Reading input and output of bridge rectifier on the same scope?

[EasyGoing1]

I have a 40 volt AC transformer that I put a bridge rectifier on, and I wanted to see the input signal and the output signal on the same screen on my scope, so I set one channel up with the test lead on one side of the input and the ground lead on the other side of the input, then I used another channel on the scope and clipped the test lead on the positive side of the output then the ground on the negative side of the output.

When I connected the transformer to power, the wire that was connected between the ground lead on the second channel and the negative side of the bridge rectifier literally caught on fire.



Why did it do this? And is there any way to achieve what I want to see on my scope?

[Per Hansson]

Re: Reading input and output of bridge rectifier on the same scope?

In short the reason is that the ground clip is connected to earth ground, and all of the channels of the scope share the same earth so you shorted primary to secondary...
To measure this safely you can use two probes instead of hooking up the ground clip: you will of course need twice as many channels to make measurements (i.e. 4x channels to take two readings).

Please watch this video: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!.

[stj]

Re: Reading input and output of bridge rectifier on the same scope?

just as well the ground clips are sold in packs of 10 on aliexpress!

[redwire]

Re: Reading input and output of bridge rectifier on the same scope?

Fixed: just as well oscilloscopes are sold in packs of 10 on aliexpress!

[EasyGoing1]

Re: Reading input and output of bridge rectifier on the same scope?

In short the reason is that the ground clip is connected to earth ground, and all of the channels of the scope share the same earth so you shorted primary to secondary...
To measure this safely you can use two probes instead of hooking up the ground clip: you will of course need twice as many channels to make measurements (i.e. 4x channels to take two readings).

Please watch this video: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!.

That guy pissed me off once ... he did a video where he fried his Weller iron because he plugged it into 220 when it was rated for only 110... and in the beginning of the video, he fully admits that it was his mistake and he took full responsibility for the damage ... then as the video went on, he started bad mouthing Weller for not building in protection to the unit to avoid damage like that ... which I felt was very unprofessional.

None-the-less, in the video you linked, his recommendation for taking readings from earth grounded equipment - was to purchase a $350+ device that isolates your scope channel so that it cant be damaged.

What I essentially did was this here ...



Which caught the wire between my scope clip and the rectifier on fire ... by shorting points A and B through the scope ...

I'm lucky I didn't damage the scope.

[EasyGoing1]

Re: Reading input and output of bridge rectifier on the same scope?

So basically, what I ended up doing by connecting the scope the way I did, was making this circuit...



Which means that for half of the AC cycle, I created a dead short?

[stj]

Re: Reading input and output of bridge rectifier on the same scope?

remind me again, you have 2 channels or 4 on your scope?

[Per Hansson]

Re: Reading input and output of bridge rectifier on the same scope?

Which means that for half of the AC cycle, I created a dead short?

Yes, to get around that instead of using an isolation probe (the proper tool for the job) you can utilize two oscilloscope probes as I alluded to in my post above.
What you do is simply hook one to positive, and the other to negative.
Then you use the math function of the scope to subtract one from the other.
This is also a trick you can use to look at really sensitive signals.
But it is not without its caveats, namely that the waveform could be a bit off depending on how complex the harmonics are.

P.S: I'm completely with Dave on how stupid it is to not have a fuse on a transformer.
All my Weller soldering stations do, removing it is just asking for a product recall after a house fire claim from an insurance company!

[stj]

Re: Reading input and output of bridge rectifier on the same scope?

did it even have a thermal fuse in the transformer?
i cant remember.

[Per Hansson]

Re: Reading input and output of bridge rectifier on the same scope?

did it even have a thermal fuse in the transformer?
i cant remember.

Not even that!

[redwire]

Re: Reading input and output of bridge rectifier on the same scope?

OP it's always good to have a fuse/1A circuit breaker because transformers can put out much more current into a short circuit.
Check the scope's internal ground did not melt open, I have seen that. The common BNC ground between channels can melt open because they aren't designed for many amps of current. And the internal connection to PE ground, should be OK but I did see a guy ground his scope to an old TV chassis and blow it. Just check continuity.

When breadboarding a bridge rectifier, it does need a little load like 10k ohm to swamp out the diode leakage currents. With no load the diode's waveforms can look weird due to their leakage currents and the scope at 1MEG input is little current. It also helps to Line trigger the scope, as the source.

Weller did screw up , and their safety agency too - in having no primary fuse. Funny is the double fusing on the secondary they have to thwart the standard. No, that won't stop any smoke. But who wouldn't check the label for 120/240V before plugging it in?
I just took apart my old Weller station and added a big red LED for power on and it has a mains fuse.

Class II "energy limited" power transformers, used in HVAC (furnaces, zone valves) and doorbells do have an internal 105°C thermal fuse that also pops due to high current. Then toss'em in the garbage though.
One old apartment building, a staple had shorted the 24VAC zone valve wiring but 15A fuse and so the wiring/building caught on fire! I made a replacement panel with circuit breakers for that. Scary.

[stj]

Re: Reading input and output of bridge rectifier on the same scope?

the BNC's on chinese scopes are mounted through tin shielding - your not burning that out!

[eccerr0r]

Re: Reading input and output of bridge rectifier on the same scope?

the BNC's on chinese scopes are mounted through tin shielding - your not burning that out!

