You correct that misapprehension that some techs have, who say that they do not need an isolation transformer when working on a valve guitar amplifier, because it has it's own transformer in its power supply and is therefore 'already isolated'. They fail to understand that the secondary of the PSU transformer is grounded, and thus not isolated from the mains. They need to watch your well-informed video to put them right.
Using an isolating transformer does not help in this scenario. The isolation transformer would only 'protect' you if you came in contact with the circuitry on the primary side of the power transformer. On a guitar amplfier this circuitry is normally fairly trivial and not exposed. As you say, one side of the HV secondary is normally grounded - so this is akin to having an isolation transformer with the ground still connected, and increases the shock hazard. Using an isolation transformer does not change this.
Although a brief description still very well detailed and can be well understood with the drawings provided. Well done and keep up the good videos coming. In my case I have a 1.5KW isolation toroidial transformer which I modified from a commercial pro amplifier. It gave 100v-0-100v in the secondary and I added a winding in the middle of the secondary to add the missing 40v. This powers my work bench and all repairs are plugged into it including my test equipment except the oscilloscope which has its own isolating transformer. My system is all floating from ground and to get electrocuted I have to touch both wires simultaneously which is not very likely to happen.
I am trying to size a small isolation transformer. A 220V source will be connected to my transformer, which in turn will supply the electric to my 220V 40W 0.45A induction motor with gear head to be controlled with a speed controller. Can you advise what VA transformer do I need for this purpose?
The wall outlet RCD only protects for leakage current up to the primary of the transformer there is no earth leakage protection on the secondary. But if the outlet from isolation transformer has an earth pin, you may use a RCD plug on the secondary.
This issue with this, is that when it comes to a piece of equipment where a 0V of the Unit under test, is directly tied to Earth/Chassis. When your scope is earth referenced, and you clip the earth of the probe to lets say +50V supply, then your scope will connect that 50V to earth, which then goes to the Earth in the building, then back through the earth of the isolation transformer, to the 0V of the UUT. So you have shorted out that supply in a big loop. So depending on the appliance, it may be required to break the earth to the UUT, so that you can freely probe anywhere on the circuit. But IMO is less safe.
The earth symbol you use at 4:27 is chassis ground isn't it? Shouldn't it be the earth ground symbol? Or maybe you were just looking for a way to show the surface of the earth :-)
Nice video - Some questions: I was taught that test equipment (scope, etc.) should not float, while the DUT via a iso transformer can. You suggest floating both, is that safe? Furthermore what are the consequences of the other was around (float the scope via iso transformer and connect the DUT direct to mains? Finally, A friend has his bench setup like this: mains to GFCI (ground-fault circuit interrupter) receptacle then a iso transformer plugged into that. Isn't this self defeating in the sense that the GFCI will never trip because of the iso transformer? What do you think?
Ken Cosi Floating the scope is fine provided you understand what you're doing - firstly you need to ensure the scope still has a protective earth, meaning that the chassis should typically be at the potential of one of the incoming line conductors so that if there is a fault in the unit the user is still safe. Secondly, if your DUT is mains earthed reference and you're probing across a component that's above a few tens of volts referenced to mains earth, you need to understand that the chassis of the scope will be at this potential and could be dangerous. Regarding your GFCI question, the GFCI will only protect against shock from the transformer itself, which is not necessarily an unreasonable precaution. Thanks for watching!
I worked in electronics for most of my life in both test and engineering. We floated 90% of our test equipment so we could accurately see what was going on inside the DUT. You had to be careful when doing this but in all those years we never lost anyone, a couple of folks got woken up but that was it. If you don't know how to do this safely you shouldn't be in the lab. When working on something like a radar transmitter power supply the equipment was all grounded just for extra safety - you don't want to lean on a scope thats floating at 24,000 volts
Nice detail in this video with less of the hand-waving I've found in other videos on this topic - although you almost fell into that trap by not noticing that you were connecting ground to neutral and claiming that would cause a short :-) When others have mentioned this "50:50" chance of causing a short then I've been confused by what they mean but your video makes this clearer.
