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Old 17th September 2003, 03:07 PM   #1
SimontY is offline SimontY  United Kingdom
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Exclamation Isolation transformer and varistors

Hello one and all,

Yesterday, I got a 2nd hand isolation transformer to use with my integrated amp (2000va/8.2A).

I stuck a large 250v-rated varistor across live and neutral to soak up spikes, but when I turned it on I blew the 5A fuse on my mains reel which it was plugged into. Any ideas why? I have used varistors across L+N (on other equipment) with no problems, and an increase in sound quality. These were smaller ones bought from a different place however...

The sound btw is quite markedly different, with a much richer tone, deeper bass etc. I have lost some attack and excitement, maybe I can put this down to crap wire from transformer to wall, which I will change to something better ASAP...


TIA for suggestions
-Simon
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Old 17th September 2003, 03:14 PM   #2
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2 kVA, have you considered inrush currents? Not small I think.
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Old 17th September 2003, 06:33 PM   #3
SimontY is offline SimontY  United Kingdom
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Default possible inrush current

It is quite large!

I had thought about inrush current, but where is the current rushing to get to? It wasn't connected to any load! Will a transformer pull some current just to sort of 'load' the wiring? And would that be enough to blow a fuse? (I noticed the internal fuses are rated at 32A/400v!!) Ooh, also, why would a varistor make it draw extra current?


-Simon
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Old 17th September 2003, 07:08 PM   #4
Rudy is offline Rudy  Belgium
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if a transformer doesn't have any load on the output the outputvoltage rizes till ... who knows ... so the VDR wil do its job, but if the VDR have to work ik wil act as a short-circuit, actually it will destroy itself in that short time, so you need to add a load to test or just use a higher voltage VDR

( this if you placed the VDR on the secundairy side, dunno if on primairy )

just my thought
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Old 17th September 2003, 08:00 PM   #5
SimontY is offline SimontY  United Kingdom
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Hi Rudy,

Quote:
if a transformer doesn't have any load on the output the outputvoltage rizes till ... who knows
Why will the output voltage rise? It's a 1:1 mains transformer.

Quote:
this if you placed the VDR on the secundairy side, dunno if on primairy
It was on the primary ie. just across mains live and neutral.


-Simon
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Old 17th September 2003, 08:10 PM   #6
tiroth is offline tiroth  United States
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Inrush current was just recently discussed, try searching. I believe figures of 150A were being bandied about for large toroidal transformers.
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Old 17th September 2003, 08:22 PM   #7
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I think your varistor was rated for 250 DC not AC

they do not need a load drawn on them.....every outlet in my house has 2 across....... live and neutral and live and ground ....they are rated at 250 VAC


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Old 17th September 2003, 08:24 PM   #8
SimontY is offline SimontY  United Kingdom
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Quote:
I think your varistor was rated for 250 DC not AC
LOL! In a way, I hope you're right! Better go check what I actually bought...

-Simon
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Old 17th September 2003, 08:59 PM   #9
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Default Is there such a thing as a DC varisistor?

I had a similar issue, I think, w/ a 220 step down transformer I threw together.

It was a big Hammond step down that I put in a box w/ a pair of duplex outlets on it. When I first plugged it in, BLEWY! All the lights went out. I should have tested it before leaving town. After the hotels floor matron showed me where the circuit breaker was, I found that I could operate the transformer, if I switched off the circuit breaker, plugged in the transformer, and then switched the circuit breaker back on. Inrush currents being the culprit, this was a mild work around.

I don’t think the varisistor should be giving you issues unless its just specked out wrong like Joe-D was suggesting. (Is there really such a thing as a DC varisistor, or is that a zenor-diode?

I’ve heard other interesting reports about iso-transformers here before. I don’t know if I’ve seen anyone hypothesize about the change in the source impedance between the transformer and the amplifier.

Without the iso transformer the amplifier in most cases will be facing a rather large transformer feeding a myriad of homes, and apartments. This to the amplifier will look more or less like an ideal voltage source (methinks?). The iso-transformer will have a substantially lower source impedance with respect to the amplifier, subsequently looking much more like a current source.

I think this may start to explain some of the perceived differences in sound quality that are observer when these devices are used. If we extrapolate a bit from here we may be able to make some parameters for ideal audiophile grade isolation transformers for various applications. (KVA, K ratings ect.)

Fred Dieckmann did you catch any that? I'd love to know what you think. Am I close to the issue, or talking out my back end (again)? If I got any of it wright, I think it was with your assistance.

Thanks.

-Dave
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Old 17th September 2003, 09:12 PM   #10
SimontY is offline SimontY  United Kingdom
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Hi Dave,

Fascinating reply. I searched and found a recent thread on inrush currents/transformers/blown fuses. The general concensus appears to be that any transformer can pull a lot of current when first connected/turned on - and 2kva is quite large!!! It's like having my own little substation

I suppose my main point originally was concerning the MOV/VDR as it only blew once in several times being plugged in - and that was when the MOV was attached.

The varistor is rated at 250vac, and it says: "max 8000 A Energy, transient 140 J, Voltage, varistor at 1mA 360 V Voltage, clamping max 650 V." It is quite a large one, I think. Here is the url...:
http://uk.farnell.com/jsp/search/sea...Y&QText=580247

Interesting musings about why a transformer can change the perceived sound. I always thought, personally, it was just because of the potentially massively reduced common-mode and 'whats the other type?' noise reduction.

For UK readers, there are more of these babies going on Ebay now, though I think they'll end up less of a bargain this time around


-Simon
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