Transformer overvoltage

[aeronaut]

Greetings,

I'm building a variable voltage power supply using Rod Elliott's circuit from Elliott Sound Products audio. It all came together well, except when I plug it in, the 25-25 volt dual output on the transformer, after rectification, produces an output voltage of 41.2 V. Since I want to regulate with LM317 and LM337, which have an absolute maximum input voltage of 40 V, this is a small problem. A previous version of the circuit burned out the adjust/feedback resistors.

I've thought about using a zener between the rectifier and the regulator to limit the voltage, but I'm having trouble with the calculations, as there is no formal resistance on the source side of the zener, and I worry about the current through it getting crazy high. It will self limit at some point, but I want to make sure that point is before failure.

Any advice appreciated. A screenshot of the circuit is attached, attribution to Rod Elliott, from sound-au.com, specifically https://sound-au.com/project44.htm

 

[bertus]

Hello,

Using a zener in series, all current must pass the zener.
Have a look at the tracking regulator on the following page:

how does tracking pre- regulator work?

How much voltage does the first lm338 drop ? How does it actually track the voltage, I thought there should be 1.25 V across adj and output? I am not able to understand the logic of using the pre

electronics.stackexchange.com

Bertus

 

[crutschow]

Have a look at the tracking regulator on the following page:

One thing to be aware of is that the tracking regulator has near the full voltage across it in the event of an output short to ground.

 

[crutschow]

Here's a tracking preregulator using a high-voltage MOSFET and Zener, that will tolerate an output short.

 

[jason022]

Overvoltage in transformers can be caused by various factors, such as lightning strikes, switching surges, or issues with load balancing. Prolonged overvoltage can lead to insulation breakdown, overheating, and reduced lifespan of the transformer.

If you're dealing with overvoltage issues, it's crucial to ensure proper grounding, install surge protection devices, and have the system inspected by a professional.


[Mod edited to remove advertising link]

 

[Bluejets]

Overvoltage in transformers can be caused by various factors, such as lightning strikes, switching surges, or issues with load balancing. Prolonged overvoltage can lead to insulation breakdown, overheating, and reduced lifespan of the transformer.

If you're dealing with overvoltage issues, it's crucial to ensure proper grounding, install surge protection devices, and have the system inspected by a professional.

Transformers typically go high on no or low low current just as they will go low voltage on overloads.
Doesn't mean there is any fault, just they are being operated outside of their design parameters.

 

[WHONOES]

You must bear in mind that the specified output voltage of a transformer is at full load.
All transformers have a regulation factor expressed in percent (%) and varies greatly depending on its size and VA rating.
The regulation factor on a small device can be a large as 25% or more whereas larger types will be in the order of 2 to 3% or less.
Having said all that, manufacturers don't always express the regulation factor in the same manner so a little bit of interpretation may be required.
The essence of all this is that a 10V transformer for example with 10% regulation will be at 12.5V with no load. This also assumes that the input Voltage is exactly as specified by the manufacturer.

 

[danadak]

LM317HV (60 volts) -

https://www.digikey.com/en/products/filter/power-management-pmic/voltage-regulators-linear-low-drop-out-ldo-regulators/699?s=N4IgTCBcDaIDYFsDMBGA7ACwG4gLoF8g

LM317HV data sheet, product information and support | TI.com

TI’s LM317HV is a 1.5-A, 60-V, adjustable linear voltage regulator. Find parameters, ordering and quality information

www.ti.com

Regards, Dana.

 

[WHONOES]

Correction to previous post. 10% of 10V is 1V not as inferred at 2.5V. So 10V transformer with 10% regulation would read 11V!!!!!!!.

 

[Harald Kapp]

the 25-25 volt dual output on the transformer, after rectification, produces an output voltage of 41.2 V.

The 25 V stated for the transformer are RMS. The peak value is [imath] rms \cdot sqrt 2[/imath] which evaluates to [imath]V_peak = 25 V \cdot 1.4142 \approx 35 V[/imath]. Add to that the regulation factor as shown by @WHONOES plus some overvoltage on the input (mains can have +-10 % tolerance or more, depending on where you live) and there you have the over 40 V you see.
Your most simple solution is probably using a regulator spec's for higher input voltage as posted by @danadak.