I estimated the start up current by using 240Vac/48r and that equals 5Aac
But that current is a transient current averaged over one half cycle of the mains waveform.
Thus it lasts for 8.3ms or 10ms.
Do the NTC manufacturers tell you what the transient current rating can be?
After start up the maximum continuous current is transformer VA rating divided by Primary voltage.
The 48r can be made up from NTC and/or resistors plus the primary winding and the other wiring resistances.
But that current is a transient current averaged over one half cycle of the mains waveform.
Thus it lasts for 8.3ms or 10ms.
Do the NTC manufacturers tell you what the transient current rating can be?
After start up the maximum continuous current is transformer VA rating divided by Primary voltage.
A friend who makes amplifiers and other stuff said that he uses 2.5-5 ohm 5A NTC Thermistor for 600VA toroid.
bigger transformer need lower added resistance.
110/120Vac transformers need half the added resistance required for a 220/240Vac transformer.
so a 600VA 110/120Vac transformer would require ~ 25% of 48r i.e. 12ohms or ~ a CL60.
when you do the calculation for a 600VA 115Vac transformer the primary circuit resistance comes out at 11r to limit the current to ~ 10.4A
and they can use a T5A fuse with a CL60 and a bit of primary and wiring resistance..
110/120Vac transformers need half the added resistance required for a 220/240Vac transformer.
so a 600VA 110/120Vac transformer would require ~ 25% of 48r i.e. 12ohms or ~ a CL60.
when you do the calculation for a 600VA 115Vac transformer the primary circuit resistance comes out at 11r to limit the current to ~ 10.4A
and they can use a T5A fuse with a CL60 and a bit of primary and wiring resistance..
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so 4.7ohm 5-10A NTC connected in series with toroid primary winding will be good to stop inrush current?
im not moving but the 24r or 6r, i know that its hard with me, but, please - bare with it
and friend is not from usa he uses 2.5-5om thermistor 5-10A on 230v mains in europe
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i have the same problem as the OP but in my case I know the cause. I tried to reduce output voltage of my toroid from dual 42 to dual 22 so it involves removing covers and unwinding. Problem is since I'm not really that good with manual jobs, I was unable to keep the windings tight and I put the covers back on with some windings still a bit loose. Actually even the covers were not that tight right now.
I haven't listened to it yet after putting the covers back but while it was lying on my table uncovered, I heard it hum. I'll try to listen to it later to see if it still hums.
Now, if it still does and I find it acceptable, can I just leave it like that or would it be disastrous? I can accept a bit of hum from traffo since I do know I really can't make the windings any tighter, or even the cover. Although, I'll be trying my best to keep it safe mechanically. So, can I leave it like that or hum means mechanical and then electrical disaster instantly?
I haven't listened to it yet after putting the covers back but while it was lying on my table uncovered, I heard it hum. I'll try to listen to it later to see if it still hums.
Now, if it still does and I find it acceptable, can I just leave it like that or would it be disastrous? I can accept a bit of hum from traffo since I do know I really can't make the windings any tighter, or even the cover. Although, I'll be trying my best to keep it safe mechanically. So, can I leave it like that or hum means mechanical and then electrical disaster instantly?
Hum due to mains DC comes and goes. It is not constant.I listened to it and hum was gone now. Guess it was just the fact that it was lying on my table uncovered that made it hum
Maybe some neighbour is using a motor that is causing the DC effect and you only hear the symptom when their motor is running.
thanks for the reply Andrew
However, most transformers are actually wound tightly. But after reducing voltage output, winding is now loose. I did however wrap it in its original cover (that plastic one, or is it? )
And also, a thick cover of electrical tape coz I don't trust that original cover now. I wasn't able to put it back the way it was before. If the transformer will be fixed and secured tightly in a chassis, would it still be safe?
However, most transformers are actually wound tightly. But after reducing voltage output, winding is now loose. I did however wrap it in its original cover (that plastic one, or is it?
