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Old 21st October 2004, 11:13 PM   #21
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Quote:
Originally posted by MikeB
EVA !!!! Where do you live ? I only read "destroyed by lightning"...
Sounds scary ! In my whole live i never had something damaged
due to lightning....

Mike

Lightning can be a real bummer. I remember when I had an old 8-bit Nintendo that was destroyed. It wasn't even plugged in....to the wall. The cable was though. When lightning hit the power pole 50 ft. from my house, it fried. Oh well, mother nature can be a ***** some times.
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Old 22nd October 2004, 10:14 AM   #22
MikeB is offline MikeB  Germany
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Hmm, my only scary experience with lightning was some ELMS-fire.
As i was a child, i found it funny and exciting, now i know how critical
this situation was. The lightningbolt came down ~50meters away.
I think my ears lost at least 1khz...

Mike
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Old 22nd October 2004, 01:44 PM   #23
quasi is offline quasi  Australia
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I would still be interested in looking at your soft start circuit. But most i have seen use banks of resistors and a relay for the soft start. this works well, but i know i have seen some that ramp up the voltage by controlling the on/off times of the incoming sine wave.

Hey Zero Cool,

Sorry about the delay, but I gotta draw it again.

Basically it's just a resistor feeding a capacitor on the base of a darlington transistor swithing a relay that has contacts across fat resistors. In my amp this circuit is powered by a dedicated power supply from a control board fed by a seperate (tiny) transformer.

My friend has tried a similar circuit to the one posted by EchoWars but this cct has one major flaw. The voltage across the main capacitor bank rises too slowly for the relay to operate with a snap. On his the contacts touch a second or so before the relay armature has closed fully. Relay contacts hate this and will fail earlier. Relays like operating with a snap so that the contacts have full pressure applied as soon as possible.

In the transistor circuit the relay closes quickly with a clear snap.

I have another idea about this using a quad schmitt trigger with voltage dividers across the capacitor feeding the different gates. This would allow soft turn on for say 4 seconds, speaker connect after 6 seconds etc.... I've not built this though.

Whan I get around to drawing the other circuit I'll sketch this out too.

Cheers
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Old 22nd October 2004, 02:28 PM   #24
Bricolo is offline Bricolo  France
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Default Re: Big is beautiful....well in some places ;)

Quote:
Originally posted by Magura
How about a pair of 5lb PP film caps to go with the torroids??

http://www.briangt.com/gallery/magura/PICT0001



Magura

OMG!!! Someone did it: a DIY capacitor made of a beer can
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Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
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Old 22nd October 2004, 02:36 PM   #25
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I have found some Velleman Brand kits that have several types of light dimmers that operate from one touch.

One touch and they ramp from off to on. they even make a 1Kw model! all are suitable for use with transformers.

This is more along the lines of what i am wanting to build. something that literally ramps the voltage up from 0 to 120VAC over a period of about 1 second. then a large relay can kick in and bypass the soft start if need be.

http://www.velleman.be/

Is the website and here is a construction manual for one of the kits with a schematic:

http://www.velleman.be/Downloads/0/Manual_K8024.pdf

the kit i have shown here is not large enough to handle this task as is. BUT, im sure it could be modified with a larger triac.

There are several other dimmer module kits available. any one of them look like they would perform the function i am looking for.

They use a triac to control the duty cycle of the incoming sine wave to ramp the voltage up. If used for an audio amp. this would allow a large bank of capacitors to charge up very slowly without large surges. the only question then becomes will a relay bypass be needed once full power is reached? Will a triac inline cause any additional noise??

I have no doubt that a resistor/relay system would work as well as this has been done many times! I even considered doing some type of multi step/resistor ramp type system. but a phase control system just seems to fit the bill. and they dont appear to be that hard to make.

Anyone have any reason a Dimmer type ramp slow start shouldnt be used????


Zero
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Old 22nd October 2004, 02:46 PM   #26
quasi is offline quasi  Australia
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Hey Zero Cool,

Yep that should work the way you want it.

I would go the fatest Triac you could find though. This torroid of yours is going to pull some juice on switch on.

Will the triac generate any noise ?

