Replacement Power Supply Project

[melodyjester]

Hi people

Ok, my project is to replace a power suppply unit. The old unit had the following spec;

INPUT: 240V AC UK MAINS SUPPLY
OUTPUT: Dual 18v AC 0.62A
MAX POWER CONSUMPTION: 22watt
Delivered to a 3 pin connector: 18v AC - GND - 18v AC

The replacement i had in mind has the following spec;

"AC Dual 18V 8VA Output EI Core Open Frame EI-4120 Type Power Supply Transformer"

LINK:
http://www.ebay.co.uk/itm/AC-Dual-1...al_Components_Supplies_ET&hash=item1c3464ee9a

It has 3 output wires: 2 x 18v AC and a GND (which is perfect)
However, it kicks out 8VA when i think i only need 0.62A

Would this transformer be a suitable replacement and also is there anything that i need to be concerned about that i am missing?

Thanks in advance

 

[KMoffett]

Assuming the output is used with a center-tapped-transformer rectifier circuit, 18VAC*0.62A=11.16VA (watts)
That transformer is likely too small.

Ken

 

[hevans1944]

... However, it kicks out 8VA when i think i only need 0.62A

As @KMoffett said, the replacement transformer you selected is likely too small. You are confusing power (or VA) with current.

The original transformer delivered 36 V AC at a maximum current of 0.62 A from a center-tapped secondary winding. This represents a maximum power (or volt-ampere) consumption of 22.32 W (or VA ) for a transformer rated to consume a maximum of 22 watts. These two numbers are in close enough agreement to imply that 0.62 A can be drawn from either side or both sides of the center-tapped winding.

The replacement transformer doesn't "kick out" 8 VA. That is the maximum power it can safely deliver. What it actually delivers, and the VA consumption, is determined by the load on the secondary. Your 8 VA transformer will deliver a maximum of 8 VA / 18 V = 0.44 A. Note this Chinese import is rated 8 VA per secondary winding, so presumably you could draw 0.44 A from either side or both sides of the center-tapped winding. Either way, you are 0.18 A short of the maximum current provided by the original transformer.

Another consideration: 8 VA (watts) is considerably smaller than 22 watts. You might compare the volume occupied by the original transformer to the volume occupied by the proposed replacement to appreciate the difference.

 

[KMoffett]

Right you are!...I need more coffee...or less.

Ken

 

[melodyjester]

Cheers Thanks for your responses

I have just checked and it is indeed 36 VCT. (i wondered what the CT bit meant).

The original power supply had two jobs.... firstly to provide continuous power to the main amplifier and secondly to provide 48v DC as and when needed (condenser mics take 48v phantom power).

So basically, you are saying that i would need it to be at least 11.13VA (0.62 X 18) if it was a CT transformer to be any where near suitable?

 

[hevans1944]

No, you need about 22 VA: 11.13 VA from each half of the center-tapped winding. Most of this will be needed to provide continuous power to the main amplifier. The 48 V DC "phantom power" for condenser mics requires virtually no power.

 

[melodyjester]

Ah! i get it now cheers for clearing that up for me!

 

[melodyjester]

OK, i have another contender but it does not state whether it is AC/DC output... so before i start to bombard them with questions etc, i just wanted your thoughts on the following contender (this one is from Maplins)...


2x18V 50VA Chassis Mounting Mains Transformer
50VA Clamp Mounting Transformer
Mains: 0 – 115V or 0 – 230V
Double Secondary Winding
Temperature Class B (130°C)
Dielectric Strength: 3750 Vrms
Regulation: 9%

here is the link....
http://www.ebay.co.uk/itm/2x18V-50V...al_Components_Supplies_ET&hash=item5d526d6cdd

Once again, thanks in advance :

 

[hevans1944]

The writing on the picture shows two 15 VAC windings while the blurb advertising this transformer says 2x 18V. Who knows which is correct? If you can live with the supposedly reduced voltage (from 18 VAC to 15 VAC) this transformer has the power (VA) capacity to replace your original 22 watt transformer. It is probably physically somewhat larger. Note the two primary windings are rated at 115 VAC, so connected in series they would have to have 230 VAC to produce the rated 15 VAC output from the two secondary windings. If your line voltage in the UK is really 240 VAC, this will produce a slightly larger voltage on the two secondary windings, perhaps enough to account for the "2x 18V" specification in the advertising blurb.

I think this transformer is a good choice to replace your original transformer.