I have problem with op amp used as comparator. It should detect when a signal exceed reference voltage (impressed by voltage divider). The signal have range about 0v to -30v (it is negative!).
I supposed to use LM358 (truly common). But this device can be powered below 30v in single configuration and in dual mode it must not exceed +15v -15v. The range is twice the dual power limit, and I can't use single mode because it is negative.
So, can I provide a 0V to the positive supply pin and -30 to the negative one? In this manner I can cover entire rage of the input signal.
Can you use a voltage divider on that signal.
If so, then connect the divider's fixed point not to ground but to a positive voltage.
Then your signal can become even positive on the tap of the divider.
Leo..
I have problem with op amp used as comparator. It should detect when a signal exceed reference voltage (impressed by voltage divider). The signal have range about 0v to -30v (it is negative!).
I supposed to use LM358 (truly common). But this device can be powered below 30v in single configuration and in dual mode it must not exceed +15v -15v. The range is twice the dual power limit, and I can't use single mode because it is negative.
So, can I provide a 0V to the positive supply pin and -30 to the negative one? In this manner I can cover entire rage of the input signal.
Can you suggest me other solutions?
Some opamps are not designed to handle a large voltage difference between inputs (they are designed
for negative feedback). Often the input current increases radically above the nominal in this circumstance.
So best to use a comparator.
You can reduce the input voltage with a voltage divider, note, to keep the devices in a sensible range.
Perhaps you can give more details of what the signal is and what you are trying to achieve,
so we can avoid the xyproblem.
This is a simple part of the entire project. The main line should supplies fixed voltage ( -1.25 to -30v negative). LM358 should detect when signal fall below -0.1V.
I have supposed to power LM358 with -33V and 0V. The signal to the positive input is within the range -1.25V to -30V (approximately). The reference voltage (about -0.1V) is applied to negative input.
With Zener I think to obtain 30V on op amp’s supply. In the attachment you can find my solution, it is a particular of previous schema, focused on LM358.
Is it ok? Can op amp be powered with 0 -30v (fixed by zener) and with input signal in range -1.25v -30v?
The last image is a particular referred to the first one. It's the same like in the first case, but I have added zener to op amp power pin, so I can impress 30V instead 33V or 32V.
I need to detect when the power line below 100mv: short circuit detection or other risk due to the load.
To do this you don’t need to run your comparator with -30v … I suggest -5. Use eg a 7905 regulator.
Run a resistor of eg 47k to the voltage to be compared and to one comparator input. Then just clamp this pin to 0v with a simple diode - eg 1N4148. This pin cannot then go lower than -0.7v. - but you’re only interested if it’s </> -100mV, so who cares.
Run the other to a -0.1v reference - eg with a 4k7 / 100 ohm divider from the -5v supply. This gives -104mV
If you want it more accurate make one of the two adjustable.
Alternatively, as Wawa suggests, use a +ve offset so you can run your comparator at > 0v - in this case you could use an arduino a/d to do the comparison if you don’t want a super quick response time. . Again I’d suggest using a diode clamp to 0v.
I have problem with op amp used as comparator. It should detect when a signal exceed reference voltage (impressed by voltage divider). The signal have range about 0v to -30v (it is negative!).
Hey.. hold your horses.
You are using a LM317 to regulate a NEGATIVE supply????
Also why are you using a LM358(Audio Power Amp) when a LM393(Comparator) will do the job a whole lot better?
I suppose you could rig a LM317 to control a -ve supply, but it’s not straightforward.
The LM358 is not an audio power amp - it’ll only drive a few mA.
It’s a very old not very good dual opamp - and useless here as it’s common-mode input range only goes to within -1.5v of it’s positive supply. No good if you want to look at Vcc -100mV.
The LM393 dual comparator suffers the same limitation.
TI also warn they may be discontinuing it in the near future.
I’ll hack up a circuit which doesn’t require any active components with a -ve supply…
