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if I need +/- 30VDC from an ALWSR

J

jarthel

Member
what is the recommended transformer secondary?

some people have said, 30VDC is too high for the opamps but the ALWSR article PDF has said 35VDC is the maximum.

Thank you.
 
30V is OK, 35V is absolute max IIRC - so probably pushing it.

You need an input voltage >=5V higher than the output voltage, so 35V.

35V / sqrt2 ~= 25V transformer secondaries.
 
Remember that 5 V is absolutely min excluding ripple voltage. 25 V AC may be on the edge here.
 
just another question, since the LM317 outputs 2.5V, does the it really get hot?

~35VDC (assuming cap input filter and 25VAC transformer) is a lot of voltage to input an LM317 when the output is only 2.5VDC.

Thanks again
 
jarthel said:
just another question, since the LM317 outputs 2.5V, does the it really get hot?

~35VDC (assuming cap input filter and 25VAC transformer) is a lot of voltage to input an LM317 when the output is only 2.5VDC.

Thanks again
Click to expand...

It's not that it outputs 2.5VDC but rather that it outputs 2.5VDC plus the ALWSR regulator output voltage. So if your target ALWSR output voltage is 30V, then the pre-reg output would be 32.5V.
 
jarthel said:
just another question, since the LM317 outputs 2.5V, does the it really get hot?

~35VDC (assuming cap input filter and 25VAC transformer) is a lot of voltage to input an LM317 when the output is only 2.5VDC.

Thanks again
Click to expand...

You have missed some parts. You will have 2.5 volts across the pass transistor, always! Between IN and OUT of the LM317 you will have Input voltage- output voltage - 2.5 volts. Min voltage across LM317 is around 2.5 - 3 volts.
 
jarthel said:
just another question, since the LM317 outputs 2.5V, does the it really get hot?

~35VDC (assuming cap input filter and 25VAC transformer) is a lot of voltage to input an LM317 when the output is only 2.5VDC.

Thanks again
Click to expand...

In addition to Patrick and Per's comments above, I'd add that the amount of current drawn will affect the heat dissipation of those ICs. Try drawing a constant 1A through an SR for an example of this :eek:

If, on the other hand, you're drawing only 10mA, things will be a lot cooler.

Carl
 
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