ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2]  All   Go Down

Author Topic: DC current spike?  (Read 997 times)

Jay Barracato

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1958
  • Solomons, MD
Re: DC current spike?
« Reply #10 on: September 26, 2018, 02:27:40 pm »

Nathan,

Back in the "bad old days" there were many woofers with suspensions that would easily allow a voice coil to crush itself on the backplate with a large transient signal ("spike"), instant destruction. Back then Crown had an entire series of "DC" amplifiers, happy to reproduce plenty of DC to cook the coils of drivers even if they survived a backplate (or phasing plug) crash.

Assuming the spike was only DC, a transformer would have eliminated it, as transformers don't pass DC.

Art
Pulses act weird on many circuits, which is why they are often used in circuit testing.

Funny (at least to me), I was working on a relatively simple circuit in multisim (a modeling program) and I wanted to put a simple DC motor as a load at one end just to show something was happening. It would eventually be replaced by a stepper and controller. The program has 30000 modeled devices, both real and theoretical, but apparently doesn't have a simple DC motor.

But it does have RCI load loops, it I wanted to define the load.

If it had an induction motor, I could do a model, but at this point that doesn't look like it is in my package

Sent from my Moto Z (2) using Tapatalk
Logged
Jay Barracato

Art Welter

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1441
Re: DC current spike?
« Reply #11 on: September 26, 2018, 04:19:34 pm »

The transformer would have eventually stopped passing the DC, but based on some parameters of the transformer and the rest of the circuit, it would have passed the beginning of the DC - maybe the first 100ms or so - and then the amount of the “DC” signal fed through will decrease if the voltage level the transformer is seeing on its primary stays at the same level.

Depending on the DC voltage, it may burn out the primary of the transformer itself, since a transformer being fed with DC isn’t acting as a transformer, it’s a resistor.
TJ,

I've always understood audio transformers as Bill Whitlock of Jensen Transformers explained in Chapter 11 of the
Handbook for Sound Engineers, Third Edition Glen Ballou, Editor:
According to the law of induction, a voltage will be induced in any conductor (wire) that cuts flux lines. Therefore, if we place two coils near each other as shown in Figure 4, an ac current in one coil will induce an ac voltage in the second coil. This is the essential principle of energy transfer in a transformer. Because they require a changing magnetic field to operate, transformers will not work at dc. .

I witnessed a lightning strike in a natural amphitheater while “camping” overnight on the front of house scaffolding mix tower. The lightening flash and crack of thunder seemed simultaneous, my guess was the strike occurred between the mix tower and stage.

The next morning we attempted to turn on the lights to dry them out, but found the lighting console was no longer working. A quick check inside the console revealed every transistor had blown it’s top!
The lighting snake was run along side the audio snake, but none of the circuitry of the transformer coupled audio consoles (Yamaha PM 1000s) suffered any damage.

Whether the transformers “saved the day” or we just got “lucky” can’t be proved, but my bet would be a non-transformer input audio console would have been damaged like the light board.

Art
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2067
Re: DC current spike?
« Reply #12 on: September 26, 2018, 05:33:24 pm »

But even DC creates a changing magnetic field when switched on and off.  That was the basic premis of the old points/ distributor/ coil that was and is used to "transform" 12 VDC into a 15,000 volt "spike" to fire a spark plug.

We often make the mistake of over looking the dimension of time in a circuit.
Logged
Steve Swaffer

John Roberts {JR}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16113
  • Hickory, Mississippi, USA
    • Resotune
Re: DC current spike?
« Reply #13 on: September 26, 2018, 05:53:19 pm »

But even DC creates a changing magnetic field when switched on and off.  That was the basic premis of the old points/ distributor/ coil that was and is used to "transform" 12 VDC into a 15,000 volt "spike" to fire a spark plug.

We often make the mistake of over looking the dimension of time in a circuit.
The "Kettering" ignition system is pretty well known. A DC voltage creates a magnetic flux in an ignition coil and when that circuit is interrupted (by the points opening) the field collapses, creating a voltage in the secondary, with a significant step up turns ratio.

This is more like a transformer*** passing an impulse than AC even thousands of RPM is less than 100Hz rep rate.

