DC measurement is magnetizing core ?

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murrayatuptown

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Off topic a bit but I recently saw, a tube heater scheme iirc, and it had a diode hanging off one of the transformer terminals. Said something about helping to keep the transformer from magnetizing. I can't find where I saw it. Any insight on this?
I've seen a Zener in Blencowe writing as a method of producing DC to elevate filament ac by the DC level. Funny I don't remember filtering for it...but maybe it's not necessary, considering it's usually fully ac. I just remembered this after thinking about halfwave rectification producing significant DC flux. That always made me think adding a second diode (or even 3 more!) to a HWR has more benefits...lower ripple, higher ripple freq., and might result in a cooler transformer...but that's all hunch and no data to back that up.
 

jokeramik

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Hello,

I know that DC voltage can permanently magnetise the core of a transformer.

But if I have an unknown (audio) transformer and I use a Fluke multimeter to measure the resistance to somehow figure what is what on the transformer...

Will this 'damage' the transformer ?

I ask this as someone mentioned I screwed up two transformers using my Fluke on the windings.
Does a multimeter for a few secs produce enough current to permanently magnetize a transformer core ?

Is this really so ?
Use a generator and a scope.:))
Best regards!
jokeramik
 

coil_man

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A Fluke DMM will not hurt the transformer - even mic input transformers. These DMM's do not pass enough current to saturate the core (which would permanently damage it). In fact, the induced field is so low that you would have a hard time seeing an increase in THD with the best test equipment.

Using a meter will only tell you the DCR of the windings. A sine wave generator (typically set at 1kHz for an audio transformer) to drive the primary and a AC meter on the secondary will get you reasonably close to the turns ratio, being the ratio of the voltages. If you want to use an oscilloscope, it will work but with less precision unless it is a digital scope that can gives digital voltage measurements. Be sure that the oscillator level is low enough not to run the transformer into saturation. Saturation causes the sine wave peaks to clip and you would not get a good turns ratio reading.
 

Ivan K.

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I disagree with coil_man on his 1st sentence. Deane Jensen (Jensen Transformers), was a friend and also made custom mic transformers for my company's products. He claimed the the laminations used in his mic transformers, once annealed and assembled in his proprietary manner, resembled a giant nickel crystal. Any DC would magnetize the core and up the THD. He said that the core could not be demagnetized. Why take a chance?

AC measurements (Scope or good TRMS, AC DMM) will give you an accurate idea of the actual impedance at your operating frequency. DC can be off by a lot. I also recommend sweeping or spot checking frequencies between 20Hz & 20kHz (minimum), to check the flatness of the impedance curve and frequency response-a quick quality check of the transformer and an indication if you need an RC network on the output to control transformer resonances.

As a reality check I measured the Ohms range current on the following:
AVO meter 9 Mk II, Ohms -/- 100 range: 71.5mA. On straight Ohms or Ohms * 100: 0.73 mA
Agilent U1253B 4.5 digit DMM, 1.0277 mA
Agilent 34401A 6.5 Digit Bench DMM, 0.998 mA
Keysight 34461A 6.5 Digit Bench DMM, 1.002 mA
Keithley 2002 8.5 Digit Bench DMM, 943.2 uA
Owon 3.5 Digit DMM, 208 uA

Will ~1mA (using modern DMMs) magnetize a mic or moving coil transformer? Any transformer manufactures out there?

All measurements were done on Keithley 2002 except itself, done on Keysight. Measurements all crosschecked on other meters.
 

abbey road d enfer

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I disagree with coil_man on his 1st sentence. Deane Jensen (Jensen Transformers), was a friend and also made custom mic transformers for my company's products. He claimed the the laminations used in his mic transformers, once annealed and assembled in his proprietary manner, resembled a giant nickel crystal. Any DC would magnetize the core and up the THD. He said that the core could not be demagnetized.
This is quite a statement. I wonder what Bill (CMRR here) has to say about it.
If the core has been magnetized by such a tiny current as that from a DMM (typically less than 1uA), I would think the next signal it receives would demag it. Even a not-too sensitive dynamic mic would produce at least 10 times that.
 

Ivan K.

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Hi d'enfer, you are certainly getting your wish these days regarding devilish UK weather!

I measured about 1mA-not 1 uA. Still not a huge amount of current, but enough? Maybe Deane was exaggerating, or since this was the late 70's / early'80s, he was thinking of Simpson or AVO 20,000 Ohms per Volt meters? I don't have a Simpson to measure, but AVO's 71 mA on low Ohms wouldn't be healthy!
 

Monte McGuire

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As a reality check I measured the Ohms range current on the following:
AVO meter 9 Mk II, Ohms -/- 100 range: 71.5mA. On straight Ohms or Ohms * 100: 0.73 mA
Agilent U1253B 4.5 digit DMM, 1.0277 mA
Agilent 34401A 6.5 Digit Bench DMM, 0.998 mA
Keysight 34461A 6.5 Digit Bench DMM, 1.002 mA
Keithley 2002 8.5 Digit Bench DMM, 943.2 uA
Owon 3.5 Digit DMM, 208 uA
FWIW, on the Keysight 34461A, you can set the test current used for the ohms test with the ohms range control. For the 100Ω and 1kΩ range, the meter uses a 1mA test current, as you state. However, if you set the range manually to a higher setting, you can choose 100µA, 10µA etc. all the way down to 500nA for the 10MΩ range. Of course, the tradeoff is resolution, but you can throw away some of the digits and still get a good reading. For basic continuity and ballpark measurements, the 1MΩ range would use only 1µA, and seems absolutely safe to me. You should be able to get a usable number at 100kΩ as well.
 

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