Soft Start for EI Transformer necessary ?? - diyAudio
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Old 9th September 2013, 11:19 PM   #1
Minion is offline Minion  Canada
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Default Soft Start for EI Transformer necessary ??

Hi , I hear a lot about needing a Soft Start for Toroidal transformers over 300Va but are they needed for EI transformers ??

I have a 600va EI transformer that I salvaged from a dead 300W class A/B PA amp that I wanted to use to power a couple 350w into 4 ohm L15D amps ...

Thanx
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Old 10th September 2013, 12:18 AM   #2
dangus is offline dangus  Canada
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It may depend on the size of the filter caps. I've got a 60wpc DIY amp with 26,000 uF per rail; the lights dim momentarily when I plug that in.

It should be healthier for the power switch, the capacitors, and bridge rectifiers if you limit the inrush current.
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Old 10th September 2013, 12:31 AM   #3
DUG is offline DUG  Canada
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Quote:
Originally Posted by dangus View Post
It may depend on the size of the filter caps. I've got a 60wpc DIY amp with 26,000 uF per rail; the lights dim momentarily when I plug that in.

It should be healthier for the power switch, the capacitors, and bridge rectifiers if you limit the inrush current.
I would agree.

When one amp I built powers up it sounds like I hit the box with a hammer.
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Old 10th September 2013, 01:05 AM   #4
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At some point, you can not avoid a soft start. As the transformer gets larger, the instantaneous current draw when you connect power can blow a 15A breaker (typical line fuse in the USA) but might have a maximum of 5A steady state draw. If you want to protect the amp, you should install a 5A fuse, but you can't without using a soft start because it would blow at every power on.

I don't believe that the transformer construction really changes this reality much...
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Old 10th September 2013, 03:10 PM   #5
Minion is offline Minion  Canada
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Okay guys , Thanx ... You guys know of a simple softstart circuit ??
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Old 10th September 2013, 06:10 PM   #6
Minion is offline Minion  Canada
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Hi , a thought just occurred to me .. Could I possibly use a 3 position power switch with one position as power off , the second position would apply power to the transformer through a high power resistor to limit the inrush current , and the 3rd position bypasses the resistor ??
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Old 10th September 2013, 06:12 PM   #7
macboy is offline macboy  Canada
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Quote:
Originally Posted by Minion View Post
Hi , a thought just occurred to me .. Could I possibly use a 3 position power switch with one position as power off , the second position would apply power to the transformer through a high power resistor to limit the inrush current , and the 3rd position bypasses the resistor ??
What happens if you/wife/guest leaves the switch in the second position with the resistor in-line? Hot resistor and fire? Probably.
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Old 10th September 2013, 06:16 PM   #8
Minion is offline Minion  Canada
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So you are saying it would work but there are risks involved ?? Okay , ill try a Thermistor as I think I have one around here somewhere .... Thanx
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Old 11th September 2013, 10:45 AM   #9
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In soft start circuits, indeed series resistors are used to momentarily limit the current to the transformer. But, they stay in circuit for milliseconds.

If you calculate the power that they will consume, you would say "ok just dim the lights every time" instead of buying such a thing. Thus, you choose a lower wattage resistor and use it for a brief period of time in series with the transformer. This is well pointed out in one of the above posts - fire is possible. No manual soft start in my opinion.

Too low a resistor, heavy current will go through it and you need massive power rating. Too high, and I doubt you will soft start your equipment. If tiny voltage gets through to the secondary, then after removing the resistor the caps will again need heavy currents to charge.

I don't say there is no solution. Many soft start circuits are published around. Chech out Rod Elliot's site for a proper first approach to the matter.
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Old 13th September 2013, 02:20 PM   #10
AndrewT is offline AndrewT  Scotland
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CL60 makes a good soft starter and it is "fail safe" in that if the bypass fails the Thermistor's NTC effect ensures it never sets fire to itself.
CL60 are expensive in the UK and we need two or three for each 240Vac transformer. I use a string of 20r 5W resistors bypassed in ~200ms by a timed relay.
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