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 Post subject: How Did I Blow This Fuse?
PostPosted: Jun Thu 10, 2021 4:29 pm 
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This is a TRIAC fan controller with an additional DC power supply that powers another part of the device.

In this diagram the ground in the red circle is created only by the diode bridge and associate components. The ground is not connected to mains ground. The circuit was working until I clipped the ground lead of my scope to it without using my isolation transformer, duh. Doing so blew the fuse and maybe the rectifier. I'm trying to figure that out before replacing the rectifier.

Replacing the fuse and hooking up the isolation transformer and variac, at 6 VAC the 220uF filter cap is 12 VDC. Higher than 6 VAC causes the diode bridge to get hot. The diode bridge does not appear to be shorted to ground, but I suspect it to be damaged. Why would touching the ground lead of the scope blow the rectifier?

In normal operation the filtered DC is 168 VDC, which is correctly 1.4 times119VAC. At 6VAC, why is the filtered DC double the AC value? Meter not accurate on AC?


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Thu 10, 2021 5:04 pm 
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This can happen if the 3rd prong (ground) is still connected on your oscilloscope power cord. Dont ask how I know. :-)


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Thu 10, 2021 5:51 pm 
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When the hot side of the power line is in it's negative half cycle, that circled circuit ground point will be up to -160 volts relative to earth ground. If the scope is grounded through the safety ground pin on the power plug, you will certainly blow the fuse and possibly a diode in the bridge. You probably have a partially melted diode in the bridge.

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Thu 10, 2021 6:32 pm 
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Airway Radio Fan wrote:
This can happen if the 3rd prong (ground) is still connected on your oscilloscope power cord. Dont ask how I know. :-)



+1. Isolation transformers are your friend. A friend I don't have. Arcs and sparks have happened if I am not careful....

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Thu 10, 2021 8:09 pm 
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Tom Schulz wrote:
When the hot side of the power line is in it's negative half cycle, that circled circuit ground point will be up to -160 volts relative to earth ground. If the scope is grounded through the safety ground pin on the power plug, you will certainly blow the fuse and possibly a diode in the bridge. You probably have a partially melted diode in the bridge.


That's what I was afraid of. Well here I am now using an isolation transformer, which admittedly is probably the appropriate way to do things. However if I were lazyer, could I have avoided the problem simply by using one of those three prong to two prong adapters on my scope power plug?


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Thu 10, 2021 8:55 pm 
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My understanding is that's unwise...

It has electrical risks for the scope itself, and if you were to touch the wrong exposed bit of scope, you could become the path to ground... (Such as say that exposed BNC connector while plugging/unplugging the probe...)

David


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 12:06 am 
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Macrohenry wrote:
That's what I was afraid of. Well here I am now using an isolation transformer, which admittedly is probably the appropriate way to do things. However if I were lazyer, could I have avoided the problem simply by using one of those three prong to two prong adapters on my scope power plug?


Many years ago I worked for a company testing temp controllers and we had the 2 to 3 prong adapters with the ground cut off to float the scopes to measure across an AC load. We looked at AC waveforms for zero crossing up to 480V. Our scopes were Tek scopes in plastic cases but it was still dangerous. Of course a triac exploding in your face was also. Those were good times.

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 12:59 am 
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I never use an isolation transformer with any of my scopes, but I don't plug them into a grounded receptacle either.


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 4:24 am 
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Tom Schulz wrote:
When the hot side of the power line is in it's negative half cycle, that circled circuit ground point will be up to -160 volts relative to earth ground. If the scope is grounded through the safety ground pin on the power plug, you will certainly blow the fuse and possibly a diode in the bridge. You probably have a partially melted diode in the bridge.


Tom, you nailed it. I tested the individual sections of the bridge rectifier unit and sure enough, the diode from ground to hot was shorted. I made a new bridge with 1N4007s, replaced the fuse, and she's running again. I did use the variac with an ammeter to monitor for any other shorts, so all good.

I have another unit that's drawing too much current so at least I have this unit as a standard to compare it to. Glad to have it working again. ARF RULES!


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 4:29 am 
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In your diagram, are "hot" and "neutral" the power source?

I'm definitely outside my limited knowledge or experience, but…
why not keep the scope grounded and use two probes in A+(-B) mode?

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 4:52 am 
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SteiniteFan wrote:
In your diagram, are "hot" and "neutral" the power source?

I'm definitely outside my limited knowledge or experience, but…
why not keep the scope grounded and use two probes in A+(-B) mode?

For those not familiar with this, you do not use the ground leads on the probes.
Many scopes can not do this at all. And of those that can, I think that it takes a very high quality scope to do it well enough.
If you want to measure power supply ripple you probably want to accurately measure down to a few volts, perhaps one or two. And the scope would have to accurately subtract out a 170 volt waveform and not overload the internal amplifiers with the gain set to see a few volt signal.

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 1:50 pm 
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Blustar1 wrote:
I never use an isolation transformer with any of my scopes...
That's correct. An isolation transformer is used only for the device you are testing.

Dave


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 3:45 pm 
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SteiniteFan wrote:
In your diagram, are "hot" and "neutral" the power source?