Very good video. Isolation transformers are hard for people to understand. The main thing, I think to remember, is the main purpose of the isolation transformer for a tech bench is to separate the neutral and the earth ground. That way if the DUT is earth grounded, the neutral is not and the circuit is only the hot and neutral of the output of the isolation transformer. Be aware most isolation transformers for sale today have the neutral and the earth ground bonded together at the output receptacle defeating the separation and for a tech bench one would have to modify the isolation transformer for tech use, in my opinion. Tom
I bought an $250 I/T for my 14 year old future electrical engineer. It will protect the scope and other equipment but not his life. I have 22 circuits in my house and all circuits each have a RCD (residual current device) which trips in 30 m/s and at a 15 mA imbalance. They are mandatory in new homes in Australia where we have 240 vac and the common ground (neutral) is tied to the ground stake as in may other countries for safety. I can with bare feet, and bare hands place an metal object into the active on any power outlet and the RCD trips. The RCD costs about $15. I am afraid to let my son use the I/T,,, and I am bewildered why I bought it for him... perhaps because I watched so many UA-cam videos on them... Can anyone please comment, thanks.
If the outlet from isolation transformer has an earth pin, you may add a RCD plug on the secondary. Isolation transformer with a RCD plug on the secondary can power DUT and an oscilloscope can be safely used on a DUT. The RCD plug on the secondary of the I/T can protect one's life in case of inbalance. Nothing will protect a person if that person grabs one wire with one hand and the other wire with the other hand.
Yes, although there is no live/neutral really on the secondary side until you introduce the earth conductor again. The "live" would essentially become neutral because it is now at earth potential. The other terminal would be "live".
hi. How do you tell which end of the secondary is the live one? Or are they floating until one of them gets connected to earth ground e.g. by connecting the ground clip of a oscilloscope?
SDG Electronics But if you provide an earth reference, e.g. connecting the ground clip of oscilloscope, then aren't you going back to the same situation as a non-isolated transformer? This means the circuit is earthed and if the +ve end of probe touches any part of the circuit which is earthed or grounded, it will create a short via the oscilloscope. So how can we do any probing in this case since we can't tell whether what we are probing is negative or positive? Could you please help to clarify. Thanks.
Thank you for the video, a good refresher for me.
You correct that misapprehension that some techs have, who say that they do not need an isolation transformer when working on a valve guitar amplifier, because it has it's own transformer in its power supply and is therefore 'already isolated'. They fail to understand that the secondary of the PSU transformer is grounded, and thus not isolated from the mains.
They need to watch your well-informed video to put them right.
Using an isolating transformer does not help in this scenario. The isolation transformer would only 'protect' you if you came in contact with the circuitry on the primary side of the power transformer. On a guitar amplfier this circuitry is normally fairly trivial and not exposed. As you say, one side of the HV secondary is normally grounded - so this is akin to having an isolation transformer with the ground still connected, and increases the shock hazard. Using an isolation transformer does not change this.
Although a brief description still very well detailed and can be well understood with the drawings provided. Well done and keep up the good videos coming.
In my case I have a 1.5KW isolation toroidial transformer which I modified from a commercial pro amplifier. It gave 100v-0-100v in the secondary and I added a winding in the middle of the secondary to add the missing 40v. This powers my work bench and all repairs are plugged into it including my test equipment except the oscilloscope which has its own isolating transformer. My system is all floating from ground and to get electrocuted I have to touch both wires simultaneously which is not very likely to happen.
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I am trying to size a small isolation transformer. A 220V source will be connected to my transformer, which in turn will supply the electric to my 220V 40W 0.45A induction motor with gear head to be controlled with a speed controller. Can you advise what VA transformer do I need for this purpose?
The cats doing this, it's making him do this video.
Great video can you reccomend a good 1-1 transformer and where to get it ?
If your house is fitted with and RCD though, isn't using an isolation transformer less safe as you would loose the protection given by the RCD...
The wall outlet RCD only protects for leakage current up to the primary of the transformer there is no earth leakage protection on the secondary.
But if the outlet from isolation transformer has an earth pin, you may use a RCD plug on the secondary.
When building your own isolation transformer do you still connect the earth of the mains electricity to the earth on the output of the second winding?
It depends on the circumstances. You can, and the other alternative is to link the N and E on the secondary
This issue with this, is that when it comes to a piece of equipment where a 0V of the Unit under test, is directly tied to Earth/Chassis. When your scope is earth referenced, and you clip the earth of the probe to lets say +50V supply, then your scope will connect that 50V to earth, which then goes to the Earth in the building, then back through the earth of the isolation transformer, to the 0V of the UUT. So you have shorted out that supply in a big loop.
So depending on the appliance, it may be required to break the earth to the UUT, so that you can freely probe anywhere on the circuit. But IMO is less safe.
Nice cat!
The earth symbol you use at 4:27 is chassis ground isn't it? Shouldn't it be the earth ground symbol? Or maybe you were just looking for a way to show the surface of the earth :-)
Nice video - Some questions:
I was taught that test equipment (scope, etc.) should not float, while the DUT via a iso transformer can. You suggest floating both, is that safe? Furthermore what are the consequences of the other was around (float the scope via iso transformer and connect the DUT direct to mains?
