# Help with mosfets

#### ian field

Joined Oct 27, 2012
6,539
I have a meter to test capacitors. They all read 2 or 3 microfarads over what is printed on them. Don't have a scope or isolating transformer. Wouldn't know where to put a scope probe on it. Just seems a waste to dump it.
If you don't know what you're doing - don't put your scope anywhere near the TV!

The scope should be earthed, parts of the TV PSU are equal to the mains peak value, the bit you might think is earth/common is actually about 320V negative (for 230V TV).

#### cmartinez

Joined Jan 17, 2007
6,599
This practice has killed more than a few engineers in the past!
That practice killed one of the channels in a Hantek USB scope that I have, it also killed the USB port in the laptop to which it was connected to... fortunately it didn't kill more things, such as myself.
Lesson learned... thanks for the advice @recklessrog... too bad I had to learn things the hard way

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#### recklessrog

Joined May 23, 2013
985
It seems that my meaning of "do not float" test equipment has been misunderstood, so to be absolutely clear, here it is in plain English.
DO NOT UNDER ANY CIRCUMSTANCES REMOVE THE "EARTH CONNECTION" FROM THE MAINS/LINE PLUG OF ANY TEST EQUIPMENT AND THEN CONNECT THE TEST EQUIPMENT TO ANY EQUIPMENT THAT IS DIRECTLY CONNECTED TO THE INCOMING MAINS SUPPLY AS THE TEST EQUIPMENT WILL NOW BE "FLOATING" AT LINE VOLTAGE AND BECOMES A SERIOUS SHOCK HAZARD. USE AN ISOLATING TRANSFORMER TO POWER THE EQUIPMENT UNDER TEST AND LEAVE THE EARTH WIRING CONNECTED TO THE TEST EQUIPMENT.

IF YOU HAVE NOT GOT AN ISOLATING TRANSFORMER, THEN LEAVE THE REPAIRS TO SOMEONE WHO HAS!

"FLOATING" IN THIS INSTANCE HAS NOTHING TO DO WITH IMMERSING IN WATER AS ONE PERSON SEEMS TO THINK!

#### ian field

Joined Oct 27, 2012
6,539
It seems that my meaning of "do not float" test equipment has been misunderstood, so to be absolutely clear, here it is in plain English.
DO NOT UNDER ANY CIRCUMSTANCES REMOVE THE "EARTH CONNECTION" FROM THE MAINS/LINE PLUG OF ANY TEST EQUIPMENT AND THEN CONNECT THE TEST EQUIPMENT TO ANY EQUIPMENT THAT IS DIRECTLY CONNECTED TO THE INCOMING MAINS SUPPLY AS THE TEST EQUIPMENT WILL NOW BE "FLOATING" AT LINE VOLTAGE AND BECOMES A SERIOUS SHOCK HAZARD. USE AN ISOLATING TRANSFORMER TO POWER THE EQUIPMENT UNDER TEST AND LEAVE THE EARTH WIRING CONNECTED TO THE TEST EQUIPMENT.

IF YOU HAVE NOT GOT AN ISOLATING TRANSFORMER, THEN LEAVE THE REPAIRS TO SOMEONE WHO HAS!

"FLOATING" IN THIS INSTANCE HAS NOTHING TO DO WITH IMMERSING IN WATER AS ONE PERSON SEEMS TO THINK!
But it would blow the fuse and render the equipment safe...........................

#### recklessrog

Joined May 23, 2013
985
But it would blow the fuse and render the equipment safe...........................
WRONG!!!! if you remove the earth from the test equipment then it will not "blow the fuse" it will be at whatever potential the ground clip of the probe is connected to.

Don't spread dangerous mis-information! Someone could well get hurt!!!
For anyone wanting a verification of my post, then go to Tektronix training web pages
where there is a whole article on that very subject!!!!

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#### ian field

Joined Oct 27, 2012
6,539
WRONG!!!! if you remove the earth from the test equipment then it will not "blow the fuse" it will be at whatever potential the ground clip of the probe is connected to.

!!
For a start, I didn't think anyone would be daft enough to throw their scope in the bath.

And most TVs don't have an earth - but they still go bang when they fall in.

Maybe you're using absolutely pure distilled water, which isn't electrically conductive and doesn't short anything out.

#### recklessrog

Joined May 23, 2013
985
Please don't make a farce out of such a dangerous practice of removing test equipment earths, someone is going to think it's ok to do it and have one hand on the "Floated" piece of equipment, then touch an earthed object and likely get killed.

