wierd rectifier output

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eccdbb

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Joined 2011
I have a problem with my rectifier output. The transformer output is:

An externally hosted image should be here but it was not working when we last tested it.


and after rectifying, it becomes:

An externally hosted image should be here but it was not working when we last tested it.



I'm using a Avel Y236653 transformer and kbpc2504 rectifier.

I have tested the rectifier with a 9v battery + DMM, and my connections should be correct, so um... what's going on?
 
E

eccdbb

Member
Joined 2011
I put the DMM in diode mode unconnected, and it says 3.14v. Then:

red | black | reading
AC1|DC+ | .46
DC-|AC2 | .46
AC2|DC+ | .46
DC-|AC1 | .46
DC-|DC+ | .84

All other connections are 3.14.

Looking at the terminal side of the rectifier, if DC+ is on the bottom right corner, AC1 is the top right, and AC2 is the bottom left. DC- is the top left.

What to make of this?
 
Mooly

Mooly

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Readings look OK on the face of it. All a DVM does on diode range is display the volt drop between the meter probes (3.14 volts open circuited) and when connected to something, the volt drop that passing a small constant current between the probes develops.

As was commented on, add a load and then scope it. Also is there a reservoir cap ?
 
Mooly

Mooly

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You must have a cap on the output to feed the 7912. At least 470uf for the current you mentioned. A diagram and photo would help. The 7912 is a negative regulator so the scope and ground would be taken (conventionally to understand the circuit) from the DC positive output of the bridge.
 
Mooly

Mooly

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Most obvious problem seems to be the lack of a reservoir cap. That's essential. Other than that and it looks OK .

You have AC going into the bridge. The - of the bridge is the ground or "zero" point and this is where your scope is clipped. The + of the bridge goes to the 7812 input. The middle pin of the 7812 goes to "zero" or ground. You need the caps.
 
Mooly

Mooly

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Its a bit difficult interpreting the scope readings without having the actual instrument in front of me. I've just put that transformer part number into google and it says its a 30-0-30 transformer. First point if that is correct is that its way to high for a 7812. It will almost certainly be zapped or otherwise damaged.

You should read as high as 35 volts per winding off load (rms on a DVM). That equates to around 42 volts peak to peak per winding. 70 volts and 84 volts respectively for both windings in series.

When you look at the unloaded output of a bridge on a scope it can look "wrong" because there is slight stray capacitance and leakage through the diodes of the bridge.
Try feeding just the bridge and place a resistor across the - and + output of the bridge and scope across that resistor. Mind the wattage at that voltage. You are probably looking at 5K6 or higher for a small resistor. Try that and see what you get :)
 
E

eccdbb

Member
Joined 2011
Ok, it works now. I guess the problem comes from diodes being in reverse, and capacitance not being drained.

First, output with a 10k resistor:
An externally hosted image should be here but it was not working when we last tested it.


Then 10k || 150uF cap, trigger at turn on:
An externally hosted image should be here but it was not working when we last tested it.


and a zoom:
An externally hosted image should be here but it was not working when we last tested it.
 
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