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Transformer Goof?

epsilon24

Transformer Goof?

Postby poseidonsvoice » April 21st, 2012, 11:29 pm

Ti,

Hi,

Great forum, and probably the best written DIY Audio website out there. Kudos.

I think I goofed on my transformer selection for the Epsilon 24 (with the Sigma 24 transformer board). My relay I am using is the Omron G8P-1A2T-F-DC12 relay, you recommend on your site (As a 12V relay, the rated current here is 77mA on the data sheet: http://www.components.omron.com/compone ... T_0911.pdf).

The transformer I have is this one:

http://www.alliedelec.com/search/produc ... U=70037414

It has a single 12V secondary, 125mA current and is 1.5VA. I bet it is underrated for the job, yes? Your schematic shows a capacitor input filter, and as such only 60% of the 125mA is available, or about 75mA).
I figure at least 3VA is warranted. I still want something encapsulated though, but both Mouser and Digikey are out of the Pulse EI-30's. Allied had the Zettler unit, but not enough current.

I might have to go for a toroid, but seriously don't want to deal with any more EMI.

I'll keep looking around.

Best,
Anand.
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Re: Transformer Goof?

Postby amb » April 22nd, 2012, 12:20 pm

The transformer should work... are you having problems with it?
The "input filter" you described is just part of the bridge rectifier circuit that converts AC to DC.
The transformer secondary current rating only has to be 1.414 times the DC load current for a bridge rectifier, so with a 125mA rating the supply is able to provide 88mA and stay within the transformer's load rating. Granted that's not much headroom, but with a 12V transformer, the rectified voltage is ~17V, minus a couple of volts for diode loss we have 15V, so there is 3V voltage headroom for the regulator. All should be well...
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Re: Transformer Goof?

Postby poseidonsvoice » April 22nd, 2012, 1:42 pm

amb wrote:The transformer should work... are you having problems with it?
The "input filter" you described is just part of the bridge rectifier circuit that converts AC to DC.
The transformer secondary current rating only has to be 1.414 times the DC load current for a bridge rectifier, so with a 125mA rating the supply is able to provide 88mA and stay within the transformer's load rating. Granted that's not much headroom, but with a 12V transformer, the rectified voltage is ~17V, minus a couple of volts for diode loss we have 15V, so there is 3V voltage headroom for the regulator. All should be well...



Okay. The best thing to do then is to fire it up and see how it functions. I'll report back.

Incidentally, the power amplifier I am using has its own integrated fuse in the power supply. Of course, I'll need a fuse for the Sigma 24 as well. I figure if the secondary current draw is a maximum of 125mA, then the fuse should be: Total VA rating/Line Voltage X 133% = 1.5VA/120V X 133% = a very tiny fuse!

Thanks,
Anand.
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Re: Transformer Goof?

Postby amb » April 22nd, 2012, 3:25 pm

Yes, it would be a small-value fuse, but you need to account for turn-on surge current (energizing the transformer fields and charging of the bulk capacitor).
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Re: Transformer Goof?

Postby poseidonsvoice » April 22nd, 2012, 4:06 pm

Ti,

You are correct. I wouldn't know what the turn on surge current would be for such a small transformer, unless I modelled it on PSUD2. About the smallest fuse I know of is 0.1A slow blow. Hopefully that is enough?

Best,
Anand.
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Re: Transformer Goof?

Postby amb » April 22nd, 2012, 4:10 pm

I used a 250mA fuse on my ε24 (used in the β24 power amp). If your AC line voltage is 115V then that works fine. If it's 230V, then use half the current rating.
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Re: Transformer Goof?

Postby poseidonsvoice » April 22nd, 2012, 4:15 pm

Perfect, I have got a boatload of 250mA fuses, and 120V steady where I live.

I will report back when everything is set up.

Thanks for supporting diyers with your designs :mrgreen:

Best,
Anand.
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