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Old 22nd September 2002, 10:25 PM   #1
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Question Elektor soft start circuit problem

I have built a soft start circuit that was published some time ago in Elektor. I am using this circuit to power 1500VA 230V toroid for Super Leach amp. The relay is 8A constant and 15A max or 2000VA rated.
But after some time the contacts stick together. I increase the time delay to 3-4 second, but it happens again!

Any suggestions how to solve this problem?

Supernet
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Old 23rd September 2002, 12:04 AM   #2
skaara is offline skaara  Slovenia
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Mabe the relay is too weak, if the contacts still sticks together..
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Old 23rd September 2002, 01:01 AM   #3
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For the circuit to work the contacts suppse to stick together. When you remove supply voltage the contacts should separate. If they don't do it, it's either still power supplied to relay (the only explanation for this would be really big caps and small current draw from relay), or the relay is no good.
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Old 23rd September 2002, 08:02 AM   #4
skaara is offline skaara  Slovenia
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I meant thet the relay has non sufficient ratings, and current holds the contacts together..
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Old 23rd September 2002, 10:32 AM   #5
yeti is offline yeti  Germany
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Default Relay

Another possibility would be that the inrush isn't finished when the relay closes its contacts, so go for more delay, or decrease the value of the limiter resistors.

Arne
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Old 23rd September 2002, 11:24 AM   #6
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To be honest, I think your main problem is the relay. I have sub with 240 VA (2 x 120 VA) toriod and the relays are normal 2-pole 8 A. I have two relays, one for the main switch and one for the bypassing of a resistor (100 ohm/10 W). The main relay stick now and then but never the soft start relay.

In my second sub I have a 500 VA toriod and the relays are ELESTA SKR135 (3 x 10 A) together with a time module, very neat and very well functioning (not very cheap though).

You can always connect the relays contact in series in order to improve switching life time.
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Old 23rd September 2002, 02:30 PM   #7
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Actually, you want to wire relay contacts in parallel, not series, for increased life.

Supernet:

You don't specify what you mean by the contacts sticking together. Do you mean that they permanently stick and you have to replace the relay? The contacts may be weldeding themselves closed due to excessive switching current.

Without a schematic of the circuit it's hard to be specific but in general, a relay for soft start should not engage while there is a potential for high inrush current to flow through the contacts. Perhaps you PS caps are so large and the limiting resistor such a high value that the caps have not had time to charge by the time the relay closes.
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Old 24th September 2002, 10:23 AM   #8
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Smile howdy bill..

...actually contrary to intuition, wiring relay contact in parallel does nothing to increase service life i am afraid....see app. notes:
http://relays.tycoelectronics.com/application.stm
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Old 24th September 2002, 11:05 AM   #9
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Quote:
Originally posted by Bill Fitzpatrick
Actually, you want to wire relay contacts in parallel, not series, for increased life.
It's quite the opposite! Why? You will decrease the time when you will have an spark. The most wearing thing for the contacts is switching of BUT also swithing on because of the contact bounces. The result is faster contact speed and that a good thing. The contact gets less effected by the hot spark.

If you connect the contacts in parallel don't exceed the current rating for one contact, because you will probely not get equal currents in the contact because of unequal contact resistances.

If you have 2 or 3-pole relay, use every pole in series if you can. This is a known trick out in the industry.

The other solution of the relay problem is to use some sort of contact protection. C or R+C across the contact. Sometimes it's not safe because the circuits isn't totally switch off but in many cases it's OK. The C in this case must be a X-type, approved for X-application.

BTW: Very good link, never come across them before.

Cut from the text:
Never parallel relay contacts to double the contact rating. Unless the relays are specially adjusted, they will not pick up and drop out simultaneously. Even if they were to be specially adjusted, they would not hold this adjustment over life.
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Old 24th September 2002, 12:58 PM   #10
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Nice link to Tyco and I stand corrected.
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