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Old 20th March 2005, 09:48 PM   #1
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Default testing without inrush protection

I'm building ESP's project 36 and capactive multiplier power supply. I've completed the power supply but am still waiting for my current inrush limiter boards from ESP.

I'm still struggling to understand what the inrush current limiter is protecting. Is it protecting my diodes? What risks am I running wiring up the supply and measuring it's output voltage without protection?

I'm using a 400VA toroid with each secondary feeding a gbpc3501 35A rectifier.
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Old 20th March 2005, 10:50 PM   #2
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You need to be patient and wait for the current limiter.

A toroidal transformer sucks a lot of amps in the first split second its magnetic is energized. Plus, power supply caps look like a dead short until they are charged. A lot can happen in the first 1-2 seconds when you turn on a power supply.

Some find that a thermistor in series with the incoming AC is sufficient, something like a CL-30. Others, like me, use a time delay relay and a 50 watt 50 ohm resistor to allow 5-7 seconds for the power supply to "soft start." After the initial few seconds the relay closes and removes the power resistor from the circuit and puts the AC directly to the amp.

Of course, some find their diode bridge and house branch electrical circuit husky enough to withstand initial turn-on abuse. Most don't, especially if your cap bank in the amp is of any good size.
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Old 20th March 2005, 10:59 PM   #3
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Darn, more waiting. Your explanation makes sense.

My supply has a pair of 4700uF 63V caps before the capacitance multiplier.

Where's the cutoff point when deciding whether or not to use an inrush limiter?
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Old 20th March 2005, 11:17 PM   #4
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It's been some time since I last looked at the ESP cap mult. circuit, but most cap multipliers use a husky pass transistor with its base tied to a fairly large capacitor -- the one that get "multiplied." Before and after the pass transistor can also be sets of fairly large capacitors. One must consider the circuit as a whole. You don't want to fry the pass transistor or the diode bidge or the electrical branch circuit in your home. Without a circuit diagram and component values it is difficult to be more precise, but if Rod Elliot says to use an inrush current limiter then I would do so -- or make a circuit similar to his.
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Old 20th March 2005, 11:59 PM   #5
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Are you really impatient? Then wire a 100W light bulb in series with one leg of the mains feed. It will glow nicely at power on, then fade out as the caps charge up. And it will severely limit the inrush current to a safe value.

If the light bulb stays lit, power down immediately and figure out what you wired wrong.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 21st March 2005, 12:44 AM   #6
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Yes, I am really that impatient. I'm just getting excited to finish this project. This has been a very challenging project putting all the pieces together.

I must confess, I actually tested one of the power supplies before posting my question but then decided to not to proceed further. I was delighted that it worked.

Actually, the inrush limiter was recommended by someone on Rod's board due to the size of the transformer I'm using. I thought I had seen Rod mention it but going back through the project 36 articles, I don't see any mention of it. Given that his softstart circuit is project 39, that may be why.

In the project 36 construction article Rod recommended a 300VA 30-0-30 transformer. I already had a 400VA 25-0-25 without a home so I decided to use it. All other component values are identical to what Rod published. Here's the schematic I'm working from:
Click the image to open in full size.
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Old 21st March 2005, 04:16 AM   #7
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The pass transistors will not begin to conduct until the big cap attached to their base charges so you have a sorta' soft start already available. The initial 4700 uF caps are not that large so you should be able to just turn on the PS without a problem. The part that might create a problem is the big toroid transformer sucking a huge amount of initial current until its magnetic field is established. Do some research in the search engines here. There is a complete thread (or two) devoted to the use of an inrush current limiter and one message has the URL for a table of values to use to select which thermistor to use. The CL-30 is a good safe choice for you, I believe. Many amps that use toroidal transformers use this CL-30 inrush current limiter.

In the meantime use the light bulb hook up suggested by SY and you will be safe.
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Old 29th March 2005, 02:49 AM   #8
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I don't want to hijack ultrachrome's thread, but it seems pointless to start a new one since I have the same question to ask - only my components are different.

I'm making dual single ended power supplies, one for each t-amp, with the following components:

(1) 120VA toroidal transformer with dual 15VAC secondary
(2) 25A bridge rectifier
(2) 22000uF filter cap

The supplies for the t-amps only need to be single ended, so each secondary will be connected to a bridge rectifier and a filter cap. The unregulated DC will be fed to a voltage regulator (LT1083) closer to the amp PCB for good voltage regulation.

While the VA rating of the transformer is quite below the 300 VA or so rating that I've read would require a soft-start circuit, the filter caps are quite large. Do you guys think this supply would require a soft-start circuit or will the inrush current be kept to an acceptable level?

back to the original topic - ultrachrome, did you ever get/install the soft-start circuit?

edit: I forgot to mention that there will also be a 6A rated EMI filter before the transformer, if that makes any difference. I know that they filter out the EMI going back into the line but I don't know if they would have any impact on inrush current.
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Old 29th March 2005, 05:16 PM   #9
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I received the boards for ESP's inrush current limiter yesterday but I'm actually going to start testing without it this week.

I went back and reread the project and construction articles for the project and Rod does not mention inrush current protection anywhere. Additonally a recent thread suggested I wouldn't need protection unless my capacitance was dramatically higher than what it currently is.
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Old 29th March 2005, 05:50 PM   #10
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Default Re: testing without inrush protection

Originally posted by ultrachrome
I'm still struggling to understand what the inrush current limiter is protecting. Is it protecting my diodes? What risks am I running wiring up the supply and measuring it's output voltage without protection?
The main purpose is not to blow the fuses in the wall.

The second reason is to prolong the lifetime of the mains switch.
/Per-Anders (my first name) or P-A as my friends call me
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