I'm designing a small PSU for a headphone amplifier, I just need a single secondary that can handle currents in miliamp range.
I've small PCB with IEC socket, line filter, inrush limiter and fusing with an EI transformer in the end, salvaged from old network equipment. It's a very nice board, that I'd like to use, but I'm not sure of the power rating of the transformer.
It has 2x115 primary windings for 115/230V operation and 18V secondary winding. Voltage without load is 21V. I get about 26Vdc after rectification on that.
The only reference to what it can handle is the label that says to use a 125mA slow-blow primary fuse. And since it was driving quite massive circuit, I'm assuming it should be able to drive 3 low-power opamps and 3 buffers, which power the headphones.
Am I right?
I've small PCB with IEC socket, line filter, inrush limiter and fusing with an EI transformer in the end, salvaged from old network equipment. It's a very nice board, that I'd like to use, but I'm not sure of the power rating of the transformer.
It has 2x115 primary windings for 115/230V operation and 18V secondary winding. Voltage without load is 21V. I get about 26Vdc after rectification on that.
The only reference to what it can handle is the label that says to use a 125mA slow-blow primary fuse. And since it was driving quite massive circuit, I'm assuming it should be able to drive 3 low-power opamps and 3 buffers, which power the headphones.
Am I right?
Hi,
based on Ritchie's (good) estimation your transformer could be anywhere between 9VA and 25VA.
The 21/18 -1 (17% regulation) indicates it is towards the higher end of this range. Assume 18VA for an example.
18VA and 18Vac gives 1Aac on the secondary.
For DC duty after a capacitor input filter the maximum continuous current must not exceed 500mAdc.
Your 2channels of 3 opamps and 3 buffers will need around 2 * [3*6mA +3 *15mA] ~ 126mA quiescent current. This is 25% of maximum continuous rating for an 18VA secondary.
What is your real quiescent current?
based on Ritchie's (good) estimation your transformer could be anywhere between 9VA and 25VA.
The 21/18 -1 (17% regulation) indicates it is towards the higher end of this range. Assume 18VA for an example.
18VA and 18Vac gives 1Aac on the secondary.
For DC duty after a capacitor input filter the maximum continuous current must not exceed 500mAdc.
Your 2channels of 3 opamps and 3 buffers will need around 2 * [3*6mA +3 *15mA] ~ 126mA quiescent current. This is 25% of maximum continuous rating for an 18VA secondary.
What is your real quiescent current?
A slightly more scientific approach is to look up the typical VA rating of the core in a table. From one I found at http://www.jogis-roehrenbude.de/Transformator.htm :
Core ... VA rating
EI48 ... 5
EI54 ... 10
EI60 ... 15
EI66 ... 20
EI78 ... 35
EI84a ... 50
EI84b ... 75
EI106a ... 100
EI106b ... 140
EI130a ... 230
EI130b ... 280
You can find the core dimensions in a table on that page as well. In general EIxx has xx millimeters on the longest side. The /a and /b versions differ in core thickness.
With multiple secondaries, you have to judge by the wire diameter. Smaller transformers use higher specific currents (A/mm2), see the "Stromdichte" graph on the above page (http://www.jogis-roehrenbude.de/Stromdichte-T.jpg) and the wire gauge tables. For example, in a 20VA transformer, assume 3.5 A/mm2 from the graph; an 18V 1A winding might use a 0.29mm2 wire i.e. around 0.6 mm diameter.
Core ... VA rating
EI48 ... 5
EI54 ... 10
EI60 ... 15
EI66 ... 20
EI78 ... 35
EI84a ... 50
EI84b ... 75
EI106a ... 100
EI106b ... 140
EI130a ... 230
EI130b ... 280
You can find the core dimensions in a table on that page as well. In general EIxx has xx millimeters on the longest side. The /a and /b versions differ in core thickness.
With multiple secondaries, you have to judge by the wire diameter. Smaller transformers use higher specific currents (A/mm2), see the "Stromdichte" graph on the above page (http://www.jogis-roehrenbude.de/Stromdichte-T.jpg) and the wire gauge tables. For example, in a 20VA transformer, assume 3.5 A/mm2 from the graph; an 18V 1A winding might use a 0.29mm2 wire i.e. around 0.6 mm diameter.
I'll need to calculate that, based on the op-amps I choose. I'll check for OPA134, since I'll probably experiment with those. The buffers will be BUF634. I intend to use a PIMETA PCB : http://tangentsoft.net/audio/pimeta/ . BTW, that is a total of 3 opamps and 3 buffers for both channels + ground. It's an interesting circuit.
Based on the input above, the trafo should be around 16 to 18 VA, let's assume the lower.
Based on the input above, the trafo should be around 16 to 18 VA, let's assume the lower.
Just to confirm the results - this trafo is perfect for what I'm trying to use it for. Driving the PIMETA trough a 24v regulated supply, in Class-A operation for the L/R channel opamps is not a problem. The trafo gets warm, which means it's probably down in the lower range of our estimates, but nothing to be really concerned with. Maybe if I stack some more buffers and run it full power for half a day, then it'll be a reson for concern. I've no intention to do that though, even half power can be waaaay louder than necessary.
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