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Old 29th September 2011, 10:38 AM   #1
Legis is offline Legis  Finland
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Default Inrush current limiter and choosing a correct relay

Hi,

I'm changing the relays of the inrush current limiter. I recently modded the amp and by dropping the line resistance (bypassing fuses) and rising the amp stage load (higher bias than before) the inrush current got bigger to the extent that a 10A relay burned after 3 weeks of use. I'm thinking to change all the relays to 16A versions and maybe change the inrush NTC's (that are relay bypassed) to sturdier power resistors with optimal value, that could inhibit the inrush current and 120,000F capacitor bank charging burst a little. Right now there are 2pcs of 10R NTCs in parallel, I'm thinging to buy 15-20R 50W aluminum power resistor to replace them (tranformer is 1,5kVA).

Now the difficulty of choosing a correct replacement relays. Naturally the operating voltage should be the same, but what about the relay's coil resistance, if I cannot find a perfect match from upper amperage relays? Original 10A relay has resistance of 280ohm. If I can choose the 16A relay from 220ohm (slightly harder load) and 365 ohm (slightly easier load), which one is safer bet or does it really matter? Also is there some possible issues replacing a 200ohm relay with 900-1100ohm relay?

Thanks for sharing the info.

Regards,

Legis
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Old 29th September 2011, 11:21 AM   #2
AndrewT is offline AndrewT  Scotland
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the relay contacts in the soft start bypass generally do not have a severe duty to meet.
The contacts should never fail.
Have you wired the soft start correctly?

The statement
Quote:
there are 2pcs of 10R NTCs in parallel,
leads me to think you don't know what you have built.
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Old 29th September 2011, 11:41 AM   #3
Legis is offline Legis  Finland
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Quote:
Originally Posted by AndrewT View Post
the relay contacts in the soft start bypass generally do not have a severe duty to meet.
The contacts should never fail.
Have you wired the soft start correctly?

The statement leads me to think you don't know what you have built.
Hi Andrew,

could you also answer to the relay coil resistance problem that I have?

Design is commercial. The burned 10A relay connects the power to the primary of the tranformer actually, it is not the relay that connects the NTC's nor the relay that bypasses them. There are two primaries and 10A relay for each. The other of these relays had burned it's contacts.

There are 2pcs big 26mm diameter, 10R NTC-thermistors in parallel (= series resistance 5R at 25 degrees) that are bypassed with relay after couple hundred milliseconds (I don't know exactly when). They do the inrush current limiting, and work perfect on stock configuration (no peaks whatsoever), but I get the feeling that the increased bias needs more current limiting. The cold startup with empty capacitor bank makes some transformer hum and approx. 350W power peak is also visible on my power meter (but that apparatus cannot measure the true highest peak due to the low sampling frequency). The formula here leads me to think that 15-20R is correct for 1,5kVA transformer.

edit. with inrush current I mean both the tranformer inrush and the capacitor inrush. They both could be somewhat less so I can power up without a fear of blowing a fuse even with high base load on the electricity line.

Last edited by Legis; 29th September 2011 at 11:53 AM.
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Old 29th September 2011, 12:04 PM   #4
AndrewT is offline AndrewT  Scotland
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Read up on soft start circuit for starting a big transformer off the mains.
Read up on slow charge circuit for slowly charging capacitors off the rectified secondary.

Once you understand the difference between these two duties you can then see what you need to do to solve your contacts problem.

Some builders think they can combine the soft start with the slow charge duties into one circuit. I do not believe that this combined circuit gives the required performance for both duties. Yes, it muddles through and it allows lower rated fuses to be used, but it is not right.

If you need further help then I think you will have to let us see the schematic for what you have bought. Do you need permission to post that here?
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Old 29th September 2011, 02:56 PM   #5
Legis is offline Legis  Finland
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By changing the original 5A relay of the limiter NTC's to high guality looking Finder brand 16A-relay, all the startup related tranformer hum and strange relay "cracling noise" is completely gone! Transformer stays completely silent even during a totally cold start up. I can barely hear that the amp is started, only the sound of relays is audible.

However the measured spike stayd the same, 300-350w. Still very good outcome.
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Old 30th September 2011, 06:41 AM   #6
Mooly is online now Mooly  United Kingdom
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Relay coil resistance...

It all depends on what the feed to the coil is.

A constant voltage feed and it doesn't matter as long as the loading isn't below what the drive can supply.

If it uses constant current where the relay coil is perhaps in series with a large value resistor fed from high voltage then it matters greatly and some modification may be needed.

Good designs fire the relay at full voltage and then drop down to a "holding voltage" which generates much less heat. The holding voltage may be as little as 10% of the nominal coil value.
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