Commercial battery charger/starter... rectifiers?

[skeer]

Question for you ladies and gents:

The big battery chargers, the ones with handles and wheels.. how does the rectification work on these? I opened one a few days ago and saw a neat start configuration just like here: https://www.centurytool.net/141_176_Century_40_2_200_Charger_W_Test_s/66.htm

But looking at the schematic.. only Negative is being rectified.

I've seen others with whom the negative is coming off the transformer directly where the positive is fed from the rectifier. Can anyone ELI5?

 

[FredWanaker]

a diode only lets current travel in one direction. Whether it is in the positive or negative lead is irrelevant to the electrons/holes traveling thru it. They can only travel one way regardless which part of the circuit it is in.

 

[dogdog]

But looking at the schematic.. only Negative is being rectified.

I've seen others with whom the negative is coming off the transformer directly where the positive is fed from the rectifier. Can anyone ELI5?

looking at that diagram posted.. you have a Center Tap transformer and two output from center. ...you can google the diagram. typical in most charger to get more amp. With this configuration all you need is two diodes instead of 4 to be "full bridge" rectifier. so if you put the meter in AC mode, the put of the two lead reference center is 1/2 if you test the two lead that goes into the diode it will be 1, usually about 28Vrms.

*** in your center tap transformer with two diode case, the center tap is the negative return of electrons. so It's the Negative on the battery charger.

so if you are looking to replace those diodes, probably two single diodes with large current rating. they do have them on Ebay cheapest....

Full wave rectifier - Center tapped full wave rectifier

A full-wave rectifier is a type of rectifier which converts both half cycles of the AC signal into pulsating DC signal.

www.physics-and-radio-electronics.com

*** I would double check and verify your schematic against the device as well... Usually the center is the Negative in most scenerios... you can have D1 D2 flipped and have center to be Positive. not usual.

 

[American Locomotive]

As mentioned, if you use a center tapped transformer, you only need two diodes. A lot of battery chargers seem to use this method. Presumably to save money on diodes and have slightly better efficiency since you're only dropping voltage across one diode.

 

[skeer]

I opened the small, old-school battery charger in the shop last night. Switchable 6/12vdc, 2/6 amp.. the incoming Line ran into the housing, through the amp meter, to the breaker then out to the Positive cable. The t-former in this one does not have a center tap, but both secondaries hitting the rectifier plate via two large diodes.

Just, my brain can't compute how this ends up working correctly on a DC device. I'm assuming in a configuration such as battery chargers the output is rather dirty. Not something you'd wan't to really 'power' anything by.

 

[iron block]

Question for you ladies and gents:

But looking at the schematic.. only Negative is being rectified.

I've seen others with whom the negative is coming off the transformer directly where the positive is fed from the rectifier. Can anyone ELI5?

Chances are that those big stud-mount rectifiers have their anodes internally connected to the stud. So it is convenient for the manufacturer to mount them on a single plate together -- that way the two anodes get connected by the mounting plate to form the negative output rail.

If those same diodes were to be used for a positive output, then the two studs would have to be isolated from each other. That would complicate the mechanics, and might require separate plates or mica washers, etc.

It does not matter whether the rectifiers produce a negative rail (as is done here) or a positive rail (as shown in dogdog's post) -- as long as the black output lead goes to the negative rail and the red output lead goes to the positive rail everything will work fine.

 

[skeer]

Understood. I guess my ultimate hangup is how is Line used as DC positive?

 

[dogdog]

Understood. I guess my ultimate hangup is how is Line used as DC positive?

look at that physics diagram posted... switching both the diodes around and it will check valve the electron flow to the other direction... in theory that would make the center tap positive and the other two negative... again that is a weird arrangement, Don't think I have ever know the reason the advantage of arranging it this way. I am thinking it provide two return path or was that electron flow is from neg to pos..thingy.. some physics guy might be able to answer you why they do it this way instead of the conventional way.

If you are confused, just forget why the center is negative for now and read that article on how the rectifier and the center tap AC transformer works..

 

[nadogail]

If it works with fewer components it will be less costly to produce resulting in a lower price point and very probably more sales and more profits.

 

[dogdog]

If it works with fewer components it will be less costly to produce resulting in a lower price point and very probably more sales and more profits.

no I am talking about the Charger use of center tap as Positive in that diagram the OP posted. That is an odd one.. normally center tap is the negative in most chargers.

 

[nadogail]

Current flow is current flow, doesn’t make a lot of difference because it’s all about moving electrons.

 

[American Locomotive]

Understood. I guess my ultimate hangup is how is Line used as DC positive?

That could work if it was setup as an autotransformer. But more likely you misinterpreted how it was wired. It would be very dangerous to couple the output directly to the line, which would happen with an autotransformer.

 

[dogdog]

The Transformer isolate you from line voltage... the diagram never shows any connection between the primary side of the line and the secondary side of the transformer... a some point it looks like it was cross to the center tap but it didn't, it skips over in that drawing... if it was connected it would have a solid dot.

 

[MBfreak]

If you can read schematics, Dogdog has it absolutely correct.
His drawing is correct. And drawn easy to read.
The century drawing is made by an idiot. They show the centertap on the secondary winding coming out of the primary leg.

Ola

 

[TractorJeff]

I looked at the Century drawing. The center tap is on the secondary winding! If there were wire labels on the drawing, you would see it. Don't get hung up on the way its drawn! The drawing was probably done by a first year draftsman. He drew it the way he visually saw it on the Production floor! When a transformer is wound as a primary coil with the secondary wound over it. It is very common for the center tap to end up on the opposite side of the coil. Hence the drawing is as visually observed. I know, because I saw lots of those drawings in my younger years working for a transformer manufacturer!

 

[dave*99]

Chances are that those big stud-mount rectifiers have their anodes internally connected to the stud. So it is convenient for the manufacturer to mount them on a single plate together -- that way the two anodes get connected by the mounting plate to form the negative output rail.

If those same diodes were to be used for a positive output, then the two studs would have to be isolated from each other. That would complicate the mechanics, and might require separate plates or mica washers, etc.

It does not matter whether the rectifiers produce a negative rail (as is done here) or a positive rail (as shown in dogdog's post) -- as long as the black output lead goes to the negative rail and the red output lead goes to the positive rail everything will work fine.

It's also possible that the stud rectifiers are mounted on a metal heat sink plate that is screwed to chassis ground. That plate, and in turn the chassis are the negative rail. The rectifiers each have 2 studs. One carries the heat out from the diode junction in addition to providing an electrical connection. Less insulators are needed if the heat sink end can hard bolted to the chassis.