Why does a terminstor reduce transformer buzz? - diyAudio
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Old 28th January 2010, 04:47 PM   #1
Eric is offline Eric  United States
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Default Why does a terminstor reduce transformer buzz?

Please help me learn something.

I just installed a CL-60 thermistor in series with the primary leg of the transformer on my A40 amp and noticed that the mechanical buzz from the transformer has been reduced.

The amp is powered by a 600VA transformer (custom wound by Victoria Magnetics some years ago) that feeds two channels, each of which draws about 3.2A from 32v rails. The transformer has always had a small buzz to it, but now it is much less. With the thermistor in place, the rail voltage drops about 0.5v DC (I expected some drop - this is not a problem).

Why does installing the thermistor reduce mechanical buzz?

Thanks,
Eric
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Old 28th January 2010, 05:12 PM   #2
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Default Reduced buzz

Hi Eric,
As you have placed a thermistor in series with the primary you have reduced the voltage and current through the primary.
It is possible that without the thermistor there was too much voltage/current on the primary causing the core to saturate and buzz.
I have found this problem to be very common in the UK where items brought in from abroad are wound for a nominal 230Vac whereas in the UK the nominal line voltage is 240Vac or higher in a lot of cases. One customer of mine regularly measures his line voltage at 253Vac so we have put a couple of Thermistors in series to cut this down and also reduce the switch on "surge"

John Caswell
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Old 28th January 2010, 05:19 PM   #3
Blues is offline Blues  United States
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One reason is the thermistor prevents at turn-on the initial large surge (then constant after a while for Class A A40) of current demanded by your power supply/amp...the other is the equivalent R of your thermistor is dropping some of the DC voltage present on your primaries.
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Old 28th January 2010, 05:26 PM   #4
Eric is offline Eric  United States
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Hmmm... This is curious. At one time I did build and install a DC blocker on the transformer primaries thinking that was the cause of the buzz, but it had no effect, so I removed it.

Measuring the wall outlet, I'm getting a pretty consistent 126v whereas the transformer is designed for 120v. The thermistor drops about 0.6-0.7 VAC to the transformer, so it could indeed be at/near saturation without it.

Thanks!
Eric
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Old 29th January 2010, 06:48 PM   #5
Eric is offline Eric  United States
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Wow! Just got around to measuring the temperature of that thermistor: 112c

I didn't know that ran that hot! I'm only pulling about 2A at 120v across it...
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Old 5th February 2010, 12:02 PM   #6
Snokker is offline Snokker  Netherlands
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Quote:
Originally Posted by Eric View Post
Measuring the wall outlet, I'm getting a pretty consistent 126v whereas the transformer is designed for 120v. The thermistor drops about 0.6-0.7 VAC to the transformer, so it could indeed be at/near saturation without it.

Thanks!
Eric
Hi Eric, I don't think that the extra 6 Volts (only 5%....) causes the saturation(?)/buzz of the toroid. A common problem in the EU is odd harmonics on the neutral conductor. For 50 Hz mains these are mainly 150Hz and 250Hz, caused by switched-power stuff and make high mu cores saturate.
One of your own finding confirms this; drawing 2A through the CL60; with a voltage drop would only dissipate:
P=UxI = 0.7 x 2 = 1.4W while your CL60 is near boiling point...
You can try to borrow a poweranalyser and check the harmonics on the Live and Neutral lines.
Adding extra thermistors in series only lowers your Circuit impedance, resulting in less "control" and peak current flowing through the power supply.
I always short my thermistors with a power relay after rush in.
Succes!
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Old 5th February 2010, 12:23 PM   #7
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If you short out the CL60 after turn on does the hum come back?
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Old 5th February 2010, 12:31 PM   #8
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by Snokker View Post
Hi Eric, I don't think that the extra 6 Volts (only 5%....) causes the saturation(?)/buzz of the toroid. A common problem in the EU is odd harmonics on the neutral conductor. For 50 Hz mains these are mainly 150Hz and 250Hz, caused by switched-power stuff and make high mu cores saturate.
One of your own finding confirms this; drawing 2A through the CL60; with a voltage drop would only dissipate:
P=UxI = 0.7 x 2 = 1.4W while your CL60 is near boiling point...
You can try to borrow a poweranalyser and check the harmonics on the Live and Neutral lines.
Adding extra thermistors in series only lowers your Circuit impedance, resulting in less "control" and peak current flowing through the power supply.
I always short my thermistors with a power relay after rush in.
Succes!
Schematic of your layout please ......
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Old 5th February 2010, 12:48 PM   #9
Snokker is offline Snokker  Netherlands
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Quote:
Originally Posted by a.wayne View Post
Schematic of your layout please ......
Well, here's the least safe one (the 24V relay is direct on the mains with a voltage divider)
Much safer is the use a small transformer for the inrush limiters
BTW in this schematic there are powerR's instead of thermistors

inrushlimit.gif

Last edited by Snokker; 5th February 2010 at 12:51 PM.
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Old 5th February 2010, 12:55 PM   #10
Snokker is offline Snokker  Netherlands
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And here's the more safe version
Nb. only the relay contacts are shown, they are supposed to short the thermistor.

inrush.bmp
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