15V from 25VAC/36VDC

[jimbo1968]

In my chipamp I've got dual25V secondaries feeding carlosFM snubber board givng approx +-36V

Now I want to add an electronic crossover requiring +-15V.

I had hoped to just use 15V reglators to drop the 36V, but the National ones only take input up to 30V.

What are my options (without adding a suitable trafo)?

Are there 15V regulators that will take my 36V?

Should I cascade regulators, say 24V reg followed by 15V.

Or do I just need some other circuitry around the 15V regs.



Something I can build on strip board is required.

Thanks
Jim

[DoctorJ]

You could use diodes in series to drop the voltage.

[ErikdeBest]

Hi Jimbo

You can, as DoctorJ said, use diodes. 4007 will drop about 0,7V. You can also use the LM317 and LM337. Those take up to 37V in- and output. I would surely put any regulator on a small heatsink!

Erik

[quasi]

I generally used a pre-regulator either a transistor type or a LM78XX / 79XX i.e, using the 24v ones and take advantage of further ripple reduction. Plus the heat is spread over a larger area.

Or you could use a series zener (work out the power rating) or a string of diodes as suggested.

Cheers

[jimbo1968]

I've just had another look at some specs, eg LM7915 will take 30V input-output differential.

But LM340/7815 says +35V absolute max.

Sounds like that limit shouldn't be pushed?

[sawreyrw]

Jimbo,

The proper approach to this depends on the load current. For example, if it's a few mA, you might be able to use a three terminal regulator with a zener in the input lead.

If you need an amp, and the high voltage supply can support this, then you need a regulator with a series pass transisor and maybe a dropping resistor in the collector to drop the excess voltage. You also need heatsinks to keep these parts from overheating.

Rick

[jimbo1968]

Heatsinking is no problem. I need about 100mA.