John said:
John said:
On 31 Aug 2005 13:13:19 -0700,
[email protected] wrote:
Thanks for you all your help - I have all the parts to build the op-amp
solution at home - the only thing that puzzles me a bit is the
+15V/-15V power to the 741 - how do I create +15 and -15V from a
standard transformer.
---
Since the 741 can take an input supply of plus and minus 22 volts,
max, you can easily get less than that that from a simple,
unregulated, half wave supply using a 12V transformer, like this:
+--[IN4001>]----------+---->+17V
| |
| |
MAINS>--+ || +--[<1N4001]--+-------|---->-17V
| || | | |
P || S | |+
R || E [C1] [C2]
I || C |+ |
| || | | |
MAINS>--+ || +-------------+-------+---->GND
Since your output is only going to swing between +5 and -5V, and the
741 needs about 3V above and below that to let that happen, you've
got about 9V of headroom to either rail so C1 and C2 can be fairly
modest, depending on what else you're going to use the supply for.
What can you tell us about the load?
Mr. Fields has given the best solution. The OP might want to add a
couple of protection diodes to keep the inputs in the range of the
supplies and the caps from being reverse-biased at turn-on, like this
(view in fixed font or M$ Notepad):
` D
` o------o-------->|----o-----o------o+14VDC
` | +| |
` | C --- - D
` o------|---. --- ^
` | | | |
` 10.2VAC | '----------o-----o------o
` from | +| |
` Wall Wart | C --- - D
` | --- ^
` | D | |
` '--------|<----o-----o------o-14VDC
---
Since we have current being steered into either one cap or the other
by the main rectifying diodes, I don't see how either of them could
become reverse-biased, since there's no available path.
Likewise, I don't understand what you mean by "to keep the inputs in
the range of the supplies."
Hi, Mr. Fields. It's possible the two extra diodes are "gilding the
lilly", but possibly not.
First, the whole circuit without the extra diodes:
` ___
` .--|___|--.
` ph A D | |
` o-----o----->|---o-----------------------. |
` | +| ___ | |\| |
` | C1--- in(-)o-|___|-o--|-\ |
` o-----|---. --- ___ | >---o---o
` ph B | | | in(+)o-|___|-o--|+/
` | '------o---. | |/|
` | +| | .-. |
` | C2--- === | | |
` | --- GND | | |
` | D | '-' |
` '-----|<---o---------. | |
` | === |
` | GND |
` '-------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Let's assume that, at turn-on, ph A is more positive than ph B. That
means C1 will start charging up in a positive direction. Several mA of
current will start to flow through the op amp Vcc pin to the Vee pin.
For that half cycle, the - of C2 will be more positive than GND. As ph
B goes negative with respect to ph A, C2 will start to charge
negatively with respect to GND, relieving the problem.
Since there are 22K resistors between the inputs and GND, the
possibility of latchup on turn-on is not there for an LM741. It
becomes more of a hazard if one of the op amp inputs is grounded. And
it becomes more of a hazard as the DC current draw of the circuit
increases. Obviously the several mA of one op amp won't charge up the
470uF cap to a big voltage in 1/2 line cycle. But for some op amps or
4000-series logic using unregulated dual supplies, or for heavier DC
loads, this can be a problem.
The problem is exacerbated when you have two different circuits with
two different power supplies. If the signal generator turns on when
this one is still off, this phantom reverse charging of the power
supply can be a more significant failure mode, because it can be
continuous. The 0-10VDC ramp voltage would cause this phantom reverse
charging on C2 through the non-inverting input to the Vee pin if the
signal generator is on before the wall wart is turned on -- that is,
unless you had the extra diode to keep the cap at less than -0.7V.
Again, with the series resistors, I don't believe you'd get latchup at
turn-on with most op amps. _This_ was my main concern, and the reason
I threw in the extra diodes. Any reverse charging is current limited,
so the cap would also probably be OK, too.
The thing is, once old dogs like me learn good tricks (usually the hard
way), we tend to stick with them. But it's good to get some feedback
from somebody who's thinking things through. It's probable the two
extra diodes aren't necessary, if the OP uses an LM741 (and he could be
using a CMOS op-amp, which would mean the diodes just might be
necessary -- they sometimes latch up at lower currents). I guess I
just like to use them on unregulated dual supplies like the above,
especially if there's more than one power source lurking around. And
after all, the OP's buying a pack of 25 diodes. That means 23 spares
-- might as well.
Thanks for the heads up, and the obviously more efficient solution.
Chris
