Why don\'t inductive spikes get through a transformer?...

[Commander Kinsey]

Consider a simple power supply / wall wart. Mains voltage in, 12V out. When you switch it off at the mains, while the load was drawing current, you\'d get a massive voltage spike across the primary. So wouldn\'t you also get that voltage passed across to the secondary (with the relevant ratio), destroying the load? Or does the transformer not store enough joules to raise the voltage much through a load which is still connected?

 

[John Larkin]

On Fri, 03 Mar 2023 22:19:19 -0000, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

>Consider a simple power supply / wall wart. Mains voltage in, 12V out. When you switch it off at the mains, while the load was drawing current, you\'d get a massive voltage spike across the primary.

Well, a spark, not necessarily a massive voltage spike.

>So wouldn\'t you also get that voltage passed across to the secondary (with the relevant ratio), destroying the load? Or does the transformer not store enough joules to raise the voltage much through a load which is still connected?

There is a voltage spike on the secondary of the transformer. In a
classic transformer-rectifier wart, it is reduced by

Leakage inductance

The 10x or 20x turns ratio

Any secondary load

the rectifier/filter on the secondary, if present.

 

[John S]

On 3/3/2023 4:19 PM, Commander Kinsey wrote:

Consider a simple power supply / wall wart.  Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary.  So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load?  Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

Load current has very little to do with the energy storage because the
current in the primary and the current in the secondary are opposing. It
is the primary magnetizing current and any attendant leakage current
that are the storage mechanisms.

--
Dogs make me happy. Humans make my head hurt.

 

[Sylvia Else]

On 04-Mar-23 9:19 am, Commander Kinsey wrote:

Consider a simple power supply / wall wart.  Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary.  So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load?  Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

You\'d only get a spike on the primary to the extent that the flux is not
linked to the secondary where current can flow. But, since it\'s not
linked to the secondary, there\'s nothing to create a spike there.

Sylvia.

 

[Commander Kinsey]

On Sat, 04 Mar 2023 01:27:40 -0000, Sylvia Else <sylvia@email.invalid> wrote:

On 04-Mar-23 9:19 am, Commander Kinsey wrote:
Consider a simple power supply / wall wart. Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary. So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load? Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

You\'d only get a spike on the primary to the extent that the flux is not
linked to the secondary where current can flow. But, since it\'s not
linked to the secondary, there\'s nothing to create a spike there.

I assumed if there was 1000 volts generated on one side, the transformer would convert that into the relevant voltage on the other side. But I\'ve been told elsewhere this spike would happen so fast it would be the equivalent of very high frequency, which transformers don\'t pass too well.

 

[John Larkin]

On Mon, 06 Mar 2023 19:45:05 -0000, \"Commander Kinsey\"
<CK1@nospam.com> wrote:

On Sat, 04 Mar 2023 01:27:40 -0000, Sylvia Else <sylvia@email.invalid> wrote:

On 04-Mar-23 9:19 am, Commander Kinsey wrote:
Consider a simple power supply / wall wart. Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary. So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load? Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

You\'d only get a spike on the primary to the extent that the flux is not
linked to the secondary where current can flow. But, since it\'s not
linked to the secondary, there\'s nothing to create a spike there.

I assumed if there was 1000 volts generated on one side, the transformer would convert that into the relevant voltage on the other side. But I\'ve been told elsewhere this spike would happen so fast it would be the equivalent of very high frequency, which transformers don\'t pass too well.

Same effect. A primary spark is usually created by energy stored in
leakage inductance, and leakage inductance reduces high frequency
coupling into the secondary.

You can Spice that.

 

[John S]

On 3/3/2023 4:19 PM, Commander Kinsey wrote:

Consider a simple power supply / wall wart.  Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary.  So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load?  Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

Have you even looked at a model of a transformer? You can find them
online. If you know as much as your question seems to intimate, then you
could probably see the answer yourself.


--
Dogs make me happy. Humans make my head hurt.