)And also, a thick cover of electrical tape coz I don't trust that original cover now. I wasn't able to put it back the way it was before. If the transformer will be fixed and secured tightly in a chassis, would it still be safe?
The insulation tape is usually safe to kV.
As long as it is not cracked/split/torn, it will stop you touching any live parts.
Most insulation will be wound on so that there is sufficient overlap to guarantee at least two layers so that if one is damaged then the other still insulates.
The wire will always have some unsupported lengths. These lengths ring/vibrate like a stringed instrument rings.
As the current passes back and forth the force exerted in the wire makes it vibrate. That is normal. The hum you hear is that normal vibration. Making the wire tighter does not stop the forces, nor does it stop the vibration. A different tightness/tension will change the natural frequency for any fixed length.
Completely encasing every wire in a non resonant solid will almost completely stop the "free wire" vibration. But the majority of toroids do not have that. Some will have a dipped core to help stop that humming.
As long as it is not cracked/split/torn, it will stop you touching any live parts.
Most insulation will be wound on so that there is sufficient overlap to guarantee at least two layers so that if one is damaged then the other still insulates.
The wire will always have some unsupported lengths. These lengths ring/vibrate like a stringed instrument rings.
As the current passes back and forth the force exerted in the wire makes it vibrate. That is normal. The hum you hear is that normal vibration. Making the wire tighter does not stop the forces, nor does it stop the vibration. A different tightness/tension will change the natural frequency for any fixed length.
Completely encasing every wire in a non resonant solid will almost completely stop the "free wire" vibration. But the majority of toroids do not have that. Some will have a dipped core to help stop that humming.
@bepowell - in adding extra turns in series with the primary, are you aiming to wind in phase or out of phase with the existing primary, and how do you know which way is which?
I would assume that to reduce the core saturation, you would be adding turns out of phase to the primary?
I think I have a core saturation issue with mine. DC blockers no help.
I would assume that to reduce the core saturation, you would be adding turns out of phase to the primary?
I think I have a core saturation issue with mine. DC blockers no help.
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Hi. First measure a secondary output voltage on no load. Use some insulated wire to add say five turns around the core. Connect this in series with the mains primary. Measure the secondary voltage again. When the secondary voltage reduces slightly then the phase is correct for reducing the magnetising current and noise. Add turns until the transformer noise is low, but not so low as to reduce the secondary voltage too much. When the turns and direction are decided upon make the job neat and SAFE. The secondary is on the outside layer in general. Thus will mean adding a good layer of tape, before the extra turns are wound on, to get adequate insulation. Also stay with insulated wire for the few extra noise reducing turns, for its insulation too. Then overwind with more tape. Make sure any clamps do not cut through insulation. If the secondary side is mains grounded, this will be safe. If a two wire system is being envisaged then the modified transformer MUST BE 4kV flash tested.
Where the wires are joined they need to be soldered and heat shrink sleeved with two layers.
Do not do this this unless you are fully competent with hazardous voltages.
Where the wires are joined they need to be soldered and heat shrink sleeved with two layers.
Do not do this this unless you are fully competent with hazardous voltages.
Most of these cheap toroids are not wound with audio on mind. Time ago i bought 2 300w on RS rs pro marked (Scandinavian Transformer Poland)instant hum without nothing placed and when i saw the secondaries had the same thin wire than the primary ,i ordered new ones and returned them.
Simple, no-math transformer snubber using Quasimodo test-jig. look at attachment PDF/Quasimodo_jig_revA.pdf
information to check the transformer phase with scope and you can try a snubber on secondary .10nf and 150nf +20R half W just to try if there is an attenuation .
Simple, no-math transformer snubber using Quasimodo test-jig. look at attachment PDF/Quasimodo_jig_revA.pdf
information to check the transformer phase with scope and you can try a snubber on secondary .10nf and 150nf +20R half W just to try if there is an attenuation .
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