60Hz transformers/et light bulbs

[PDRUNEN]

Hi Group,

Consider the 1VA 120 to 10 V transformer designed for 60Hz. Can I use this as
a audio transformer for frequency range of 60Hz to 10KHz ?

I would be stepping up the voltage so the signal would be on the 10V side of
the transformer.

If my sig generator can output 10V at 60Hz with an output impedance of 600
ohms, and I step this up to 100V on the other side of this transformer, would
the output impedance be also stepped up by a factor of 10 or 10 squared, or
other?

I would expect to be able to measure it by first measuring the unloaded output
then loading down this output until the measurement was 1/2 of the unloaded
measurement, is this true?

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

Tnx de pdrunen.

 

[Tim Wescott]

Hi Group,

Consider the 1VA 120 to 10 V transformer designed for 60Hz. Can I use this as
a audio transformer for frequency range of 60Hz to 10KHz ?

Probably not up to 10kHz, and certainly not with good audio quality,
unless its way overdesigned. You can get fairly good audio transformers
fairly cheap from the toob suppliers -- why dink with AC mains transformers?

I would be stepping up the voltage so the signal would be on the 10V side of
the transformer.

You mean input signal on the 10V side? Then yes, you'd see a 12:1
voltage step-up.

If my sig generator can output 10V at 60Hz with an output impedance of 600
ohms, and I step this up to 100V on the other side of this transformer, would
the output impedance be also stepped up by a factor of 10 or 10 squared, or
other?

The voltage goes up by N:1, the current goes down by N:1, Z_in = V/I so
the new impedance will be Z_out = (V/N)/(N*I) = Z_in/N^2.

I would expect to be able to measure it by first measuring the unloaded output
then loading down this output until the measurement was 1/2 of the unloaded
measurement, is this true?

Well... You're talking a pretty crappy transformer, but in theory yes.

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

Good question. I don't know -- but I believe that _some_ filaments will
degrade more on one end than the other with DC drive; with AC drive the
degradation is evened out & such a filament lasts longer. I have no
idea if this effect is prevalent in a light bulb, however.

 

[Tam/WB2TT]

Hi Group,

Consider the 1VA 120 to 10 V transformer designed for 60Hz. Can I use this as
a audio transformer for frequency range of 60Hz to 10KHz ?

I would be stepping up the voltage so the signal would be on the 10V side of
the transformer.

If my sig generator can output 10V at 60Hz with an output impedance of 600
ohms, and I step this up to 100V on the other side of this transformer, would
the output impedance be also stepped up by a factor of 10 or 10 squared, or
other?

I would expect to be able to measure it by first measuring the unloaded output
then loading down this output until the measurement was 1/2 of the unloaded
measurement, is this true?

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

Tnx de pdrunen.

I ran audio through a small 60 Hertz 120:12V transformer, and listened to it
by ear, No way was 10KHz getting through. The source signal was bandlimited
to 2.8 KHz from an SSB receiver, and there was a definite reduction in high
frequency response. Actually, a rather pleasing mellow tone. Perfectly
adequate for commuinications quality speech.

The impedance goes as the square of the voltage ratio.

If you FULL WAVE rectify AC, and run it to a light bulb current will flow
during BOTH half cycles of the AC wave. You really don't want to bother
doing this, buy a dimmer.

Tam

 

[Tam/WB2TT]

I should have mentioned that I used the 12V center tapped winding as an
autotransformer at about 8 Ohms impedance. Primary was floating.

Tam

 

[Pooh Bear]

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

Why would DC operation prolong life ?

Incandescent bulbs lose material from the filament due to evaporation. When the
filament gets too thin the bulb fails. That is not going to change.

Graham

 

[Pooh Bear]

Hi Group,

Consider the 1VA 120 to 10 V transformer designed for 60Hz. Can I use this as
a audio transformer for frequency range of 60Hz to 10KHz ?

Power transformers are not designed for audio use.

You will get *something* but likely at 'telephone quality'.


Graham

 

[Spehro Pefhany]

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

Probably somewhat less. There will be an increase in life due to the
slight reduction in RMS voltage and a reduction due to the DC on the
filament causing filament "notching".

If your "AC main" is 120V and your rectifier drops 2V, then you have a
22% increase in life using the commonly quoted 12th power relation,
balanced off against perhaps a 50% reduction due to notching, so you
might have a net decrease of 30-40%.

Best regards,
Spehro Pefhany

 

[David Lesher]

PDRUNEN wrote:
Why would DC operation prolong life ?

Incandescent bulbs lose material from the filament due to evaporation. When the
filament gets too thin the bulb fails. That is not going to change.

