Zei,
did you read the datasheet?
Did you understand what you read, or do you need help in understanding?
If you need help then ask the question.
Your 26-0-26Vac transformer question is so far away from what National tell you in the datasheet, that I can see you have done dozens of mental calculations and somewhere along the way you have come up with an incorrect conclusion.
What do you not know?
Ask that specific question.
did you read the datasheet?
Did you understand what you read, or do you need help in understanding?
If you need help then ask the question.
Your 26-0-26Vac transformer question is so far away from what National tell you in the datasheet, that I can see you have done dozens of mental calculations and somewhere along the way you have come up with an incorrect conclusion.
What do you not know?
Ask that specific question.
Hi Andrew
Yes, i have read the datasheet
No, i did not understand completely and therefore i asked here..
What i could read was:
Supply Voltage |V+| + |V−| 20V to 84V
But i remembered something about 24V being the max recommended voltage so i turned to this forum.
I already asked the questions i had.
1. Is 26-0-26V safe to use with LM3875? And what i can tell from the answer it is not safe.
2. Is there a way to lower the voltage on the transformer so i can use it?
Add your own answer here.
Maybe my English isn’t making any sense?
/JZ
Yes, i have read the datasheet
No, i did not understand completely and therefore i asked here..
What i could read was:
Supply Voltage |V+| + |V−| 20V to 84V
But i remembered something about 24V being the max recommended voltage so i turned to this forum.
I already asked the questions i had.
1. Is 26-0-26V safe to use with LM3875? And what i can tell from the answer it is not safe.
2. Is there a way to lower the voltage on the transformer so i can use it?
Add your own answer here.
Maybe my English isn’t making any sense?
/JZ
The majority of the data graphs are based on a 35 VDC supply. Double up on the rectifier diodes and depending on transformer quality shouldn't 26-0-26 be close? Granted with limited output below 6 ohms.
Attachments
42-0-42 usually means VAC and it is +-60VDC and that will fry the chip instantly.
As mentioned before +-36VDC is in the higher region for the chip, but it shold be fine if not used with low imp speakers. At low loads should be fine with 4 ohm.
What is the VA rating of your transformer? That is really the question needs an answer. Rail voltages will fall under load depending on the size of the transformer. You work within a window of supply voltages. Quiescent rail voltage that does not exceed the chips maximum limit and voltage under load that allows you to get the power output desired and not run into the chip's current limit. Music dynamics and load reactance tends to make a mess of the calculations.
Most are completely forgetting that 84Vdc is the MAXIMUM voltage when signal is present.
That means you should all be telling him to look at MAXIMUM output voltage from the PSU.
The transformer output is
Mains supply voltage / Rated Primary voltage * rated secondary voltage * transformer regulation.
A 300VA toroid will typically have a regulation between 6% and 8%. Let's assume 7% for the example that follows.
The harmonised voltage maximum in Europe is 253Vac.
A 230:26-0-26Vac transformer will NOT put out 26Vac much of the time.
The highest output is 253/230*26*(1+0.07) = 30.6Vac
There were a lot of assumptions to get from 26 to 30.6
Now the peak of the sinewave is sqrt(2) * Vac giving 43.27Vpk
A bridge rectifier will reduce this by one diode drop in a dual polarity supply.
At very low current the diode drop can be as little as 500mV
The MAXIMUM voltage from a 7% regulation 230:26-0-26Vac transformer fed with 253Vac will be around 42.77Vdc
Double this for the total voltage across the supply pins and you have >85.5Vdc
about 1.5V ABOVE the allowed.
There were a lot more assumptions to get from 30.6 to 85.5
Stop misleading and get the questioner to think !
A PSU needs to be designed, or follow a working recipe.
There is no "in between".
The half baked recipes offered so far were potentially damaging.
That means you should all be telling him to look at MAXIMUM output voltage from the PSU.
The transformer output is
Mains supply voltage / Rated Primary voltage * rated secondary voltage * transformer regulation.
A 300VA toroid will typically have a regulation between 6% and 8%. Let's assume 7% for the example that follows.
The harmonised voltage maximum in Europe is 253Vac.
A 230:26-0-26Vac transformer will NOT put out 26Vac much of the time.
The highest output is 253/230*26*(1+0.07) = 30.6Vac
There were a lot of assumptions to get from 26 to 30.6
Now the peak of the sinewave is sqrt(2) * Vac giving 43.27Vpk
A bridge rectifier will reduce this by one diode drop in a dual polarity supply.
At very low current the diode drop can be as little as 500mV
The MAXIMUM voltage from a 7% regulation 230:26-0-26Vac transformer fed with 253Vac will be around 42.77Vdc
Double this for the total voltage across the supply pins and you have >85.5Vdc
about 1.5V ABOVE the allowed.
There were a lot more assumptions to get from 30.6 to 85.5
Stop misleading and get the questioner to think !
A PSU needs to be designed, or follow a working recipe.
There is no "in between".
The half baked recipes offered so far were potentially damaging.
The chip's no-signal design maximum under normal operation is 94 volts. The power supply will easily sag 1.5 volts under load and, capitalization notwithstanding, won't explode in Quinn Martin fashion if it does momentarily exceed 2% of maximum rated PS voltage.
The above analysis ignores lighting strikes and nuclear EMP. Caveat emptor.
rdf, Andrew, Richie and all the rest 
Thank you all for your help.
I have tried to interpret the datasheet a bit more and as far as i can understand it should work. However since main voltage may vary some i wont take the chance of something breaking or worse. Better safe then sorry right?
Since my own technical knowledge is limited i,m depended on advice from this forum and i choose the "safest" advice.. right or wrong
So... where can i find a proper toroid.....?
Best regards
JZ
Thank you all for your help.
I have tried to interpret the datasheet a bit more and as far as i can understand it should work. However since main voltage may vary some i wont take the chance of something breaking or worse. Better safe then sorry right?
Since my own technical knowledge is limited i,m depended on advice from this forum and i choose the "safest" advice.. right or wrong
So... where can i find a proper toroid.....?
Best regards
JZ
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