Continue to Site

# full-wave bridge rectifier problem

Status
Not open for further replies.

#### MsStateDawg

##### New Member
I am having to build a full-wave bridge rectifier circuit to test another circuit. The bridge should rectify 120 VAC from a wall outlet to obtain a DC voltage. I realize that the maximum possible DC value that can be produced is 169 VDC. I am trying to acheive as close as possible to 170 VDC. I connected the circuit shown at the following site one of the 3A fuses blew:

I don't understand why the fuse would blow with a 20k-ohm load. Does anyone know what could be wrong?

The fuses shouldn't blow, unless you have some connections wrong.

Are you sure it's a 20K resistor? not 20ohm or something
got the + / - and AC connections on the bridge rectifier good?
+ / - of the electrolytic capacitor?

Also, try to measure the bridge rectifier, maybe it's shorted out for some reason. With a DMM in diode measuring mode you should be able to measure the 4 separate diodes in there. All 4 must conduct in one direction and not in the other.

Be extremely carefull when thinkering with this setup, no need to say when you accidently touch something you'll defenately feel it! If it works you'll have DC on the output wich is acutually more dangurous then AC

There will be inrush current to charge the cap up. that might have taken out yr fuse if your cap is high value. Without values for the cap a quick estimate for the inrush cannot be worked out.

Styx said:
There will be inrush current to charge the cap up. that might have taken out yr fuse if your cap is high value. Without values for the cap a quick estimate for the inrush cannot be worked out.

On the link he posted it says 220µF cap. That won't make a fuse blow

It's such a simple circuit, there can only really be three possibilities:

1) He's connected it up incorrectly.
2) He's got a faulty component.
3) He's used an incorrectly specified component and blown it.

Assuming option 1), the most likely cause is the bridge rectifier connected the wrong way, or next likely the capacitor connected the wrong way.

maybe tha cap is not rated for that voltage????
also check it for shortcircuit. the bridge might also have a problem as mentioned before......anyway, if you have some spare fuses, then try the circuit without the resistor and cap. then just without the resitor.
or, simply test them with the DMM.

Exo said:
Styx said:
There will be inrush current to charge the cap up. that might have taken out yr fuse if your cap is high value. Without values for the cap a quick estimate for the inrush cannot be worked out.

On the link he posted it says 220µF cap. That won't make a fuse blow

I have just done a quick Spice sim and I got above 10A peak inrush to the bridge rectifier - more than enough to take out the fuses. When I get to work tomorrow I will edit one of my 3ph rectifiers to these param and rerun under SABER for a more accurate result.

maybe tha cap is not rated for that voltage????
also check it for shortcircuit. the bridge might also have a problem as mentioned before......anyway, if you have some spare fuses, then try the circuit without the resistor and cap. then just without the resitor.
or, simply test them with the DMM.

If the voltage rating of the cap was less it would have blown up. Their could have been a prob with the rect but it is rated at 25A. If it isnt the inrush then it must be in the setup

Did you have any test equipment connected? A grounded instrument such as an oscilloscope will definitely blow your fuse.

Fuses (even "quick blow" types) need "some" time to blow. Typically, they take 1 - 2 secs. at twice their rated current at ambient temp. to blow, and, again the current must be continuous. The calculated 10A most probably would not produce enough I.t to blow the fuse. Some 40 - 75W switchers can have inrush currents in the order of 60A or more (due to the filter cap., and the flyback transformer. and yet are protected by 2A fuses.

As suggested earlier, try removing the load, then the cap. Just make sure to stock up on them fuses!

grounded scope and high capacitor current

Thanks alot for the help everyone. I finally got the full-wave rectifier to work. The problem was that the o-scope was grounding out the circuit. The startup current may have also been a problem. To elimate this, I used an auto transformer to turn up the ac voltage. Can anyone tell me how to make a differntial measurement where I can see the entire signal on one channel? I made a differential measurement by connecting both channel 1 and 2 negative wires to the common of the circuit being tested. Then I measured across the load using the positive wires from each channel. The o-scope showed the signal, but it had to be added to get the entire signal. I have attached a picture of the o-scope I am using. Thanks again for the help.

Re: grounded scope and high capacitor current

MsStateDawg said:
Thanks alot for the help everyone. I finally got the full-wave rectifier to work. The problem was that the o-scope was grounding out the circuit. The startup current may have also been a problem. To elimate this, I used an auto transformer to turn up the ac voltage. Can anyone tell me how to make a differntial measurement where I can see the entire signal on one channel? I made a differential measurement by connecting both channel 1 and 2 negative wires to the common of the circuit being tested. Then I measured across the load using the positive wires from each channel. The o-scope showed the signal, but it had to be added to get the entire signal. I have attached a picture of the o-scope I am using. Thanks again for the help.

If you are using an oscilloscope on this circuit you MUST feed the rectifier circuit from an isolating transformer, otherwise you can't connect the scope to it. To make differential measurements a scope normally has an option to invert at least one of the channels, you simply invert one of them, and set them to add - that gives a differential input.

There are other ways to connect a scope to a live circuit (rather than an isolating transformer), but you have to know what you are doing! - and I won't even mention it in an open forum, hopefully no one else will either!.

Re: grounded scope and high capacitor current

MsStateDawg said:
Thanks alot for the help everyone. I finally got the full-wave rectifier to work. The problem was that the o-scope was grounding out the circuit. The startup current may have also been a problem. To elimate this, I used an auto transformer to turn up the ac voltage. Can anyone tell me how to make a differntial measurement where I can see the entire signal on one channel? I made a differential measurement by connecting both channel 1 and 2 negative wires to the common of the circuit being tested. Then I measured across the load using the positive wires from each channel. The o-scope showed the signal, but it had to be added to get the entire signal. I have attached a picture of the o-scope I am using. Thanks again for the help.

Status
Not open for further replies.