I'm definitely outside my limited knowledge or experience, but…
why not keep the scope grounded and use two probes in A+(-B) mode?

Yes the hot and neutral in the diagram correspond to the unit's plug blades. The ground wire from the plug is connected to the metal parts of the chassis that's isolated from the circuit board.

My scope is one of those Packard Bell / Heathkit single trace 5 MHZ training models that doesn't even have more than one input. I should have gotten a better scope by now, but every time I start to get one there's all these knobs looming in my face. I know why I haven't bought one. Aside from my dyslexia that makes it hard enough for me to keep up with what the few knobs mine has are doing, philosophically, there is a distinct satisfaction in getting things done using simple old equipment, especially when there are workarounds such as the 3 prong to 2 prong adapter while wearing rubber soles that generally should be avoided in favor using an isolation transformer on the device being measured.


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 6:09 pm 
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Thanks, Tom. That helps some, though I'm still easily confused about grounding and shielding, which I suspect are related. Preventing ground loops…ugh!
For instance, for an isolation transformer output, It seems reasonable to me to use a grounding receptacle which is connected to the input ground. Apparently that's not the prevailing opinion.

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Fri 11, 2021 8:40 pm 
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SteiniteFan wrote:
Thanks, Tom. That helps some, though I'm still easily confused about grounding and shielding, which I suspect are related. Preventing ground loops…ugh!
For instance, for an isolation transformer output, It seems reasonable to me to use a grounding receptacle which is connected to the input ground. Apparently that's not the prevailing opinion.


Here's my system. Theoretically the neutral is connected to the ground, but in my case there is some resistance there, enough to cause a 0.266 VAC differential. I don't know the practical effect of this, but I expect for the discussion above it's negligible. So for now, let's stipulate that neutral and ground are at the same potential.

My scope probe's ground wire is connected to the system ground (green in diagram). Because the ground (green) is system-wide connected to the neutral (white), connecting the scope probe to the diode bridge ground is the same as connecting the white wire to that point. Every other cycle the diode creates a short between hot and neutral, blowing that upper left diode.

The way to prevent blowing the diode is to use an isolation transformer on the circuit, which effectively disconnects the white from the green that's still connected to the scope probe. With the scope connected, the circuit ground is at the same potential as the green wire. For the rest of the circuit following the power supply shown, the diode bridge provides isolation from the mains. Do I have this right?


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Sat 12, 2021 1:19 am 
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The diode bridge does not provide isolation from the mains because each diode conduct every half cycle if it were a short.

Dave


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Sat 12, 2021 2:56 am 
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Dave Doughty wrote:
The diode bridge does not provide isolation from the mains because each diode conduct every half cycle if it were a short.

Dave

So this brings up a question about so called hot chassis. I always thought hot chassis necessarily meant that the metal chassis of a radio was connected to the mains. It's that correct?

So here the metal parts of the device are grounded at the mains, and the circuit board acts like a hot chassis in that the circuit ground carries hot potential and can shock the daylights out of you. Is that correct?


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Sat 12, 2021 4:22 am 
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There indeed were some radios where one side was connected to the chassis. Most of them, however, had the AC lines going to the rectifier plate and the other to a common B- bus with the heater string across it. Between that bus and the chassis there would be a capacitor which usually had enough reactance to limit the current to what wouldn't kill you, but could still give you a nasty shock. The big problem is most of them switched the neutral side of the line which left everything on the other side of it hot. As long as you had it plugged in the right way you'd only get zapped when it was off; if it wasn't you could get zapped on or off if you got between the chassis and any sort of real ground. I think there's a Mr. Carlson's Lab video that touches on this subject.

Scott Todd


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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Sat 12, 2021 5:02 am 
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Macrohenry wrote:
Dave Doughty wrote:
The diode bridge does not provide isolation from the mains because each diode conduct every half cycle if it were a short.

Dave

So this brings up a question about so called hot chassis. I always thought hot chassis necessarily meant that the metal chassis of a radio was connected to the mains. It's that correct?

So here the metal parts of the device are grounded at the mains, and the circuit board acts like a hot chassis in that the circuit ground carries hot potential and can shock the daylights out of you. Is that correct?

Yes, the circuit ground would be hot. Not quite the same as if it was connected to the hot side of the line, but hot enough. Now if you use an isolation transformer and then ground circuit ground then both sides of the line coming from the transformer will be hot .

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 Post subject: Re: How Did I Blow This Fuse?
PostPosted: Jun Sun 13, 2021 8:48 pm 
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Well, I thought this thread had run its course, but once burned, twice shy I am.

I set up to do the testing I started when I clipped the scope ground lead that blew the fuse and rectifier. This time I'm using a variac and isolation transformer.

Paranoid and curious, I inserted a 220K resistor between circuit ground and the scope ground lead to measure any differential. and yes, there's 6.5 VAC and 30mA across that resistor. I would have thought that with the floating AC, the circuit ground would quickly equalize to the scope ground. But 6.5VAC difference? Why? What does this imply?

My bottom line question is, is 6.5 VAC and 30 mA within the normal bounds enough to where it's ok for me to go ahead and clip that scope ground lead to the circuit ground?


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