Confused about a bridge rectifier

Thread Starter

drbenne

Joined Jul 30, 2013
48
Hey guys,

So, I'm troubleshooting this linear power supply (12Vdc) by doing the basics first. Tested primary side of transformer, tested fine (120Vac). Tested secondary output, seems fine (33.6 Vac). This is then connected directly to a what I'm pretty sure is a bridge rectifier (part no. 70CT01P, couldn't find a datasheet). Tested the DC output of the rectifier and get 0V

Unplugged all connections to the rectifier and used the diode test function of my meter. I'm getting a forward voltage drop across each diode and no voltage drop in reverse bias across each. Using these conditions I then drew out a schematic according to my tests and I'm getting confused as to how this particular rectifier is connected.

I attached a schematic I drew out according to my diode tests which doesn't seem right. Also, the labeling on the rectifier doesn't specify positive or negative for the DC output, it only has 2 positives. One can be seen on the top of the picture I took. The bottom side looks the same except the sine wave symbol says AC.


Thanks,

Dave
 

Attachments

ronv

Joined Nov 12, 2008
3,770
I think you probably have a center tap transformer then everything would make sense. The center tap would be ground, the 2 +'s tied together for the positive output.
 

Thread Starter

drbenne

Joined Jul 30, 2013
48
I think you probably have a center tap transformer then everything would make sense. The center tap would be ground, the 2 +'s tied together for the positive output.
Thanks :)

I looked at the bottom of the transformer and sure enough there was the center tap. Measured from the center tap and got a reading. Did a little more probing and found two blown pass transistors (2N3771). Replaced both with two 2N3055s temporarily to see if that fixed the problem, doesn't seem to have. Gonna order some 2N3771s today.

Dave
 

AnalogKid

Joined Aug 1, 2013
8,221
Your schematic of the bridge is incorrect. It matches the pin locations in the photo IF you reverse the two diodes going to the lower left pin and mark this pin -. In your photo the upper right tab (the one that is oriented differently from the other three) is the +DC output. The upper left and lower right tabs are the AC inputs, and the lower left tab is the -DC output. In other words, AC input on diagonally opposed corners, DC output on other two diagonally opposed corners.

The bridge is the same for transformers with and without a cernter tap.

ak
 

Thread Starter

drbenne

Joined Jul 30, 2013
48
Your schematic of the bridge is incorrect. It matches the pin locations in the photo IF you reverse the two diodes going to the lower left pin and mark this pin -. In your photo the upper right tab (the one that is oriented differently from the other three) is the +DC output. The upper left and lower right tabs are the AC inputs, and the lower left tab is the -DC output. In other words, AC input on diagonally opposed corners, DC output on other two diagonally opposed corners.

The bridge is the same for transformers with and without a cernter tap.

ak

That's what I thought too but, again, using the diode test function on my meter yields the schematic I drew, unless both diode directions spontaneously changed direction which I think is highly unlikely . I don't have the supply near me at the moment but on the underside of the rectifier it has a positive sign where you said should be the negative DC output in the lower left corner.
With the black probe on the center screw (ground) and the red probe on either DC output corner I get ~+22Vdc I believe. This confuses me also because shouldn't a rectified wave yield around the same as the AC input.
 

profbuxton

Joined Feb 21, 2014
418
I also believe you have a center tapped tx. That would make sense of the diode layout.
Your measurement of 22v dc sounds about right for a measurement to ground from one side of the output. Were the filter caps in circuit? As I understand your measurement you are only measuring half of the output. Is your 33vac across the total sec winding or from the center tap to one side?
 

Thread Starter

drbenne

Joined Jul 30, 2013
48
I also believe you have a center tapped tx. That would make sense of the diode layout.
Your measurement of 22v dc sounds about right for a measurement to ground from one side of the output. Were the filter caps in circuit? As I understand your measurement you are only measuring half of the output. Is your 33vac across the total sec winding or from the center tap to one side?
Yes, the filter caps are in the circuit. The transformer is actually a dual secondary output. One output is 10.6 Vac and the other is 33.6 Vac. They are both center tapped with the two centers tied to each other. I had to desolder one center tap from the board to confirm this. The 33 Vac is across the sec winding. From center to one side is 16.7 Vac.
 

ronv

Joined Nov 12, 2008
3,770
Seems all as it should be then. It sounds like your transformer is probably 32 volt center taped that is reading a little high so 16.7 volts to the center tap. The meter measures RMS on ac, but when the voltage is applied to the caps it charges them to the peak of the AC waveform so 1.41 X as high. 23.57 volts, but the diodes drop it by .6 X 2 or 1.2 volts less at 22.4 or so.
The 3055's probably should have worked so there might be something else going on. Is there an adjustable voltage regulator as well (Maybe like an LM317)? What ever drives the big transistors may be bad. If you have a schematic it could be a life saver.
 

Thread Starter

drbenne

Joined Jul 30, 2013
48
Seems all as it should be then. It sounds like your transformer is probably 32 volt center taped that is reading a little high so 16.7 volts to the center tap. The meter measures RMS on ac, but when the voltage is applied to the caps it charges them to the peak of the AC waveform so 1.41 X as high. 23.57 volts, but the diodes drop it by .6 X 2 or 1.2 volts less at 22.4 or so.
The 3055's probably should have worked so there might be something else going on. Is there an adjustable voltage regulator as well (Maybe like an LM317)? What ever drives the big transistors may be bad. If you have a schematic it could be a life saver.
unfortunately I don't have a schematic. I tried looking but couldn't find one anywhere. It's a Tripp-lite PR-20 if someone happens to know where one is :confused:
 

Thread Starter

drbenne

Joined Jul 30, 2013
48
No, this is one is pretty different. I couldn't identify a regulator on this one. I did see a LM614CWM and LM393N though.
 

Thread Starter

drbenne

Joined Jul 30, 2013
48
Here is somewhat of a crude schematic I drew up to help me diagnose. I checked as best as I could to ensure accuracy. I'll probably have to go back and add resistor values, perhaps at a later time as needed. The ICs are as follows:

2n3771
LM614CWM
2n6388
LM393N
TL431
MCR692G
 

Attachments

Thread Starter

drbenne

Joined Jul 30, 2013
48
Wow guys.. :eek:

I put it back together to start probing in different areas and went to check the output and lo and behold, there is a nice 13.8Vdc . Put a load on it and it stayed the same. Not sure what the problem was. I know for sure those 3771s were no good. :confused:
 

LDC3

Joined Apr 27, 2013
924
Your schematic of the bridge is incorrect. It matches the pin locations in the photo IF you reverse the two diodes going to the lower left pin and mark this pin -. In your photo the upper right tab (the one that is oriented differently from the other three) is the +DC output. The upper left and lower right tabs are the AC inputs, and the lower left tab is the -DC output. In other words, AC input on diagonally opposed corners, DC output on other two diagonally opposed corners.

The bridge is the same for transformers with and without a center tap.

ak
The way this bridge is used is the outer taps from a center tap transformer are connected to the 2 AC inputs and the DC ground is connected to the center tap. That way you get a positive output from each 1/2 for a half cycle. If you look at circuit that was drawn by drbenne, you can see that is how it is wired.
 

inwo

Joined Nov 7, 2013
2,419
I think we all know how a full wave center tap supply works.:cool:
The question seems to be about a rectifier package that has two common anode and two common cathode.

That couldn't rightly be called a bridge.:confused:

I can't think of a reason for designing that package, nor have I found another.
If the only use is parallel diodes. Then why not a half bridge of sufficient rating?

I guess maybe a dual supply from the same ac source. But what would be the advantage?
 
Top