JR

*** I am not sure if ignition coils are isolated winding transformers or auto-formers (probably), which could pass DC... not that it matters to this (let me show you vague off topic information) veer. That I am party to.  :o
Logged
On the internet people tell you everything "they" know, not the answer to "your" question.....  http://circularscience.com/

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4148
  • St. Paul, MN
Re: DC current spike?
« Reply #14 on: September 26, 2018, 06:13:13 pm »

But even DC creates a changing magnetic field when switched on and off.  That was the basic premis of the old points/ distributor/ coil that was and is used to "transform" 12 VDC into a 15,000 volt "spike" to fire a spark plug.

We often make the mistake of over looking the dimension of time in a circuit.
yes - that was my point. The transformer will pass some amount of voltage for a while after a change in voltage at the primary. The secondary voltage will decay a short time after the primary voltage stops changing (becomes DC). Once the primary voltage stabilizes, it’s not a transformer anymore - it’s just a piece of wire with a resistance across a supply voltage.

You can do the following experiment- disconnect your doorbell transformer from the AC supply and do a simple resistance measurement on the primary winding. I forget the exact value I measured, but it was quite a bit lower than I expected - something like 50 or 100 ohms. Fed DC, the transformer turns into a couple-hundred watt heater and it will burn up. Connect it to AC and it’s impedance rises such that it only draws a few milliamperes when the secondary is open.
Logged

Chris Grimshaw

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 793
    • Grimshaw Audio
Re: DC current spike?
« Reply #15 on: September 27, 2018, 03:40:24 am »

TJ,

I've always understood audio transformers as Bill Whitlock of Jensen Transformers explained in Chapter 11 of the
Handbook for Sound Engineers, Third Edition Glen Ballou, Editor:
According to the law of induction, a voltage will be induced in any conductor (wire) that cuts flux lines. Therefore, if we place two coils near each other as shown in Figure 4, an ac current in one coil will induce an ac voltage in the second coil. This is the essential principle of energy transfer in a transformer. Because they require a changing magnetic field to operate, transformers will not work at dc. .

<snip>

Art

True, but a "pulse" is made up of many frequency components, most of which will get through, down to the LF cutoff slope inherent in the transformer itself.

If you had a perfect transformer and fed it a very-low-frequency square wave, what you'd get out would be more like a sawtooth waveform - the transformer will support all the information that gives the leading edge, but when you apply a highpass filter, the horizontal bits of the square wave won't be so horizontal any more.

Chris
Logged
Sheffield-based sound engineering.
www.grimshawaudio.com

Jay Barracato

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1958
  • Solomons, MD
Re: DC current spike?
« Reply #16 on: October 12, 2018, 05:29:17 am »

True, but a "pulse" is made up of many frequency components, most of which will get through, down to the LF cutoff slope inherent in the transformer itself.

If you had a perfect transformer and fed it a very-low-frequency square wave, what you'd get out would be more like a sawtooth waveform - the transformer will support all the information that gives the leading edge, but when you apply a highpass filter, the horizontal bits of the square wave won't be so horizontal any more.

Chris
I happened to snap this pic the other day demonstrating something for a student. Even an unprocessed square wave shows a finite rise and fall time due to the capacitance of the load. The classic saw tooth form is when the sum of the rise and fall time exceeds the duty cycle of the square signal.

Sent from my Moto Z (2) using Tapatalk

Logged
Jay Barracato

Jay Barracato

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1958
  • Solomons, MD
Re: DC current spike?
« Reply #17 on: October 12, 2018, 11:44:01 am »

I happened to snap this pic the other day demonstrating something for a student. Even an unprocessed square wave shows a finite rise and fall time due to the capacitance of the load. The classic saw tooth form is when the sum of the rise and fall time exceeds the duty cycle of the square signal.

Sent from my Moto Z (2) using Tapatalk
Here is the saw tooth


Sent from my Moto Z (2) using Tapatalk
« Last Edit: October 12, 2018, 03:11:20 pm by Jay Barracato »
Logged
Jay Barracato
Pages: 1 [2]  All   Go Up
 


Page created in 0.096 seconds with 21 queries.