I've got a circuit where I'm feeding the output of a 230vac dimmer into a bridge rectifier and then the DC output of the rectifier is filtered with a huge inductor and a fairly large capacitor (I also have 2 inline surge limiting thermistors).

Anyway I got the circuit hooked up, tested it carefully and everything was working dandy. As a test load, I have 720 watts of incandescent bulbs - nice and bright. The highest I've had it up to is 150V across this load.

Since I had it all working, I wanted to measure the DC ripple and see how close I got it to what I modeled in LTSpice. So I got out my oscilloscope - a Tektronics 2215A that's been in a box at least 10 years. When I bought it (used) it came with 2 probes so I just grabbed one of them. I don't remember what the difference was between the probes. I attached the probe across the load with the red on the positive side and black on the negative.

As soon as I applied power, the probe actually exploded! The black probe popped off and flew apart.

Was this probe designed to measure current perhaps - so I just created a short circuit? Or is there something else that I obviously did stupid? I am certain that there weren't any wires touching - whatever short that was came from the probe.

Of course I shut off power right away, and then after checking things out I reapplied power with no probe. As soon as I started bringing up the dimmer from 0, the dimmer exploded quite loudly. The best I can guess is the original short damaged the dimmer.

Needless to say, I'll need to order a new dimmer and test the various components to make sure they weren't damaged. But I wonder if anyone has any insight as to what may have gone wrong? Its pretty obvious I did something stupid, but it would be nice to know what haha.

Here's the probe I used. The shroud is missing from the black hook because it blew off. The little black plastic piece where the red/black probes come out is stamped with the name "E-Z-HOOK". There's no model number, and the web site for EZ-Hook doesn't have this model - I'm sure its too old.

 

Does this circuit have a transformer? If there is no transformer to isolate the DC output from the AC mains then you're lucky you don't have arc-flash burns.

 

That is the reason this forum forbids the discussion of transformerless power supplies.

 

Nothing fancy. Just a clip ended wire for a BNC connector instrument. That is what I use for my oscilloscope.
Not intended to measure large currents, but voltages.
Passing a large current overheated the inner probe metal and melted the plastic latch inside the black EZ clip and as it is spring loaded, ejected the casing. No explosion.

 

Does this circuit have a transformer? If there is no transformer to isolate the DC output from the AC mains then you're lucky you don't have arc-flash burns.

Thanks nsaspook & Meixner - I hadn't even thought of it that way. I've got an application where I need a variable 70-150vdc, about 6 amps (10 max), without too much ripple. I actually discussed this on this forum here. There's a pic below that's an approximation of my circuit. Is there a transformer that would work with this? (the inductor weighs 45 pounds!)

Notes on the circuit:
I show an input sine wave. Actually its an SCR based dimmer from China. Its rated for 250V, 4000W. I've been told I could model that, but I don't know how. The dimmer is plugged into a 250V single phase circuit in my garage on a 30A breaker.

The stack of diodes is the rectifier.

R2 is actually 2 large thermistors in series, that should be at about 0.2 ohms once they get cooking.

Anyway this runs pretty nice in LTSPice but of course I'm not feeding a clean sine wave due to the dimmer, so I wanted to attach the oscilloscope to see what I was getting out.

Thanks again for the input.

 

Nothing fancy. Just a clip ended wire for a BNC connector instrument. That is what I use for my oscilloscope.
Not intended to measure large currents, but voltages.
Passing a large current overheated the inner probe metal and melted the plastic latch inside the black EZ clip and as it is spring loaded, ejected the casing. No explosion.

Interesting- I wonder if the spring went into the dimmer (it has holes for airflow) causing it to blow when I reapplied power.

Dumb question though - if I had the probe across the load, how would that current even pass through the probes? If you look at the circuit I posted above, the probes were connected across the load "R1" on the diagram. Shouldn't an oscilloscope be able to measure that voltage?

Sorry for the dumb questions. I'm an electronics hack and haven't tinkered for quite some time. I'd be wise to stay away from anything dangerous, but that wouldn't be fun haha.