Err... ISTM GE Minature Lamp div. footnoted that fact that life is
[significant %] shorter on DC than AC.

But I can't recall the whys and wherefores...

 

[John Woodgate]

Hi Group,

Consider the 1VA 120 to 10 V transformer designed for 60Hz. Can I use this as
a audio transformer for frequency range of 60Hz to 10KHz ?

Yes. It may not be as good as a real audio transformer but it will work.

I would be stepping up the voltage so the signal would be on the 10V side of
the transformer.

If my sig generator can output 10V at 60Hz with an output impedance of 600
ohms, and I step this up to 100V

You will get (nearly) 120 V, not 100 V. But not if your generator really
has an output source impedance of 600 ohms. Are you sure about that? The
fact that the spec says it needs a *load* of 600 ohms doesn't mean that
its source (internal) impedance is 600 ohms.

on the other side of this transformer, would
the output impedance be also stepped up by a factor of 10 or 10 squared, or
other?

12 squared.

I would expect to be able to measure it by first measuring the unloaded output
then loading down this output until the measurement was 1/2 of the unloaded
measurement, is this true?

Loading down to half voltage is a bit harsh. It's better to load down by
maybe 10% or so and then calculate the source impedance.

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

It's not possible to tell. The voltage applied to the lamp will be less
than when it is connected directly to the mains, which will lengthen
life. But there may be effects that will shorten it. The effect of lower
voltage will probably win.

 

[normanstrong]

Why would DC operation prolong life ?

Incandescent bulbs lose material from the filament due to evaporation. When the
filament gets too thin the bulb fails. That is not going to change.

Graham

You can get all sorts of information from Don Klipstein at
[email protected]

Norm Strong

 

[Ken Smith]

PDRUNEN wrote:
Why would DC operation prolong life ?

Incandescent bulbs lose material from the filament due to evaporation. When the
filament gets too thin the bulb fails. That is not going to change.

Err... ISTM GE Minature Lamp div. footnoted that fact that life is
[significant %] shorter on DC than AC.

But I can't recall the whys and wherefores...

The ions are swept in one direction. This means that the partial pressure
of the material from the filament is higher at one end than the other.
There is always some redepositing going on. On DC it all happens at one
end and not the other. The end it happens on stays thicker and hence has
a lower resistance and runs cooler. The hotter end loses material even
faster and the situation slowly runs away.

 

[Ken Smith]

Another unrelated question, if I DC recify the AC main to full-wave and drive a
light bulb, the voltage is now unfiltered DC. Consider the 100 watt light
bulb. Would the life of the bulb be reduced now that the current flows one way
into the bulb?

It's not possible to tell. The voltage applied to the lamp will be less
than when it is connected directly to the mains, which will lengthen
life. But there may be effects that will shorten it. The effect of lower
voltage will probably win.[/QUOTE]

Recified AC has an RMS just slightly less than the AC you started with.
Running the light on the DC would yeld a short life than hooking it to the
AC would, I'll bet.

According to my high school electrical shop teacher: Many years ago, some
factories used DC for lighting. They had a reversing switch that allowed
the polarity to be inverted from one day to the next. The effect must be
big enough to matter if this is the case.

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith

Recified AC has an RMS just slightly less than the AC you started with.

Yes. But life is inversely proportional to a very high power of the
applied voltage.

Running the light on the DC would yeld a short life than hooking it to
the AC would, I'll bet.

That's why I said that one cannot tell.

 

[tmbg]

Probably not up to 10kHz, and certainly not with good audio quality,
unless its way overdesigned. You can get fairly good audio transformers
fairly cheap from the toob suppliers -- why dink with AC mains transformers?

What do you consider 'fairly cheap', and who specifically would you
suggest getting them from? So far all I've really found is Hammond (via
Mouser), and they're like $50 for smallish (15-20W) output transformers.

 

[Tam/WB2TT]

I read in sci.electronics.design that Ken Smith



Yes. But life is inversely proportional to a very high power of the
applied voltage.


That's why I said that one cannot tell.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I think that as far as lamp life goes, he would get the most bang for the
buck from an inrush current limiter. About $2 FOR THE 1.3 AMP.

Tam

 

[Ken Smith]

I read in sci.electronics.design that Ken Smith



Yes. But life is inversely proportional to a very high power of the
applied voltage.


That's why I said that one cannot tell.

I'll bet you $0.32 I'm right.

We need an impartial 3rd party to do the experiment. You'll need about 100
bulbs to get moderately good statistics. A 2 channel strip chart recorder
recording the total current to the two banks of 50 bulbs would be good
enough but I'd recommend computer monitoring.