Cheers
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Old 22nd October 2004, 03:33 PM   #27
Eva is offline Eva  Spain
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Driving inductive loads or capacitor-rectifier loads from a triac or from thyristors requires special considerations in comparison with resistive loads. Most dimmer circuits are designed to work only with resistive loads and these malfunction or blow when an inductive load is connected

I've designed a prototype of phase-sweep soft-start circuit based in two diodes and two thyristors. It's intended for high power offline SMPS applications where 2.000uF or higher capacitances must be charged directly from the mains line. Soft starting in these circumstances is almost mandatory, otherwise peak startup currents in excess of 400A would be generated. It's high enough to trip circuit breakers, blow fuses and even damage diodes and capacitors

The circuit works quite well and uses continuous triggering [10Khz gate pulses] so theoretically it's also suitable for transformer soft-start, replacing thyristors and diodes by a single triac, altough I've not tested this application yet

The bad news are that I don't have drawn full schematics yet, but these are some pictures :

Click the image to open in full size.
The control circuit itself, it uses an LM393 as a 10Khz 10% duty cycle oscillator and a PWM comparator. There are also five BC550/BC560 transistors [zero-cross comparator, etc..] and a lot of 1N4148 diodes. It also uses a 4N25 optocoupler for remote shutdown. The circuit powers itself from mains due to its low power consumption. There is also a 555 used as a 50Hz oscillator to allow for careful testing before mains applied [anyway, I allways use isolation transformers for testing offline circuits]. Mains power comes through blue/black wires, test 15V power comes through red/green wires, shutdown pulse comes from blue/green wires and thyristor drive goes through grey/yelow wires

Click the image to open in full size.
This is the thyristor/diode bridge. Black/green wires are AC input and red/black are capacitor output. A simple 6A diode bridge with both AC terminals connected in paralell is used as a 12A dual diode, together with two BT151/650 thyristors

Click the image to open in full size.
This is the pulse transformer that drives both thyristors. It has a single primary and two secondaries with turn ratios 5:1,1 allowing for proper thyristor triggering and very small current consumption at the same time
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Old 22nd October 2004, 03:35 PM   #28
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Quote:
Originally posted by quasi
Hey Zero Cool,

Yep that should work the way you want it.

I would go the fatest Triac you could find though. This torroid of yours is going to pull some juice on switch on.

Will the triac generate any noise ?

Cheers

I dont know if the triacs will generate noise at full on. thats is the question! I may have to use a large relay to bypass the traic once the soft start has ramped up all the way.

I need to build a test setup to test the traics for noise. and if that noise will affect the output of the power supply.

I dont have a spectrum analyzer however. I want to buy a HP3580A but i cant afford one even used right now!

Anyone have any suggestions on how to test for noise? I do have a nice scope. and i could visually check at least but...


Zero
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Old 22nd October 2004, 03:42 PM   #29
Eva is offline Eva  Spain
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A turned-on triac with continuous gate drive works essentially as two slow soft diodes connected back to back in paralell. It won't produce any RF ringing

Secondary side rectification is the only potential source for RF ringing

PS: Don't try connecting a big toroid transformer to a classic resistive lighting dimmer. The usual problem is asymetric drive to the transformer, causing saturation and a blown triac or no dimming effect at all
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Old 22nd October 2004, 05:39 PM   #30
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Quote:
Originally posted by Eva
A turned-on triac with continuous gate drive works essentially as two slow soft diodes connected back to back in paralell. It won't produce any RF ringing

Secondary side rectification is the only potential source for RF ringing

PS: Don't try connecting a big toroid transformer to a classic resistive lighting dimmer. The usual problem is asymetric drive to the transformer, causing saturation and a blown triac or no dimming effect at all


No noise, this is good news!

Eva, please take a look at the kits, i posted a link for. i would not use the kits as-is, only as a basis for designing my own, or maybe as a controller.

As i understand these kits are designed to work with low voltage lighting where a step down transofrmer is normally used.

Please explain more about asymetric drive.

Yes i understand a typical household dimmer doesnt have the Inductor in them needed to handle large highly reactive loads like transformers!

Thats why these kits, looked so interesting to study. surley there is some schematics out there for this sort of thing?


I have a very large variac i use for testing!

Zero
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