 

[Fred Bloggs]

On Friday, March 3, 2023 at 7:11:04 PM UTC-5, John Larkin wrote:

On Fri, 03 Mar 2023 22:19:19 -0000, \"Commander Kinsey\"
C...@nospam.com> wrote:

Consider a simple power supply / wall wart. Mains voltage in, 12V out. When you switch it off at the mains, while the load was drawing current, you\'d get a massive voltage spike across the primary.
Well, a spark, not necessarily a massive voltage spike.
So wouldn\'t you also get that voltage passed across to the secondary (with the relevant ratio), destroying the load? Or does the transformer not store enough joules to raise the voltage much through a load which is still connected?
There is a voltage spike on the secondary of the transformer. In a
classic transformer-rectifier wart, it is reduced by

Leakage inductance

The 10x or 20x turns ratio

Any secondary load

the rectifier/filter on the secondary, if present.

Is your switch opened with a nuclear detonation? If not, the leakage inductance arcs across the small gap in the switch as it opens, decaying increasing rapidly as the contacts separate. Since the switch takes all the voltage, there is nothing left for the magnetizing primary. The magnetization in the core as it collapses induces voltage in the secondary at exactly the magnitude where it left off before the switch was opened, and then decays exponentially.

 

[Fred Bloggs]

On Friday, March 3, 2023 at 5:19:27 PM UTC-5, Commander Kinsey wrote:
> Consider a simple power supply / wall wart. Mains voltage in, 12V out. When you switch it off at the mains, while the load was drawing current, you\'d get a massive voltage spike across the primary. So wouldn\'t you also get that voltage passed across to the secondary (with the relevant ratio), destroying the load? Or does the transformer not store enough joules to raise the voltage much through a load which is still connected?

By George! Sounds like you\'ve hit on something the entire planet has been doing zillions of times but failed to notice.

 

[Commander Kinsey]

On Mon, 06 Mar 2023 20:32:16 -0000, John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:

On Mon, 06 Mar 2023 19:45:05 -0000, \"Commander Kinsey\"
CK1@nospam.com> wrote:

On Sat, 04 Mar 2023 01:27:40 -0000, Sylvia Else <sylvia@email.invalid> wrote:

On 04-Mar-23 9:19 am, Commander Kinsey wrote:
Consider a simple power supply / wall wart. Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary. So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load? Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

You\'d only get a spike on the primary to the extent that the flux is not
linked to the secondary where current can flow. But, since it\'s not
linked to the secondary, there\'s nothing to create a spike there.

I assumed if there was 1000 volts generated on one side, the transformer would convert that into the relevant voltage on the other side. But I\'ve been told elsewhere this spike would happen so fast it would be the equivalent of very high frequency, which transformers don\'t pass too well.

Same effect. A primary spark is usually created by energy stored in
leakage inductance, and leakage inductance reduces high frequency
coupling into the secondary.

You can Spice that.

My car seems to think you can pass it.

 

[Commander Kinsey]

On Mon, 06 Mar 2023 20:55:33 -0000, John S <Sophi.2@invalid.org> wrote:

On 3/3/2023 4:19 PM, Commander Kinsey wrote:
Consider a simple power supply / wall wart. Mains voltage in, 12V out.
When you switch it off at the mains, while the load was drawing current,
you\'d get a massive voltage spike across the primary. So wouldn\'t you
also get that voltage passed across to the secondary (with the relevant
ratio), destroying the load? Or does the transformer not store enough
joules to raise the voltage much through a load which is still connected?

Have you even looked at a model of a transformer?

Made of lego?

You can find them
online. If you know as much as your question seems to intimate, then you
could probably see the answer yourself.

How could you possibly know the level of my knowledge?

 

[Anthony William Sloman]

On Saturday, March 18, 2023 at 7:06:18 AM UTC+11, Commander Kinsey wrote:

On Mon, 06 Mar 2023 20:55:33 -0000, John S <Sop...@invalid.org> wrote:
On 3/3/2023 4:19 PM, Commander Kinsey wrote:

<snip>

Have you even looked at a model of a transformer?

Made of lego?

Made of mathematical equations. You can find a pretty good one in LTSpice.

You can find them online. If you know as much as your question seems to intimate, then you could probably see the answer yourself.

How could you possibly know the level of my knowledge?

John Larkin isn\'t very bright, and doesn\'t seem to realise quite how enthusiastically you advertise your incorrigible pig-ignorance.

--
Bill Sloman, Sydney