# What current is that transformer (noobie question)

#### billgr

Joined Jul 11, 2019
8
Hi everyone, I'm making a linear power supply for my computer audio system, suitable for HDD or network cards.
The psu needs to deliver 5vdc and 2A minimum.

My toroid transformer Y236103 has 2 secondarys and is rated at 1A on the manufacturer's website http://www.avellindberg.com/transformers/y23_range_specs.htm

My question - is that the combined total or can each secondary deliver up to 1A?

#### nsaspook

Joined Aug 27, 2009
8,686
The load is rated at 30VA so that's 1A in each 15VAC secondary.

#### AlbertHall

Joined Jun 4, 2014
11,506
You can parallel both windings to get 15VAC at 2A. If that is fed to a bridge rectifier with a smoothing capacitor that will give you around 20V DC and an RMS transformer secondary current of 1.61A, so that's OK.

The transformer voltage is much higher than is actually needed though. The output power is 5W (5V @ 1A) and the regulator needs to dissipate 15W of heat ((20V - 5V) * 1A). You could use a smaller and cheaper transformer or, to keep the heat down, use a switching regulator.

#### billgr

Joined Jul 11, 2019
8
Wow you guys are fast. Thanks for the help.

#### AlbertHall

Joined Jun 4, 2014
11,506
You can parallel both windings to get 15VAC at 2A. If that is fed to a bridge rectifier with a smoothing capacitor that will give you around 20V DC and an RMS transformer secondary current of 1.61A, so that's OK.

The transformer voltage is much higher than is actually needed though. The output power is 5W (5V @ 1A) and the regulator needs to dissipate 15W of heat ((20V - 5V) * 1A). You could use a smaller and cheaper transformer or, to keep the heat down, use a switching regulator.
Correction [edited again] to that: You need 2A DC output but that will 3.2A RMS secondary current which will seriously overload the transformer.

#### billgr

Joined Jul 11, 2019
8
Correction [edited again] to that: You need 2A DC output but that will 3.2A RMS secondary current which will seriously overload the transformer.
That's interesting, I'd be keen to learn what's the logic behind that.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
Not so fast...

I urge you to look at this note: http://www.hammondmfg.com/pdf/5c007.pdf

AC secondary current needs to be significantly [was less] more than the DC current in full wave applications.

<Changed per Les - migraine kicked in>

Last edited:

#### LesJones

Joined Jan 8, 2017
3,498
Hi KISS,
I do not agree and neither does the information in your link. For full wave rectification with a capacitor output filter it states
V (Peak) D.C. = 1.41 X Sec. V A.C.
V (Avg) D.C. = 0.90 X Sec. V A.C.
I D.C. = 0.62 X Sec. I A.C.
So the AC current is about 1.6 times the DC current . (1/0.62)

Les.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
Your correct. Modified post. I knew that.

#### Dodgydave

Joined Jun 22, 2012
10,050
Hi everyone, I'm making a linear power supply for my computer audio system, suitable for HDD or network cards.
The psu needs to deliver 5vdc and 2A minimum.

My toroid transformer Y236103 has 2 secondarys and is rated at 1A on the manufacturer's website http://www.avellindberg.com/transformers/y23_range_specs.htm

My question - is that the combined total or can each secondary deliver up to 1A?
Just use the 5V rail from your computer Atx psu...

#### billgr

Joined Jul 11, 2019
8
Just use the 5V rail from your computer Atx psu...
That's exactly what I'm trying to avoid. I want to provide _clean_ power to these components and try not inject any noise back into the other components. ATX power is convenient but not ideal for my application.

I thanks for the replies and the links. I'm getting the gist of it now. I don't have the circuit diagram (only the pcb) and I need to determine whether its full wave rectification and then I can order a suitable transformer.

#### billgr

Joined Jul 11, 2019
8
As expected it's full wave bridge rectification but using discrete diodes.

#### billgr

Joined Jul 11, 2019
8
I need 3.22 A for supply of 5vdc 4A

A 50VA transformer with 12 vac secondarys will supply
50va / 12v = 4.17 A (more than needed)
Va = 0.9 * 12 = 6.75
Vp = 1.4* 12 = 10.5 (so I need at least this right?)

#### AlbertHall

Joined Jun 4, 2014
11,506
I need 3.22 A for supply of 5vdc 4A

A 50VA transformer with 12 vac secondarys will supply
(1) 50va / 12v = 4.17 A (more than needed)
Va = 0.9 * 12 = 6.75
(2) Vp = 1.4* 12 = 10.5 (so I need at least this right?)
(1) is true providing the rectifier isn't followed by a smoothing capacitor. If it is then the maximum continuous current is divided by 1.6, so there is only 2.6A
(2) 1.4 * 12 = 16.8V