LOL yes, same as Tek scopes or whatever, all probe shields are connected together through the chassis and most likely the places that will fail given typical scope probes:

a) most likely as OP experienced: DUT dies
b) probe clips melt down
c) BNC connector explode
d) probe wires fry... this is very tough as the short path is through the braid which is quite low resistance...

[EasyGoing1]

Re: Reading input and output of bridge rectifier on the same scope?

remind me again, you have 2 channels or 4 on your scope?

It's a 4 channel ... Siglent 1104X-E

[EasyGoing1]

Re: Reading input and output of bridge rectifier on the same scope?

LOL yes, same as Tek scopes or whatever, all probe shields are connected together through the chassis and most likely the places that will fail given typical scope probes:

a) most likely as OP experienced: DUT dies
b) probe clips melt down
c) BNC connector explode
d) probe wires fry... this is very tough as the short path is through the braid which is quite low resistance...

The interesting thing is, that the probe wire (talking about the ground lead that's clipped onto the probe) wasn't even warm. The wire that it was clipped onto was just a thin breadboard jumper wire ... about maybe 30mm long. and it caught on fire right next to me ... when it did that I realized that there was nothing I could do about the problem cause it had already done whatever it was gonna do faster than I could blink.

OP it's always good to have a fuse/1A circuit breaker because transformers can put out much more current into a short circuit.

Check the scope's internal ground did not melt open, I have seen that. The common BNC ground between channels can melt open because they aren't designed for many amps of current. And the internal connection to PE ground, should be OK but I did see a guy ground his scope to an old TV chassis and blow it. Just check continuity.

Are you saying I should take the scope apart and check these wires?

When breadboarding a bridge rectifier, it does need a little load like 10k ohm to swamp out the diode leakage currents. With no load the diode's waveforms can look weird due to their leakage currents and the scope at 1MEG input is little current. It also helps to Line trigger the scope, as the source.

Lets see ... an American Power Conversion Server class UPS unit transformer ... (about 5 lbs of transformer) ... kicking out 40 V AC rectified down to 20 volts with a 10k resistor across the output of the rectifier? And do this BEFORE looking at the signal?

Weller did screw up , and their safety agency too - in having no primary fuse. Funny is the double fusing on the secondary they have to thwart the standard. No, that won't stop any smoke. But who wouldn't check the label for 120/240V before plugging it in?
I just took apart my old Weller station and added a big red LED for power on and it has a mains fuse.

Class II "energy limited" power transformers, used in HVAC (furnaces, zone valves) and doorbells do have an internal 105°C thermal fuse that also pops due to high current. Then toss'em in the garbage though.
One old apartment building, a staple had shorted the 24VAC zone valve wiring but 15A fuse and so the wiring/building caught on fire! I made a replacement panel with circuit breakers for that. Scary.

I'll take issue with this ... I do believe that iron that he fried was an iron that is only sold in the US ... which means the likelihood that it would ever be plugged into anything larger than 110V ... is slim to none ... since the plugs aren't even compatible. That was NOT a Weller design flaw ... that was dumb-ass not paying attention and then trying to not look stupid by bad mouthing Weller - unacceptable for a professional in my opinion.

[stj]

Re: Reading input and output of bridge rectifier on the same scope?

the scope groundclip was probably decent copper - your jumper may have been copper coated aluminium or steel.

that said, pull the rubber boot back on the groundclip and inspect where the wire joins the clip.
some chinese ones (owon comes to mind) are not well crimped - they should be soldered.

[EasyGoing1]

Re: Reading input and output of bridge rectifier on the same scope?

Yes, to get around that instead of using an isolation probe (the proper tool for the job) you can utilize two oscilloscope probes as I alluded to in my post above.
What you do is simply hook one to positive, and the other to negative.
Then you use the math function of the scope to subtract one from the other.
This is also a trick you can use to look at really sensitive signals.
But it is not without its caveats, namely that the waveform could be a bit off depending on how complex the harmonics are.

Honestly, the only reason I want to show the AC signal and the UNFILTERED rectified signal on the same screen is that I have a friend who is basically a metal forger, who is very intelligent but doesn't really have any electronics knowledge. And I was trying to explain to him how we convert AC to DC and how CAPs and other components can clean up the rectified signal, and I thought a nice visual would help him comprehend it a little more.

Precision measuring in this case is by no means a requirement ... I just wanna see input and output on the same screen.

I just wanted frickin laser beams on sharks ... is that too much to ask?

[stj]

Re: Reading input and output of bridge rectifier on the same scope?

weller (and hakko, etc.) should just make world models
removeable cables and multi-tapped transformers with a selector-switch.
(and a fucking fuse!)

if the japanese hifi companies could do it in the 70's then why cant everybody else?

[EasyGoing1]

Re: Reading input and output of bridge rectifier on the same scope?

weller (and hakko, etc.) should just make world models
removeable cables and multi-tapped transformers with a selector-switch.
(and a fucking fuse!)

if the japanese hifi companies could do it in the 70's then why cant everybody else?

My cheap $125 Weller station has a fuse ... I haven't opened my newer one ... and I won't ... just to see if it has a fuse...

Also, here are both ground clips with boots off ... and the cable / would-be fuse/oscilloscope guardian angel ...