We all live and learn. It'll be OK for short term testing use but I seriously agree that you have destroyed its long term safety.
I have 100VA toroid waiting on the table. I will measure it tomorrow. I want to do it in the hard way
I'm not ready to go to 300VA, not just yet 😉It has turned out to be an ideal stress test dummy for future purposes. It would be interesting to leave it hot and see how long it takes before it fails. Days, weeks, months . . . 🙂
Hi,
Are you claiming Class A amplifiers are "typical" amplifiers ?
They are not and clearly need bigger transformers than Class aB.
Class aB is typical and at clipping the transformer dissipation is
miles below sine wave continuous or class A operation.
Given that continuous operation is a poor model for designing a
class aB transformer for an audio amplifier, and they are different.
A classic case is the NAD 3020, 20W continuous, 50W programme.
No way does it have a "standard" transformer as described.
rgds, sreten.
Last edited:
the 50VA rating is for a resistor load on the AC supply.
i.e. 2.05Aac
When the transformer feeds a capacitor input filter the transformer must be de-rated.
Most manufacturers specify thye de-rating factor.
Many are around DF=0.7
That 50VA becomes 35W
Now the rectifier + capacitor input filter outputs the AC voltage times sqrt(2) as the DC voltage.
Some of this is lost through the rectifier, but the transformer still has to use it's de-rated power to drive the rectifier as well as the DC load.
12+12Vac gives a nominal 33.9Vdc
to pull 35W from 33.9Vdc the DC current will be ~1.03Adc.
This is the continuous maximum current that can be pulled from the 50VA transformer.
The voltage available at the load is ~32.5Vdc
The maximum continuous DC output power is ~33.5W
Draw anymore than 1.03Adc and you are overloading the 50VA transformer.
i.e. 2.05Aac
When the transformer feeds a capacitor input filter the transformer must be de-rated.
Most manufacturers specify thye de-rating factor.
Many are around DF=0.7
That 50VA becomes 35W
Now the rectifier + capacitor input filter outputs the AC voltage times sqrt(2) as the DC voltage.
Some of this is lost through the rectifier, but the transformer still has to use it's de-rated power to drive the rectifier as well as the DC load.
12+12Vac gives a nominal 33.9Vdc
to pull 35W from 33.9Vdc the DC current will be ~1.03Adc.
This is the continuous maximum current that can be pulled from the 50VA transformer.
The voltage available at the load is ~32.5Vdc
The maximum continuous DC output power is ~33.5W
Draw anymore than 1.03Adc and you are overloading the 50VA transformer.
I measured a couple of more toroids. They are 20VA, 50VA and a 105VA toroid.
Here's the toroid surface temperatures plotted against the measured DC current normalised by the specified nominal current.
Now I can see if I want to target, say, at 50 C surface temperature I can have DC load as 0.5 times the nominal current rating.
So, in this case with 50W constant load dissipation 100VA transformer is enough.
Here's the toroid surface temperatures plotted against the measured DC current normalised by the specified nominal current.
An externally hosted image should be here but it was not working when we last tested it.
Now I can see if I want to target, say, at 50 C surface temperature I can have DC load as 0.5 times the nominal current rating.
So, in this case with 50W constant load dissipation 100VA transformer is enough.
Bigger is better. If 100VA is adequate just look at the cost of the next size up, if you can afford it - go for it.
- Status
- Not open for further replies.