Finally, A friend has his bench setup like this: mains to GFCI (ground-fault circuit interrupter) receptacle then a iso transformer plugged into that. Isn't this self defeating in the sense that the GFCI will never trip because of the iso transformer? What do you think?
Ken Cosi Floating the scope is fine provided you understand what you're doing - firstly you need to ensure the scope still has a protective earth, meaning that the chassis should typically be at the potential of one of the incoming line conductors so that if there is a fault in the unit the user is still safe. Secondly, if your DUT is mains earthed reference and you're probing across a component that's above a few tens of volts referenced to mains earth, you need to understand that the chassis of the scope will be at this potential and could be dangerous.
Regarding your GFCI question, the GFCI will only protect against shock from the transformer itself, which is not necessarily an unreasonable precaution.
Thanks for watching!
I worked in electronics for most of my life in both test and engineering. We floated 90% of our test equipment so we could accurately see what was going on inside the DUT. You had to be careful when doing this but in all those years we never lost anyone, a couple of folks got woken up but that was it. If you don't know how to do this safely you shouldn't be in the lab.
When working on something like a radar transmitter power supply the equipment was all grounded just for extra safety - you don't want to lean on a scope thats floating at 24,000 volts
What means DUT ???
@@incxxxx DUT means Device under test. :)
Your Cat looks awesome
is isolation trafo avail in 3 phase?
And in case of 1phase, if primery i/p is 220v P-N, what would be secondery o/p? P-P 220V?
Nice detail in this video with less of the hand-waving I've found in other videos on this topic - although you almost fell into that trap by not noticing that you were connecting ground to neutral and claiming that would cause a short :-) When others have mentioned this "50:50" chance of causing a short then I've been confused by what they mean but your video makes this clearer.
That cat is perfect
Thank you for this important safety explanation. I thought I understood it but now I have it ingrained in me.
Very good video. Isolation transformers are hard for people to understand. The main thing, I think to remember, is the main purpose of the isolation transformer for a tech bench is to separate the neutral and the earth ground. That way if the DUT is earth grounded, the neutral is not and the circuit is only the hot and neutral of the output of the isolation transformer. Be aware most isolation transformers for sale today have the neutral and the earth ground bonded together at the output receptacle defeating the separation and for a tech bench one would have to modify the isolation transformer for tech use, in my opinion. Tom
I bought an $250 I/T for my 14 year old future electrical engineer. It will protect the scope and other equipment but not his life. I have 22 circuits in my house and all circuits each have a RCD (residual current device) which trips in 30 m/s and at a 15 mA imbalance. They are mandatory in new homes in Australia where we have 240 vac and the common ground (neutral) is tied to the ground stake as in may other countries for safety. I can with bare feet, and bare hands place an metal object into the active on any power outlet and the RCD trips. The RCD costs about $15. I am afraid to let my son use the I/T,,, and I am bewildered why I bought it for him... perhaps because I watched so many UA-cam videos on them... Can anyone please comment, thanks.
If the outlet from isolation transformer has an earth pin, you may add a RCD plug on the secondary. Isolation transformer with a RCD plug on the secondary can power DUT and an oscilloscope can be safely used on a DUT. The RCD plug on the secondary of the I/T can protect one's life in case of inbalance. Nothing will protect a person if that person grabs one wire with one hand and the other wire with the other hand.
What would happen if you touch an earth against the live on the isolation transformer would it just make the earth live without blowing any fuses?
Yes, although there is no live/neutral really on the secondary side until you introduce the earth conductor again. The "live" would essentially become neutral because it is now at earth potential. The other terminal would be "live".
Please make a video on your lab tour
Serious collection of Flukes
hi. How do you tell which end of the secondary is the live one? Or are they floating until one of them gets connected to earth ground e.g. by connecting the ground clip of a oscilloscope?
bergweg Exactly that - there is no sense of live or neutral until you provide an earth reference.
SDG Electronics
But if you provide an earth reference, e.g. connecting the ground clip of oscilloscope, then aren't you going back to the same situation as a non-isolated transformer? This means the circuit is earthed and if the +ve end of probe touches any part of the circuit which is earthed or grounded, it will create a short via the oscilloscope. So how can we do any probing in this case since we can't tell whether what we are probing is negative or positive? Could you please help to clarify. Thanks.
Thank you!
The big question is... Where do you get an isolation transformer from in UK?
RS or Farnell will sell them.
Ty
good job indeed!
The dusky cat is the best ventriloquist I've ever seen ! The dummy is not very life-like, though.
yeah cat right off the bat