

I didn’t want anyone else talking about electrolysis. If I didn’t mention what it was for I knew the creativity and conversation would flow much easier. If their is a specific objective in mind everyone will just give me the exact way it’s supposed to be done and I don’t get to learn from you guys about the hardcore electrical engineering side of it(I know we haven’t discussed anything hardcore yet)!

OK, so now that the application is known here is a thought, which is that such an application does not need a low ripple voltage unless you are actually doing electroplating of something like show car chrome.
My guess had been that it was for operating old automotive accessory items. Oh Well.
The most efficient rectification system will be a center-tapped full wave arrangement, because of only one diode drop in the loop. But that does require twice as many turns in the transformer secondary, which might be a challenge. Each diode will only be dissipating half as much power, so that is a benefit as well. And there is the easy option of cutting the power simply by switching off one side.

 

Please read this response with a kind and courteous inner voice. I know you cannot see me and my social cues, which causes a lot of fights on forums in my experience.

You don't know or understand how mrbill operates. It has no bearing on you, which as a new member have been very good. It has to do with him only reading parts of a thread(usually the parts with pictures) then making comments on things that have already been discussed and worked out.

 

Why would they use it in the original transformer then?

To make it easier to put together on the assembly line.