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VK2ZRG > TECH 16.11.05 06:42l 43 Lines 1526 Bytes #999 (0) @ WW
BID : 1667_VK2ZRG
Read: GUEST DL1LCA OE7FMI
Subj: Re: Frustrating Opamp
Path: DB0FHN<DB0RGB<DB0MRW<OK0PKL<OK0PPL<DB0RES<ON0AR<ZL2BAU<VK2DOT<VK2XGJ<
VK2IO<VK2WI
Sent: 051116/0445Z @:VK2WI.#SYD.NSW.AUS.OC #:63865 [SYDNEY] FBB7 $:1667_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : TECH@WW
VK2ZRG/TPK 1.83d Msg #:1667 Date:16-11-05 Time:4:41Z
Hello Andrew and readers,
The reason why your op-amp circuit isn't doing what
you expect is that a LM741 type op-amp doesn't properly work with input
voltages that are too close to the supply rails. From memory, the input range
is from -Ve supply rail plus 1 volt to +Ve supply rail minus 1 volt.
e.g. Supply rails +/- 12v, input range +/- 11v.
Some op-amps will work with input voltages very close to the supply rails;
consult some data books Andrew, or look on the internet if you have access.
One commonly available op-amp that would probably work in your circuit is a
LM358. This is a dual op-am. A quad op-amp of the same type is a LM324.
Both of these op-amps woll work with input voltages within a few tens of
millivolts to the -Ve supply rail, but the upper limit is about 1.5 or 2.0V
below the +Ve supply rail.
Hope that helps Andrew. Looks like the exams are over...how did you go?
73s from Ralph VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
/ack
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>From: ZL4AJS@ZL4GQ.#95.NZL.OC
>To : TECH@WW
Snip
>
>I have the sensor voltage going to the + input, and the reference to the -
>input. No other components. The voltage from the sensor is for various
>reasons very low, around 0.4 V. The change in voltage between which I want
>the alarm to sound is also very small, just 20mV at most.
>
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