Capacitor/Transformer Voltage Rating for a 12V Power Supply?

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Discussion Overview

The discussion centers around the design considerations for a 12V power supply, specifically regarding the voltage ratings of capacitors and transformers. Participants explore the necessary voltage ratings for components when converting wall power to a usable 12V output, including discussions on transformer specifications and power ratings.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses a lack of knowledge in electronics and seeks guidance on the appropriate voltage ratings for capacitors in a 12V power supply project.
  • Another participant suggests using capacitors rated below 50V for a circuit utilizing a 7812 voltage regulator, with an input voltage of 15-18V.
  • There is a question about the transformer’s primary and secondary voltage ratings, with some confusion about the 240V specification in the schematic versus the typical 100-120V wall outlet in the US.
  • One participant argues that using a transformer with a secondary voltage of 35V is inefficient and suggests a lower secondary voltage of 15-18V instead.
  • Another participant explains the significance of VA (Volt Ampere) ratings for transformers, discussing the factors that contribute to transformer losses and heat generation.
  • Several participants clarify the distinction between primary and secondary voltages in transformers, emphasizing the importance of understanding these terms in the context of the circuit diagram provided.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate secondary voltage for the transformer, with some advocating for lower voltages to improve efficiency while others question the necessity of such specifications. There is also a lack of consensus regarding the voltage rating of the transformer in relation to the US wall outlet voltage.

Contextual Notes

Participants reference various voltage standards and practices in different countries, highlighting the potential for confusion regarding voltage ratings. There is also mention of the complexities involved in transformer design, including losses that affect performance.

Who May Find This Useful

This discussion may be useful for electronics enthusiasts, hobbyists building power supplies, and individuals seeking to understand transformer specifications and capacitor ratings in practical applications.

TheLaw
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Hello,

About me...so you know where I am coming from. If you don't want to read, skip to the end-ish...

I'd like to start off my saying that I know next to nothing about electronics although I am eagerly reading the All New Electronics Self-Teaching Guide right now and it is confusing but I am slowly understanding it.

There's a lot of equations in that book. Perhaps some of you may have heard of it, read it. But, I am going to attempt to do a very small project and that is building a simple 12V power supply for...anything that might require 12V.

So I am searching over at Mouser.com, the site which I have had the best luck with and I am looking for some nice capacitors. I am an avid PC hardware enthusiast trying to get into electronics and nothing draws my eye quite like aluminum organic polymer caps. These caps are pretty much worshiped on the PC spectrum. (AKA solid state caps).

But, don't really know squat-diddly about electronics. What kind of voltage rating do I need to use on a 12V supply sipping power straight from the wall. No batteries or anything.

Solid state caps only go up to about 50V or so I have seen but standard electrolytics can go up to 900-1000v.

What the heck do I need? I have some schematics, just trying to determine the parts. This is the schematic: http://www.instructables.com/files/orig/F6Z/DVT6/FKVQJ2RJ/F6ZDVT6FKVQJ2RJ.jpg

Also, and I am sorry if I am asking too much, the primary/input rating on a transformer should be like equal to the wall? And the output maybe around 35V?

Thanks.
 
Last edited:
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you are using 7812, which will give 12V output. Input side voltage should be 15-18V. so you need capacitor <50V ratings.
 
http://www.mouser.com/ProductDetail/Panasonic-Electronic-Components/EEU-FC1E472/?qs=sGAEpiMZZMtZ1n0r9vR22SCZt3APDM9SqIKsF5pj5H4%3d

Cap?
 
Last edited by a moderator:
Primary voltage on the transformer can be up to 35V?
 
^ you mean secondary voltage of transformer (i.e. voltage input to rectifier circuit) ? as primary voltage on the transformer is 240V as per circuit diagram.

why to use 240/35 V transformer? it would create problems of low efficiency, over heating, high temperature. 240/15 (or 240/18V) is sufficient.
 
aks786 said:
^ you mean secondary voltage of transformer (i.e. voltage input to rectifier circuit) ? as primary voltage on the transformer is 240V as per circuit diagram.

why to use 240/35 V transformer? it would create problems of low efficiency, over heating, high temperature. 240/15 (or 240/18V) is sufficient.

Thank you very much aks786. I read about 50 pages on transformers yesterday before I crashed for the night, in hopes of trying to make a little bit of sense.

Here's the one thing I am not getting. I understand how transformers work but on the schematic it says 240V. I live in the US and is not a standard wall outlet only 100-120V? Why does it say 240V then? Am I missing something.

Also, regarding power rating. VA = Volt Ampere. Is there a reason why they put VA instead of Watt? This is the one I am considering: http://www.mouser.com/ProductDetail/Triad-Magnetics/F372P/?qs=sGAEpiMZZMv4oUrzpPKU3Ka3fUoV036URXm0H0/Wt4U%3d

Thank you very much for your help.
 
Last edited by a moderator:
240V is also standard in some countries like in india. (higher distribution voltage is dangerous but economical)

TheLaw said:
Also, regarding power rating. VA = Volt Ampere. Is there a reason why they put VA instead of Watt?
like all other electrical machines, transformers rating is limited by temperature rise, which depends on losses. in transformer there are mainly two type of losses

1. copper losses - depends on current through windings and resistance of winding (I²R). keeping resistance constant this part is restricted by current.

2. iron losses - depends on core (size, material, lamination thickness), flux density and power frequency. here physical properties of core and power frequency is constant, so iron losses will depend on flux density. which will depend on voltage applied.

in this way transformer heat generated will depend on voltage applied on transformer and current flowing through windings, it has nothing to do with power factor. so you will find transformer ratings such as 300VA, 315KVA, 100 MVA ...
 
Thanks you have very clear descriptions.
 
aks786 said:
240V is also standard in some countries like in india. (higher distribution voltage is dangerous but economical)
Actually, outside of North America, 200V to 240V AC is by far the most common utilization voltage.

See : http://upload.wikimedia.org/wikipedia/commons/f/f5/Weltkarte_der_Netzspannungen_und_Netzfrequenzen.svg
 
  • #10
uart said:
Actually, outside of North America, 200V to 240V AC is by far the most common utilization voltage.

See : http://upload.wikimedia.org/wikipedia/commons/f/f5/Weltkarte_der_Netzspannungen_und_Netzfrequenzen.svg

I suppose its much cheaper to run higher voltage into your house, but wow, that seems really dangerous.
 

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