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FW rectifier - can't measure DC

Kaine

Nov 25, 2015
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Hello.

Building breadboard projects, including a full wave rectifier, but encountering a problem when trying to observe the voltages with a 'scope.

With half wave rectifier and an LED and resistor as a load, I can get both signals up and observe HW imposed over the AC sine wave. Yet with FW, no matter where I try to connect probes on the output I seem to burn out diodes or LEDs. Clearly I'm doing something daft, and it was getting late.

Any pointers? Thanks.
 

Harald Kapp

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What's your AC source (transformer, i hope)? Where's your ground connection? It seems your short-circuiting something by the scope's ground. Show us a schematic diagram of your circuit and the placement of the scope probes.
 

Kaine

Nov 25, 2015
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Thanks for your response Harald.
I've crudely adapted this stock image to reflect my probe positions. Wherever I attempt to measure the DC output I either blow the LED or a diode.
fwbr with probe positions.jpg
 

(*steve*)

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Make sure your multimeter is in voltage and not current mode.

if using a scope, ensure you don't connect the probe's ground clip to any point other than ground on a grounded circuit.
 

(*steve*)

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oh, if you're trying to use a dual channel scope to view both AC and DC on your circuit simultaneously, the ground leads will short things out of you connect then as shown. The ground side of the probes is connected.
 

Kaine

Nov 25, 2015
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Thanks for the reply. Are you then suggesting that it could be something as simple as not observing polarity of the scope's DC channel probes?

I can display a HW version using one diode perfectly, imposing the reduced height HW peaks over the AC trace. I really wanted to achieve the same result with FW and am sure that I'm missing something blindingly obvious.
 

Harald Kapp

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Are you then suggesting that it could be something as simple as not observing polarity of the scope's DC channel probes?
I don't think so. A scope normally has AC inputs.

When using a dual channel scope connect either channel 1 as shown in your schematic or channel 2, but do never connect both as the ground lead will short out the rectifier leading to many possibly detrimental effects.
 

Kaine

Nov 25, 2015
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OK, thanks again Harald.

I assumed that the very presence of 2 channels would lend itself to such a comparison easily, particularly as I've achieved such with the HW rectifier. I'll make do with showing the DC output on its own.
 

GPG

Sep 18, 2015
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Thanks for your response Harald.
I've crudely adapted this stock image to reflect my probe positions. Wherever I attempt to measure the DC output I either blow the LED or a diode.
fwbr-with-probe-positions-jpg.23407
Draw a wire between one of the CH1 leads an one of the CH2 leads. This is the common ground of your probes. Its a short.
 

Tha fios agaibh

Aug 11, 2014
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I can see this short circuit burning out diodes in the bridge, but why would the led burn out?
 

pgib8

Jul 26, 2015
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on my scope, the ground leads of each probe are internally connected and they are also connected to real ground. my scope has a battery so i can operate it in an isolated mode but the grounds of each channel are still connected.

could it be possible that through the ground leads, you are putting -12V below that resistor, and actually giving the LED twice the voltage or am i totally confused right now (lol)?
 

Harald Kapp

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It would help a great deal if you marked the ground leads in your diagram in post #3.

Apart from that, simply take care not to short-circuit any components by connecting the ground leads to different potentials.
 

pgib8

Jul 26, 2015
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could it be possible that through the ground leads, you are putting -12V below that resistor, and actually giving the LED twice the voltage or am i totally confused right now (lol)?

Had to think about this one and I was in fact confused. So maybe then, you put the ground lead above the resistor and effectively bridged it. In that case your drawing is inaccurate.
 
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