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Old 29th May 2001, 09:22 PM   #1
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Location: Poole, England
I want to start at least 4 toroids of 500va+ and wish to leave the circuit breakers in place on the main fuseboard.

Previous projects have used "only" 330va toroids and they vibrated (quite loudly)when powered up, though they were attached well.

I will have a lot of capacitors to charge and don't want to just dump a bucket load of current directly in to them.

I have seen a circuit that simply short circuits a resistor in series with the transformer primary, but it seems a little crude and i will need at least 4 of them (one for each transformer).

Anybody seen anything slicker?
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Old 29th May 2001, 09:34 PM   #2
Freddie is offline Freddie  Sweden
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I've seen some people using thermistors in series with the transformer primary.
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Old 30th May 2001, 01:52 AM   #3
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There aren't too many options. Thermistors are one. Relays with resistors are another. If you happen to be planning regulated rails (unlikely) you can make a soft-start regulator. It's a pain, but there's always the old stand-by of bringing up a really big circuit with a Variac.
It's not the toroids that are the problem, it's the caps behind them. You can hook up a huge transformer and switch it on. Nothing happens. The reflected impedance the AC line sees is essentially infinite. Put caps in there and the equation changes, and the more caps, the worse it gets.

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Old 30th May 2001, 07:13 AM   #4
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Thermistors are easier to use but harder to pick. Maybe the toroid manufacturer has an idea what to use there.
Another solution that works is putting series resistors between caps. This is has 2 functions
1. limits large incoming currents on turn on
2. works as RC low pass filter in the power supply. The resistors are usually 0.1 Ohms each.
This adds to the series output resistance of the power supply but I donīt think itīs so important here. Itīs used widely in a lot of amps.
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Old 30th May 2001, 02:55 PM   #5
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BAT uses 10 Ohm version thermistor for the VK60 in 240V mode, in 120V mode they use 5 Ohm version.

I am using a product from which is expensive, but good. It is digital and so works no matter how short your switch-off is. A thermistor will stay low resistance if say power goes and comes back up.

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Old 30th May 2001, 06:33 PM   #6
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You can use inrush current supressors and they come in a variety of resistances and current handling capability.
radio amateurs use these soft start circuits for their linear amplifiers and you will find these circuits in the ARRL manuals.
I thought I would use relays with a resistance across them and a resistor/cap combination for a small delay.
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Old 5th June 2001, 12:05 AM   #7
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Default Soft starting big toroids

I had this problem with soft starting two 300VA toroids and
120,000uf of caps but found a project that Elektor UK produced for soft starting . The authors page is :
and the GIF

Of course you will have to change the components slightly
for 115v or contact the author but the project is a dead
simple resistor/relay combo that powers up through the
resistors to stop the surge and then switches over to bypass. I have built several of these and they work well. Also a friend has been using one of these in his amp for nearly two years now.

Regards Kooze
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Old 5th June 2001, 07:18 AM   #8
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Would it be possible to build a current limiter out of 2 transistors, and put it between the bridge and the cap bank?
I'd set the current at 120% transformer secondary rating.

ASCII art doesn't work too well here, so I'll describe the positive current limiter. A negative limiter should be easy. If Silicon transistors are used, the voltage drop will be 0.6V - 1.2V (current dependant). Germanium transistors should drop this to only 0.2V - 0.4V, but the circuit will respond slower. All transistors, and the pass resistor, should be on a heatsink.

The collector of the pass transistor (NPN high current) is connected to the input and one terminal of the base resistor (1K). The emitter of the pass transistor is connected to the base of the regulator transistor (NPN medium current), and one terminal of the pass resistor. The base of the pass transistor is connected to the second terminal of the base resistor, and the collector of the regulator transistor. The second terminal of the pass resistor is connected to the emitter of the regulator, and the output.

The current limit is set by the pass resistor. With Silicon transistors, the pass resistor should be 0.6/limit. With Germanium transistors, the pass resistor should be 0.2/limit.

Good luck.
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Old 5th June 2001, 11:54 AM   #9
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Location: Sydney Australia
You could put transistors between the bridge and the caps
but probably not advisable as you then introduce another component between the two .My preference is KISS ( keep it
simple *) although I have seen a similar Resister/Relay combination in this position on some amp designs.
With regard to the circuit I found the info to run it on 110v

C1=680nF , C2=470nF , R1=120R
(Capacitor working voltage 400VDC/130VAC)

oh and the URL was supposed to be :

(I love the way IBM's drop the extra l at the end)
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Old 5th June 2001, 05:34 PM   #10
ppl is offline ppl  United States
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I have noticed that when i powered A 1000 Watt Plitron Troid with no load that upon startup the transformer humed for a sec and dimed the Lights momentarly. is this not normal?

While were at it is it possible to stack Troids on top of one another without any problems? Will the Bolt used to mount the Two cause any problems? TIA PPL
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