I thought any posts on here that advocated using equipment not isolated from the mains was not allowed because of the danger. trouble is, no matter how much you tell some people NOT to do something, once they've been given the idea, they will try it.

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#### crutschow

Joined Mar 14, 2008
23,532
When doing any kind of electrical testing you should always have everything plugged into a GFI or GFCI (Ground Fault Circuit Interrupter) device.
It's cheap insurance (generally <US$15 for wall outlet types and <US$25 for plug-in types) so that you or your test equipment won't get seriously zapped if there is some accidental path between the main's hot wire and earth ground.
(Minimizes Darwin Award candidates).

#### ian field

Joined Oct 27, 2012
6,539
Please don't make a farce out of such a dangerous practice of removing test equipment earths, someone is going to think it's ok to do it and have one hand on the "Floated" piece of equipment, then touch an earthed object and likely get killed.

I thought any posts on here that advocated using equipment not isolated from the mains was not allowed because of the danger. trouble is, no matter how much you tell some people NOT to do something, once they've been given the idea, they will try it.
Some people would still be a danger to themselves even if they quit electronics and took up embroidery!

Still - there's always publication in the Darwin awards for those of exemplary achievement.

#### Wendy

Joined Mar 24, 2008
21,906
Ian, I think you are trying to be funny, or at least I hope you are, but please do not advocate unsafe procedures, even in jest. We value safety very highly here, and there really is a reason for earth ground.

#### ian field

Joined Oct 27, 2012
6,539
Ian, I think you are trying to be funny, or at least I hope you are, but please do not advocate unsafe procedures, even in jest. We value safety very highly here, and there really is a reason for earth ground.
Just making it clear that some people think they will always get away with it when they most likely won't.

Apart from being an entertaining read - the Darwin Awards is very sobering. Its always better to learn by, and not make the mistakes of others.

Especially the one's that removed themselves from the gene pool.

#### recklessrog

Joined May 23, 2013
985
Just making it clear that some people think they will always get away with it when they most likely won't.

Apart from being an entertaining read - the Darwin Awards is very sobering. Its always better to learn by, and not make the mistakes of others.

Especially the one's that removed themselves from the gene pool.
Please do not continue these irresponsible posts Ian, I know you are jesting, but we have a responsibility to everyone on here to only submit safe and proper advice. While some (me included), have had severe electric shock from faulty equipment and survived, apparently there have been deaths from testing a 9volt battery on their tounge!!!
Full line voltage, even with circuit breakers can put enough current through the heart to cause fibrillation, and
without immediate medical assistance, that person is most likely going to die.
I too have laughed at some of the Darwin awards, but by correct education, we may save someone from joining that pool. Preferable than hearing later on that someone took your joking the wrong way and then ended up dead!!!

#### ian field

Joined Oct 27, 2012
6,539
Please do not continue these irresponsible posts Ian, I know you are jesting, but we have a responsibility to everyone on here to only submit safe and proper advice. While some (me included), have had severe electric shock from faulty equipment and survived, apparently there have been deaths from testing a 9volt battery on their tounge!!!
Full line voltage, even with circuit breakers can put enough current through the heart to cause fibrillation, and
without immediate medical assistance, that person is most likely going to die.
I too have laughed at some of the Darwin awards, but by correct education, we may save someone from joining that pool. Preferable than hearing later on that someone took your joking the wrong way and then ended up dead!!!
If you think that's a joke - I feel very sorry for you.

It is a warning to the "it won't happen to me" brigade, that such an attitude probably means it will.

#### recklessrog

Joined May 23, 2013
985
If you think that's a joke - I feel very sorry for you.

It is a warning to the "it won't happen to me" brigade, that such an attitude probably means it will.
It's precisely because I don't think it is something to make light of, that I am surprised at your apparent lighthearted approach to the subject, and so is Wendy the moderator!

#### ian field

Joined Oct 27, 2012
6,539
It's precisely because I don't think it is something to make light of, that I am surprised at your apparent lighthearted approach to the subject, and so is Wendy the moderator!
I think I made it clear enough that its best not to copy the mistakes of those who removed themselves from the gene pool.

But it looks like you're not too difficult to confuse!

#### recklessrog

Joined May 23, 2013
985
I think I made it clear enough that its best not to copy the mistakes of those who removed themselves from the gene pool.

But it looks like you're not too difficult to confuse!
It seems that you just can't take genuine criticism of your posts, I'm quite sure that any reasonable person can see your silly remarks for what they are.

#### ian field

Joined Oct 27, 2012
6,539
If you'd hit the mark - I'd have taken your criticism on board.

But an observation that was a deadly warning to the seriously stupid - you confused for a joke.