Min. PT sec. current spec. for stereo insert switcher?

[fazeka]

I am building a stereo insert switcher (http://www.ka-electronics.com/kaelectronics/Insert_Switcher/Insert_Switcher.htm & https://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=7&t=797) utilizing a Bud CH-14400 enclosure. I was planning to use a Triad VPT30-830 for power but I can't fit it into the Bud box without interference from the top cover touching the power transformer.

I am using 14 DS2Y-S-DC12V relays having 16.7 mA nominal operating current for a total of 233.8mA. Of course the Triad had plenty of reserve for the job at hand and it's what I had lying around. But being that it won't fit, I'll need to get a smaller (shorter) PT that will.

My question: What's the lowest current rating on the PT secondary I can get away with this given this application? Is 250mA cutting it too close? What factor (e.g., 1.4? 1.5?) do I use to ensure safe/long operation?

Thanks,
Chris

 

[trobbins]

I'm guessing you don't have a lab power supply or battery to operate the switcher and measure the actual max current being drawn with all relays engaged ?

Most transformers have a dc output nominal max current spec.  Were you going to use a 24V linear regulator after a raw DCV?

 

[ruffrecords]

Here is a link to an info page at Sowter on transformer/rectifiers:

https://www.sowter.co.uk/rectifier-transformer-calculation.php

For a regular bridge rectifier followed by a capacitor input filter they recommend the transformer current rating is 1.6 times the dc output current. I find this can run the transformer too close to saturation so I personally use a factor of 2.

Cheers

Ian

 

[moamps]

....I am using 14 DS2Y-S-DC12V relays having 16.7 mA nominal operating current for a total of 233.8mA...

IIRC, they use 24VDC power supply for that board, and 12V relays are connected always two in series so the max. current at 24V is half of that you calculated.  Also, you will loose some power on a series regulator, so IMO the transformer rated to 20Vsec/15-20VA would fit nicely.

 

[fazeka]

Hi, thanks for the info! Appreciated!

Looks like I'm going with a Talema 62053 then!

https://www.digikey.com/product-detail/en/talema-group-llc/62053/1295-1065-ND/3881350

Cheers,
Chris

 

[PRR]

> Triad VPT30-830 for power but I can't fit it into the Bud box

Putting the PT in a tight box with a low of unsheilded audio sounds like begging for hum.

I'd be thinking 24V SMPS wart dropped behind the desk.

 

[fazeka]

Hi PRR,

Yeah, you're right.

I never used a wall wart for any of my builds. Does the wall wart output 24VDC regulated that I can take direct to the relay PCB? What's fusing look like? ??? I remember I had an old Lexicon Alex that used a wall wart that had no apparent fuse on the rear of the chassis...

 

[PRR]

> I never used a wall wart

What do you use to charge your cell-phone? Laptop? Tablet? E-book?

Modern ones are somewhat self-limited to  not start fires. (Cheap ones may anyway.) While they are not the Perfect DC Source, they have some advantages.

 

[fazeka]

Hi PRR,

You missed the rest of the sentence when you quoted me: "... for any of my builds." Of course I've used a wall wart.

This is important as all my builds use linear supplies that are fused. I'm asking if I need to fuse DC.

> I never used a wall wart

What do you use to charge your cell-phone? Laptop? Tablet? E-book?

Modern ones are somewhat self-limited to  not start fires. (Cheap ones may anyway.) While they are not the Perfect DC Source, they have some advantages.

 

[trobbins]

Fusing depends on the wallwart, as it may have a current limit protection.