Thanks!
Eric

 

Sorry for the dumb questions. I'm an electronics hack and haven't tinkered for quite some time. I'd be wise to stay away from anything dangerous, but that wouldn't be fun haha.

Thanks!
Eric

1. Your circuit is dangerous. You have already seen the effects of accidental grounding it via the mains ground on the oscilloscope.
2. Being killed or badly burned is not funny.

 

1. Your circuit is dangerous. You have already seen the effects of accidental grounding it via the mains ground on the oscilloscope.
2. Being killed or badly burned is not funny.

Thanks for the blunt advice. I just read up on what you're talking about, and your are right its no joking matter. I think I read a book on power supply design when I was 14 years old and this thread has jogged my memory a bit - I do recall reading about this. Well, I'll have to rethink this whole project but I do appreciate having the forum here to benefit from everyone's experience. (Now if I told you what I was planning to do with the output voltage you'd probably have me committed for sure haha)

But I still don't understand one thing... Why would the oscilloscope probe be grounded to the oscilloscope mains? Even if my circuit was properly isolated- wouldn't it still have a potential to the mains ground? How could one measure this with an oscilloscope? Or does this indicate a faulty ground in my scope?

Again, sorry dumb questions. But I do appreciate the opportunity to learn.

 

You created a ground loop from the circuit load back to mains earth by connecting the probe ground into a load voltage. The oscilloscope's ground probe is connected directly to mains earth through the 'scope chassis via the power cord's third (ground/earth) leg. You need to get a differential probe or use an isolation transformer to run your device under test if you want to make connections like that. If you used an isolation transformer to run your device under test, then that device is not referenced to mains earth anywhere and you'd be safe. The oscilloscope will still be referenced to mains earth, but the circuit under test isn't, so no current should flow. Please note that removing the ground pin or wire from your device under test does NOT necessarily make it safe, nor does a device using only two wires necessarily not have a connection to ground, as some have a connection with neutral to ground. An isolation transformer removes all physical (conductive) connection to mains wiring, and that is why it works safely. You could very easily have been killed, and that's why no one is taking kindly to your hahas. Another way to test this if your 'scope has the capability is to use 2 probes, one on either side of your resistor, and hook your oscilloscope ground clips to the ground lug on the front of your scope. Then you can set your 'scope to display a math waveform as CH1-CH2 and get your waveform. This way is a little more noisy, but it will work in a pinch.

Here are some Tektronix Application Notes on differential measurement, FYI:

http://www2.tek.com/cmsreplive/tirep/13220/51W_10540_1_2009.01.07.14.35.59_13220_EN.pdf

http://www.testunlimited.com/pdf/an/3AW_19134_0_2010.02.19.10.47.40_3214_EN.pdf

 

eKretz nailed it: the scope negative (that BLACK clip lead) is connected to the power line ground, which connects to neutral, so it makes a pretty damn good short when you put the black lead in the wrong place.

You coulda put your eye out!

A transformer is helpful here to isolate the supply during development. Depending on your application you may not need it later on during use.

 

Hopefully you have learnt a very important lesson-NEVER connect your scope directly to any mains supply or equipment connected to a mains supply without using an isolation transformer (unless the item you are testing has its own built in isolated transformer power supply)

 

Agreed!

And ALWAYS understand that a bridge rectifier attached to the AC mains has NO safe voltages on it's output, it can NEVER be grounded. All parts of your circuit are live and very dangerous.

 

I am a bit disappointed with the earlier posters who, rather than explain the dangers in a way that could teach a lesson, decided to make derisive posts or just say "you screwed up, don't do that." In situations such as this, in my opinion we have a responsibility to educate our fellow hobbyists about such dangers when we have the ability and opportunity to do so. What if this OP were your son, daughter, brother or sister, etc. Wouldn't you want someone to help out rather than simply calling them ignorant? What if, after he had come here asking for help, he had tried again and got himself hurt or killed? I should hope that none of you are so callous that it wouldn't have bothered you.

