Maker Pro
Maker Pro

Transformer help

bradt10550

Feb 19, 2015
2
Joined
Feb 19, 2015
Messages
2
Hi guys, I'm relatively new to electronics, and I'm trying to build a 15v 1A split rail power supply. I've found a pretty good schematic, but I'm having trouble finding a transformer that I can use with it. I really don't know much about transformers, so any info would be of great help, thanks! Also the schematic says 230v, but I'm in america so that would be 120v instead right?

Schematic:
CQNn0p0.jpg
 
Last edited:

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
Hi guys, I'm relatively new to electronics, and I'm trying to build a 15v 1A split rail power supply. I've found a pretty good schematic, but I'm having trouble finding a transformer that I can use with it. I really don't know much about transformers, so any info would be of great help, thanks! Also the schematic says 230v, but I'm in america so that would be 120v instead right?

Schematic:
CQNn0p0.jpg
That transformer is a step-down transformer with a center-tap on the secondary.
Perhaps searching for similar key words will give you the result you need.
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
You can use e,g, this one. Note that this one has two primary coils which you'll need to connect in parallel.It also has two secondary coils, where you'll have to connect the center wires to create the 3-wire configuration as shown in your circuit diagram.

Observe the phases! Look at the datasheet:
  • On the primary side connect the two pins with the little dot and connect the two pins without dot. Do not mix dotted and un-dotted pins.
  • On the secondary side, connect the dotted pin of SEC1 with the un-dotted pin of SEC2.

For selecting another transformer, relevant parameters are:

  • input voltage AC 115V or 120V at 60 Hz
  • output voltage 2*15V (rms)
  • output current 2*1A or output power 30 VA (30W) respectively
Note that you need 15V (rms) at the output terminals. This will lead to a peak voltage of 21.2V (15V * sqrt(2) at the terminal,s labeled con1 in the schematic.Subtract the voltage drop across the diode (~0.7V) in the rectifier and you'll have 20.5V as indicated in the schematic.
C1 and C2 are rated 25V in the schematic. This is not a good design choice:
  • If your input voltage has 10% overvoltage (not unrealistic), the peak voltage across C1 or C2 will rise to 22.6V, very close to the max. limit of the capacitors.
  • If additionally the transformer has only a small load, the output voltage will rise additionally by 1.19V (see datasheet) and the voltage across C1 and C2 will rise to 27V which is above the limit of the capacitors.

I recommend you use 35V types for C1 and C2 for increased lifetime/durability.
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
why are you paralleling the primaries, Harald ?
Because the transformer's datsheet states 2*115V You can use the primaries in series for 230V or in parallel for 115V. To achieve the same power at 115V as at 230V, you need to double the current. If you were to use only one primary for 115V, the copper resistance would limit the input current and thus the output power to less than 30VA.
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
Hi guys, I'm relatively new to electronics, and I'm trying to build a 15v 1A split rail power supply. I've found a pretty good schematic, but I'm having trouble finding a transformer that I can use with it. I really don't know much about transformers, so any info would be of great help, thanks! Also the schematic says 230v, but I'm in america so that would be 120v instead right?

Schematic:
CQNn0p0.jpg

Just make sure you heat sink the devices if needed. At 1 Amp the device will be dissipating 5 Watt of power.
Adam
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
Because the transformer's datsheet states 2*115V You can use the primaries in series for 230V or in parallel for 115V. To achieve the same power at 115V as at 230V, you need to double the current. If you were to use only one primary for 115V, the copper resistance would limit the input current and thus the output power to less than 30VA.

yeah ok :)
 

bradt10550

Feb 19, 2015
2
Joined
Feb 19, 2015
Messages
2
Thanks for the help! So would it work if I just plugged a hammond 166J18 directly into a standard US wall outlet and then wired up the circuit as described in the schematic? I know this one puts out 18v not 15v, but I read that it would be good to give the voltage regulators a little extra voltage so that I don't have any losses coming out the DC side. In that case I would just use some standard heat sinks right?
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
Thanks for the help! So would it work if I just plugged a hammond 166J18 directly into a standard US wall outlet and then wired up the circuit as described in the schematic? I know this one puts out 18v not 15v, but I read that it would be good to give the voltage regulators a little extra voltage so that I don't have any losses coming out the DC side. In that case I would just use some standard heat sinks right?
Keep in mind that the regulators you are using are analogue regulators... so they regulate the voltage by wasting the excess voltage as heat.
 
Top