I am building an isolation transformer to put in front of my audio system. It is a 2000VA and currently in a chassis with 4 thermistors in series. I used it as-is and had some mechanical hum issues due to DC in my house wiring. I build a DC filter with the ATL PCB on eBay. It has a 33-ohm 10W resistor in parallel with the 2 caps.
Do I need the thermistors with the DC filter or will the 33-ohm absorb the inrush current? If I do, 4 in series seems a bit excessive! I'd like to keep power supply impedance as low as possible or is that a moot point on the primary side of the isolation transformer?
After the isolation transformer I am planning on using a ZeroSurge 8R15W-I since I need always on for my preamp and then I can put the rest of the gear on the switched outlets.
Do I need the thermistors with the DC filter or will the 33-ohm absorb the inrush current? If I do, 4 in series seems a bit excessive! I'd like to keep power supply impedance as low as possible or is that a moot point on the primary side of the isolation transformer?
After the isolation transformer I am planning on using a ZeroSurge 8R15W-I since I need always on for my preamp and then I can put the rest of the gear on the switched outlets.
How did you determine the need for the thermistors originally?
Why do you need in-rush current suppression - does something fail or trip in your abode?
Is the 33 ohm 10W rated for the surge current you think will pass through it ? What is the resistor dissipation when isolation transformer is operating normally and at full anticipated load.
Why do you need in-rush current suppression - does something fail or trip in your abode?
Is the 33 ohm 10W rated for the surge current you think will pass through it ? What is the resistor dissipation when isolation transformer is operating normally and at full anticipated load.
The transformer came off of a piece of equipment and already had the thermistors intact.
My understanding is that transformers that large can have large inrush currents so you need the thermistors to absorb those surges during first application of power.
I'm not sure about the 33-ohm. It is in the DC filter PCB I bought on eBay. The designer says to still add the thermistors as the 33-ohm will be in parallel with the ESR of the caps, so very little resistance.
Not sure if that is enough to handle the inrush surge or if I also need thermistor(s) ... that is why I started the thread.
Only if they are likely to cause problems in the preceding circuitry, since you are using a DC blocker, then it's advisable. I'd use proper surge protection circuitry that involves a relay to short the thermistors/resistors after a delay.
Craigtone, if you can identify the thermistor make/model then it would be appropriate to reverse engineer what the likely in-rush energy could be (assuming the thermistor string was well designed). The combined joule rating of the thermistors should be adequately above the max in-rush energy anticipated.
I'd suggest the main reason for the thermistors is to allow standard fuse and circuit breaker current levels to be used, and the surge capability of those protective devices to cope with the thermistor suppressed in-rush. Otherwise the cb's or fuses would need to be changed to allow higher in-rush levels (eg. D-curve cb's rather than typical c-curve). It would also likely suppress any turn-on audible 'boing'.
The insertion of a DC filter is a bit trickier to determine. The impedance of the capacitors needs to be estimated firstly. That impedance will depend on frequency, and the in-rush current will likely include a high harmonic content. That impedance will be in parallel with 33 ohm, and that total impedance will further suppress in-rush current peak levels. And we don't know the likely peak in-rush current levels for starters unless you want to do some mains side probing. The peak current level without thermistors can be guesstimated by the VA rating of the transformer (10x the VA current rating), but that is usually multiplied up for toroids by about 2x.
I'd suggest the main reason for the thermistors is to allow standard fuse and circuit breaker current levels to be used, and the surge capability of those protective devices to cope with the thermistor suppressed in-rush. Otherwise the cb's or fuses would need to be changed to allow higher in-rush levels (eg. D-curve cb's rather than typical c-curve). It would also likely suppress any turn-on audible 'boing'.
The insertion of a DC filter is a bit trickier to determine. The impedance of the capacitors needs to be estimated firstly. That impedance will depend on frequency, and the in-rush current will likely include a high harmonic content. That impedance will be in parallel with 33 ohm, and that total impedance will further suppress in-rush current peak levels. And we don't know the likely peak in-rush current levels for starters unless you want to do some mains side probing. The peak current level without thermistors can be guesstimated by the VA rating of the transformer (10x the VA current rating), but that is usually multiplied up for toroids by about 2x.
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Hmmm ... the isolation transformer came off of a device that draws a lot of current.
The stock supply has three 5-ohm, 20A, 200J thermistors in series.
This isolation transformer will remain on all the time and will not be switched on and off. The load I will be putting on it is only around 4A. I run low-wattage SET tube amps feeding horns.
I am considering just eliminating the thermistors all-together. On a similar note, if I am using it JUST as an isolation transformer, what is the proper fusing regiment? Should I fuse the primary, secondary, both or none? I am thinking a resettable circuit breaker on the primary that is sized according to the transformers capability (~15A).
The stock supply has three 5-ohm, 20A, 200J thermistors in series.
This isolation transformer will remain on all the time and will not be switched on and off. The load I will be putting on it is only around 4A. I run low-wattage SET tube amps feeding horns.
I am considering just eliminating the thermistors all-together. On a similar note, if I am using it JUST as an isolation transformer, what is the proper fusing regiment? Should I fuse the primary, secondary, both or none? I am thinking a resettable circuit breaker on the primary that is sized according to the transformers capability (~15A).
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