 

I am a bit disappointed with the earlier posters who, rather than explain the dangers in a way that could teach a lesson, decided to make derisive posts or just say "you screwed up, don't do that." In situations such as this, in my opinion we have a responsibility to educate our fellow hobbyists about such dangers when we have the ability and opportunity to do so. What if this OP were your son, daughter, brother or sister, etc. Wouldn't you want someone to help out rather than simply calling them ignorant? What if, after he had come here asking for help, he had tried again and got himself hurt or killed? I should hope that none of you are so callous that it wouldn't have bothered you.

My view of the OP was that he was operating out of ignorance of the danger and not stupid so his education was not the IMO first priority. Making him STOP from having a possible life changing event was.

From the OP:

Thanks for the blunt advice. I just read up on what you're talking about, and your are right its no joking matter. I think I read a book on power supply design when I was 14 years old and this thread has jogged my memory a bit - I do recall reading about this. Well, I'll have to rethink this whole project but I do appreciate having the forum here to benefit from everyone's experience.

 

I am a bit disappointed with the earlier posters who, rather than explain the dangers in a way that could teach a lesson, decided to make derisive posts or just say "you screwed up, don't do that." In situations such as this, in my opinion we have a responsibility to educate our fellow hobbyists about such dangers when we have the ability and opportunity to do so. What if this OP were your son, daughter, brother or sister, etc. Wouldn't you want someone to help out rather than simply calling them ignorant? What if, after he had come here asking for help, he had tried again and got himself hurt or killed? I should hope that none of you are so callous that it wouldn't have bothered you.

I do appreciate the constructive replies- and pointing out the ground loop was helpful. But I don't get my feelings hurt with the blunt feedback either. Once I was working under a spinning rotor of an Apache helicopter (working as a civilian contractor, just out of high school) - verifying some equipment that we were testing. When I was done I started walking slightly toward the tail rotor. The ground crewman tackled me to the ground, giving me a nasty cut where my chin met the concrete. Now, I fully knew the tail rotor was there and only planned to step 1 or 2 steps that direction. But he didn't know I knew that, and I certainly appreciate that he did his job. In building this circuit I've been extremely careful with the voltages involved. But that's not really apparent from my post. It did really bother me not knowing why that probe popped, but now its crystal clear.

Usually I tinker with low voltage stuff where the worst I'll get is a smoking transistor (I kinda like that smell). More often than not, the questions I've come to ask here have been pretty dumb, but I always learn from the full range of responses - so thanks to all for helping out.

 

I am a bit disappointed with the earlier posters who, rather than explain the dangers in a way that could teach a lesson, decided to make derisive posts or just say "you screwed up, don't do that." In situations such as this, in my opinion we have a responsibility to educate our fellow hobbyists about such dangers when we have the ability and opportunity to do so. What if this OP were your son, daughter, brother or sister, etc. Wouldn't you want someone to help out rather than simply calling them ignorant? What if, after he had come here asking for help, he had tried again and got himself hurt or killed? I should hope that none of you are so callous that it wouldn't have bothered you.

The "I read a book about power supply design when I was 14" comment rather tends to support their reaction.

When I was a lad,I was crazy about Aviation,& one of the books I read was called "Teach Yourself To Fly".
I devoured that book from cover to cover,& thought I had a pretty good handle on this flying stuff!

The years went on,I went into Electronics,& came to terms with the fact that I'd never be a "Top Gun".

In my late 20s,I joined a Gliding club.

We went up in the Blanik,the Instructor handed the controls over to me.

So far,all was well,& I flew straight & level OK.

He then said, "Bank to the right".

I banked to the right.

"Now straighten up"

I centred the stick,& much to my surprise,the Blanik stayed in its bank!

It turns out that to straighten up in a Sailplane,you have to apply opposite aileron.

So much for what I read when I was 14!

More on topic,his transformerless circuit uses a huge choke which is as heavy as the transformer he isn't using would be!

