How to power opamp ?

[Buzzy]

I have a 500VA transformer with 42Vac secondaries. Can i use this to power tl072 opamps which need +15 -15 Vcc and Icc at 1.4mA ? How should i do this ? Thanks.

[lineup]

why would you do this?
this is no good!

Get one 2x15 VAC or 2x18 VAC, like the size of 5-10 VA
to power your TL072 op-amp
Will not cost you much nothing.

After 4 rectifier diodes you use
Two IC regulators: 7815 and 7915
to make clean +- 15 Volt DC for op-amp

(2x42VAC trafo gives close to 2 x 60 VDC after rectifier)

[luka]

Hi

If you can, add two secondarys, first just few turns, so you can see how much turns will you need, then wind then up like any normal secondarys.

And what ^^ he said, 15Vac is pretty much what you need, a bit lower too, if you don't really need +/-15V... as for power 2VA will be more or less overkill, since they produce enough current for everything, but if you go for a bit more it won't hurt

[Buzzy]

I use that 500 VA transformer for my subwoofer amp. So this opamps are part of an active crossover for the amp. I don't have space to put another transformer in thats y i thght of using the existing transformer.
Can i use a resistor to drop the voltage ? But i dunno how to calculate the resistor value as i do not know how much current must be made available to the opamp.

[luka]

Hi

If you can't add secondarys, then use between + DC line, resistor and say 12 or 15v zener connected to gnd, this will produce + voltage for OPs. Same for other side...GND, zener, resistor, - DC line...As for resistor value, say you gonna load zener with 20mA so,..

R= U rail - U zener/I zener

Don't forget to calculate resistor power