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Old 23rd July 2010, 11:25 AM   #11
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tangmonster's Avatar
Join Date: Jul 2007
Location: Cape Town

Really sorry for taking over your thread!!!
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Old 23rd July 2010, 11:28 AM   #12
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We use a very simple technique in servicing equipment. Put a light bulb in series with the power. The bulb "absorbs" the inrush. When the bulb dims, flip another switch that shorts across the bulb...You could use a multi-position switch to have a "standby" mode, followed by "ON".
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Old 23rd July 2010, 12:13 PM   #13
AndrewT is offline AndrewT  Scotland
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Originally Posted by len_scanlan View Post
The power transformer has a nominally 120 ohm wirewound resistor in series with its primary winding. This limits the power-on surge current while the large capacitor bank on the secondary side charges up.
I think you have overlooked the fact that there are two separate mechanisms at work here and both need addressing.
They happen over very different time periods and I believe that two separate systems should be used to control the peak currents due to the different phenomena.

At start up the transformer core has no or little flux in it. This results in a very short term current into the primary of the transformer to establish the core flux. This is all complete in a few cycles of the mains waveform. Since each cycle lasts 17ms or 20ms, the system should be done with and bypassed at about 100ms to 300ms.
This is the soft start that all transformers and motors need if we are to avoid the typical starting fuse rating of ~3times the maximum rated transformer current.
480VA on 240Vac is 2Aac. The normal fuse without a soft start is ~6A. typically we use T4A, or T5A or T6A.
If we use a soft start system then that starting fuse can be reduced by a factor of three. Say between T1.6A to T2A for a 480VA 220/240Vac transformer.
When you have an auxiliary supply I would suggest a properly timed delay using a timer IC (555 are cheap and easy) of 100ms to 250ms to power a rely that bypasses the current reducer after the flux has been established.
You can use a combination of Power Resistors and/or Power Thermistors.
A 120r Power Resistor on the mains side of the Primary will get hot very quickly!!!

The second reason for high current soon after the transformer starts up is the charging of the smoothing capacitance. This happens over a period of hundreds of milliseconds to many seconds. The lower the current limit, the longer it will take.
I believe this second current surge is best met by a slow charge system timed to bypass after 1s to 10s depending on charge rate and limiting method.
Again an IC timer is better for controlled delay.
This slow charge current limiting can be done on either the primary or secondary of the transformer. I have never needed it, but my biggest capacitor bank is only +-75mF and it does not blow the close rated starting fuse (T4A for 1000VA).
If resistors are used then as soon as the resistor is bypassed the current rises instantaneously to complete the charging process. If this resistor is in the primary it will overheat. Don't.
If the resistor is in the secondary then I'd suggest that the resistor be progressively reduced over an extended period to avoid that third current peak after bypassing the slow charge.
If you choose to use Power Thermistors for slow charge, (I believe PTC are ideal for this duty) then they can be fitted in either the Primary or the secondary. Again they must be bypassed but in this case the resistance falls as the Thermistor heats during the charge build up and then the Thermistor resistance value increases again as the caps reach full charge. The current surge on bypassing is much less then with resistors. You can use longer delays here without overheating consequences. One CL60 on the 220/240primary will not do enough to allow reducing the fuse to close rated. Two series connected might just about let the fuse survive, But I suspect some experimentation will be required, particularly if you combine the soft start with the slow charge and try to do the whole current limiting with one timed delay system.

Sorry, this took so long, but there are many on the Forum who do not agree with me. As a result, I see alternative views on why and how to implement soft start and slow charge. Some are OK, some are plain dangerous. Read carefully, select what you want to believe, convince your self that what you adopt is safe and meets your criteria for performance. Good luck.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 23rd July 2010 at 12:18 PM.
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Old 23rd July 2010, 01:11 PM   #14
Marra is offline Marra  United Kingdom
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Would this meet your criteria Andrew;

soft start module
Soft Start Module for Mains Supply

* SSM1
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SSM1 Avondale Audio soft start module

Fully Automatic Timing Sequence - No more dimming lights, pops, clicks or blown fuses!

Operates in line fully automatically or manually with a low current switch.

Handles up to 1KW.

Includes an on-board mains current breaker / fuse, spark suppression and transient clamping.
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Old 25th July 2010, 11:31 AM   #15
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Hi AndrewT,

Thanks for your very detailed thoughts on soft-start. Yes I did overlook the transformer core issue, and your ideas about PTC on the secondary side are interesting too - I'll have to take this offline and have a think.

Tangmonster - Thanks for your suggestion of using a 555 timer. I've actually done this in the past, but I was trying to achieve a discrete design, purely because I want to use parts I already have in stock. I have a large supply of MJE350's and plenty of 24V relays, so that's why my circuit uses '24V logic'.

Andrew, I agree with your comments about using an auxilliary supply for the 'other functions' in an amp, thereby keeping the main audio supply clean(er).. Apart from the external 12V trigger, there will be some (tasteful I might add) LED lighting, possibly a fan, and definitely a thermal cut-out in case of the ventillation getting blocked. Nice to keep this stuff separately powered for the sake of a few dollars.

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Old 25th July 2010, 04:57 PM   #16
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Marra, that soft start module has very little information to make a decision, and is rather expensive. Check out the one I bought: Connexelectronic

It says in the manual that you can download (before you buy) that it's good for 24A @ 110V or 20A @ 230V (RMS).
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Old 26th July 2010, 04:31 AM   #17
ChrisA is offline ChrisA  United States
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Originally Posted by cliffforrest View Post
The major problem with the thermistor solution, which is otherwise very elegant, is what happens on multiple rapid power cycles?

The thermistor cool-down time is too long.

A relay solution should be designed for an instant disconnect and very rapid re-arm.
I used to think that would be a problem too but there is no in-rush on a rapid power cycle because the caps are still charged. You choose a themister to match the time constant on the bleeder resister. If you choose the parts correctly the in-rush current is always controlled.

The even older solution is a choke input power supply, then you don't have the problem so no solution is required.

Last edited by ChrisA; 26th July 2010 at 04:33 AM.
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Old 10th August 2010, 08:43 PM   #18
Marra is offline Marra  United Kingdom
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Thanks Tyler; that connexelectronic soft start looks a real bargain.

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Old 11th October 2010, 07:47 PM   #19
brickj is offline brickj  United Kingdom
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That soft start module from connexelectronic looks pretty neat. Has anyone used one in anger? I have a couple of 400VA transformers I need to switch remotely.


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Old 11th October 2010, 07:48 PM   #20
brickj is offline brickj  United Kingdom
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That soft start module from connexelectronic looks pretty neat. Has anyone used one in anger? I have a couple of 400VA transformers I need to switch remotely.


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