The thing he hung across the mains isn't "a probe's bootlace",it's a piece of coax with EZ Hooks at one end,& a BNC at the other.
I wouldn't have that across mains voltage levels,even using an Isolation transformer.
A proper X10 probe is the minimum requirement for this.

 

The "I read a book about power supply design when I was 14" comment rather tends to support their reaction.

When I was a lad,I was crazy about Aviation,& one of the books I read was called "Teach Yourself To Fly".
I devoured that book from cover to cover,& thought I had a pretty good handle on this flying stuff!

The years went on,I went into Electronics,& came to terms with the fact that I'd never be a "Top Gun".

In my late 20s,I joined a Gliding club.

We went up in the Blanik,the Instructor handed the controls over to me.

So far,all was well,& I flew straight & level OK.

He then said, "Bank to the right".

I banked to the right.

"Now straighten up"

I centred the stick,& much to my surprise,the Blanik stayed in its bank!

It turns out that to straighten up in a Sailplane,you have to apply opposite aileron.

So much for what I read when I was 14!

More on topic,his transformerless circuit uses a huge choke which is as heavy as the transformer he isn't using would be!

The thing he hung across the mains isn't "a probe's bootlace",it's a piece of coax with EZ Hooks at one end,& a BNC at the other.
I wouldn't have that across mains voltage levels,even using an Isolation transformer.
A proper X10 probe is the minimum requirement for this.

Thanks for the info. What electronics I do know, I kind of learned backwards. I can remember devouring books on electronics (anything I could find in the library in 1973) starting at about 8 years old. I understood very little, yet retained surprisingly a lot (young eager brain cells). But I never figured out how to apply much of it until finally taking a basic electronics course in college. Even something as simple as ohms law- I knew i=e/r before I knew basic algebra, but I never learned how to actually use it. But when I finally learned it was a huge light bulb moment. I had actually always planned to be a EE, but from the time I wrote my first line of software in about 1980 my fate was sealed. But every now and then I get the urge to fire up a soldering iron and build something I can hold in my hands.

In this case, this started out as a practical project. But when I probably should have looked at it and said, hmmm that's a bit too much (like needing a 45 pound choke that cost $300) finishing the project has become something of an obsession. But to me there's nothing more rewarding than the learning process. Now the fact that I'm able to come here and anonymously ask my stupid (and/or dangerous) questions sure helps! A few weeks ago I built a project on a tiny PCB with an 8 bit PIC MCU and 0402 resistors/capacitors (funny story actually) that I successfully SMT soldered on a $15 burner, now I'm building a high voltage circuit with a 45 pound inductor and capacitors the size of a beer can. Tons of fun. And in both cases, I couldn't have done it without the advice here on allboutcircuits filling in some of the missing pieces for me.

 

The thing he hung across the mains isn't "a probe's bootlace",it's a piece of coax with EZ Hooks at one end,& a BNC at the other.
I wouldn't have that across mains voltage levels,even using an Isolation transformer.
A proper X10 probe is the minimum requirement for this.

In this situation a 10X probe would glow as bright.

The ground of that probe is the same as the EZ hooks: wire in to wire out.

 

Yep, wouldn't make any difference at all. The probe's ground wire doesn't have any attenuation.

 

Something you could do that might be educational. In your spice model you posted in #5 move the ground node to the mains neutral - in reality this is how it's connected by the power company. Then look at the voltage on the node you connected your black probe to.

It's more useful in your simulation to put the ground where you did as it makes it easier to get results out, but never forget that in real life neutral is grounded so your negative rail can't be unless it's transformer isolated.

Other posters have suggested using an isolation transformer for development even if it's not included in the final circuit. For the power rating you want they'd be expensive but it would make testing easier and safer. I don't know about your country but here we can hire such things. There are ways of making measurements without it, but they are more tricky.

PS, if anyone ever suggests removing the mains ground connection from the oscilloscope in this kind of testing DO NOT DO IT - it makes the whole oscilloscope dangerous to touch.