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Toroid inrush current???

1/3/2005 6:20 AM
Toroid inrush current???
Sorry about this double post but I somehow posted in the wrong forum...  
I'm building a rack-mnt power distro for my Rocktron gear w/ some Hammond toroids. I have reservations about the possibility of frying all of my rack units from the possible inrush issues from using toroids (from what I've read this can be an issue).  
Since I have no experience dealing w/ toroids or current inrush limiters, what is my best approach for averting disaster? TIA!  
P.S. More ignorance: can I run an LED from the CT of the trannys to ground to get a decent Voltage drop, or would I fry the LED immediately once the units conn. to the 2ndary are turned on? Thanks!  
P.S.S. The trannys are Hammond 18-0-18V CT @3.33A, I'm using 3 of those to cover 7 units.
1/3/2005 3:41 PM

Cant speak to the toroid issue, but since most LEDs don't want more than 20ma running through them, they will be at risk as soon as the secondary tries to source more than 20ma. Also you might want to check the reverse voltage rating of the LEDs since I think that is fairly low as well.
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1/3/2005 4:48 PM

Thanks Enzo! :)  
The thought occured to me that the CT is the ground reference for the taps, and if I wanted only 20mA of capacity, I went way over-spec on the tranny selection (and wouldn't need a toroid either...LOL).  
Would MOV's last longer if a couple were paralleled? I know they don't last all that long because they're always seeing the AC. I was thinking series conn. MOV's but the first MOV will see the brunt of the action, but if it failed (hopefully closed), the second one could continue duties w/ a little longer life. TIA!
1/3/2005 5:35 PM
Don Symes

Consider Tranzorbs in place of MOVs - they're a sort of ultra-fast, high-current, looooow-duty-cycle zener. They really take a pounding. Available in bipolar and normal diode-style.  
Tranzorb is a brand name. I think the generic term is TVS.  
The inrush thing with toroids is related to the capacitive load (basically your power filters). IIRC an inrush overload can generate big enough mechanical stresses cause your toroid's core to break. Conceptually, teh thing you want here is something resistive to trickle-charge the load and something else to shunt that resistor after a few seconds.  
Lots of ways to do it, even some integrated devices. Have you tried digikey of mouser? (I hanven't)
1/3/2005 6:14 PM

Thanks Don!  
Since this is just a basic Voltage Step-Down function, and there are no capacitors anywhere (no filtering at all), should this be a moot point in this application?  
Nonetheless, I'll definitely check out the Transorbs/TVS devices. Thanks for the tip!  
1/3/2005 8:30 PM
Don Symes

Um ... the tranzorbs are for ESD (voltage surge) not for inrush (current surge).  
So, a voltage stepdown. OK, then what? In most amplifiers, you have the recto, the big filter caps, and then the preamp filter string.  
Maybe I didn't understand the application.
1/3/2005 8:40 PM
Don Symes

[quick re-read] Oh, I see. You're feeding some FX with low-voltage AC at a few tens or hundreds of mA each.  
Each FX box then has a recto and some filters, but the voltages are an order of magnitude lower than a tube amp. That makes the inrush currents (into a given capacitive load) an order of magnitude lower as well.  
You should be OK, but try hard to keep your loads well below the toroids' ratings.  
If you have the option to sequence the power to individual boxes, that would about bulletproof the thing.  
The toroids are a good choice for this application because they really don't radiate (though at these (assumed) currents EI cores sould be quiet enough).
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