OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

[indulis]

I was taking about the difference between opamps and comparators in general, not specficic part numbers.

It is the frequency compensation capacitor in the opamp that causes its slew rate to be so slow.

What I said was

Can compensation make a opamp switch slow??? Yes it can, but  it's not automatic, you you have to select a value (LARGE) on purpose for that to happen. Then, slow is a relative term, how slow is slow?

Are you suggesting that a opamp can't be compensated such that it's "fast"?

 

[audioguru2]

You can see that the LM358 dual opamp has nearly the same circuit as the LM393 dual comparator and they have a huge difference in speed.

Some older opamps are under compensated so that they must have a closed loop gain of 5 or more. Their smaller frequency compensation capacitor allows them to have a higher speed.
 
Newer opamps are much faster.

 

[walid1]

Hi
I understand that the voltage of 240 volts is a danger to the fridge , But I do not understand how the voltage when is less than 180 volts risk it. Can you please explain that to me? God bless you and thank you

 

[audioguru2]

Some electric motors draw more current when the supply voltage is low. Then if the voltage is too low the motor will burn out.

Some electric motors have a very high current surge when they start running when loaded. If the voltage is too low then they might not start running and just sit there smoking with the high current.

The circuit is from India where their electrical distribution system has problems maintaining a steady voltage.

 

[walid1]

Also here in Gaza every day hear that some devicewas burned because of Electricity . Thank you very much

 

[mvs sarma]

Also here in Gaza every day hear that some devicewas burned because of Electricity . Thank you very much

Hi walid zee,

High voltages cause insulation breakdown and low voltages cause high currents and heat up the device to burn.

regards
Sarma

 

[walid1]

Thank you Sarma
but please what the meaning of zee?
thanks

 

[ante1]

Some electric motors draw more current when the supply voltage is low. Then if the voltage is too low the motor will burn out.

This is true only under certain load conditions, not without or with light loads!

 

[walid1]

Hi
For a comparator : What will be the form of the o/p when exactly equal voltages applied on the inputs.
Also, what the less difference between the two inputs to turn the o/p of the comparator to another, thank you for your attention

 

[audioguru2]

For a comparator : What will be the form of the o/p when exactly equal voltages applied on the inputs?

It depends on the input offset voltage of the comparator.
An LM393 dual comparator has a typical input offset voltage of 1mV (5mV max) and a typical voltage gain of 200,000. Its output would try to be 200V! Its output would be saturated at the positive supply voltage or at ground. If you use a comparator with zero input offset voltage then its output would be full of full-output square-waves noise due to its extremely high voltage gain.

Also, what the less difference between the two inputs to turn the o/p of the comparator to another?

It depends on the voltage gain of the comparator and on its supply voltage.
An LM393 dual comparator has a minimum voltage gain of 50,000. So for its output to change 15V then the voltage between its inputs must be 15V/50,000= 300uV.

Its typical voltage gain is 200,000. Then the input must be only 75uV. But such low levels would be covered up with noise.

 

[indulis]

The only problem is the LM393 a "open collector" device.

 

[walid1]

hi guru

(1) Is all of an op-amps suitable for use as a comparator or is that only certain types?
An opamp has frequency compensation so it can work with negative feedback without oscillating. The frequency compensation makes an opamp slow to switch its output.
A comparator doesn't have frequency compensation so its output switches very quickly. Therfore a comparator cannot have negative feedback.
There are many slow applications where an opamp can be used as a comparator. 

It is very good answer.
I read somewhere that op-amps designed to be a comparator has an o/p stage like open collector transistor or a darlington pair. is this true?

AND what is the frequency compensation ? No thanks I found it in: http://en.wikipedia.org/wiki/Frequency_compensation
thank you guru

 

[walid1]

Hi guru

Hi Walid,
The transistor is not straining so hard without the LED. With the LED, its load was 60mA for the relay and about 25mA for the LED if it had a current-limiting resistor. The total collector current would have been 85mA so the 8mA base current is fine. Without the LED then you can increase the value of R4 a little for a base current of 6mA. But 2mA doesn't matter.

The transistor was poorly chosen as a BC547 with an absolute max allowed collector current of only 100mA. Its current gain drops above only 30mA. I would have used a relay that uses less current or a higher current BC327 or 2N4401 transistor. Their saturation voltage is much lower so the base current can also be much lower if you want.

BC547 max allowed collector current is 500mA and not 100mA
BC327 is pnp tansistor and not npn
2N4401 is 1000mA but the same power of 625mW
here is a table of some suggested transistors, i see all are suitable.
thank you guru

 

[audioguru2]

Hi Walid,
You are correct, I was wrong. A BC327 is a PNP transistor. I haven't used BCxxx European transistors for about 35 years and I forgot.

Your table does not have correct data. It has the current ratings for the transistors much too high. Check the datasheets for correct max ratings.

 

[indulis]

It depends on the input offset voltage of the comparator.
An LM393 dual comparator has a typical input offset voltage of 1mV (5mV max) and a typical voltage gain of 200,000. Its output would try to be 200V! Its output would be saturated at the positive supply voltage or at ground. If you use a comparator with zero input offset voltage then its output would be full of full-output square-waves noise due to its extremely high voltage gain.




It depends on the voltage gain of the comparator and on its supply voltage.
An LM393 dual comparator has a minimum voltage gain of 50,000. So for its output to change 15V then the voltage between its inputs must be 15V/50,000= 300uV.

Its typical voltage gain is 200,000. Then the input must be only 75uV. But such low levels would be covered up with noise.

The LM393 is an open collector device!!!

guru, you may want to clear up a few things in your statements above, in regards to gain and open collector devices, so that walid doesn't get the wrong idea!!

 

[audioguru2]

Of course a comparator that has an open collector output needs to have a load resistor. It also needs to have a power supply. I was talking about a complete circuit that works.

 

[indulis]

A load resistor?? You  mean pull-up resistor. A "pull-up" supply doesn't have to mean Vcc, and then there is the pull-up value... now explain why you think your "gain numbers" mean something!!

 

[audioguru2]

An LM393 comparator has a typical voltage gain of 200,000 when it has a load resistor (National Semiconductor calls it RL) of 15k or more and a 15V supply.
If the supply voltage for the load resistor is less then the voltage gain will be less. If the supply voltage for the load resistor is more then the voltage gain will be more. The voltage gain changes because of the change in the internal emitter resistance of the output transistor.

 

[indulis]

Explain if you would, how exactly does a "non-linear device" , that would be the LM393, have linear characteristics.

 

[audioguru2]

The LM393 is a high gain comparator. It is not a low distortion linear amplifier.

None of its transistors have negative feedback for linearity. The circuit in the IC probably is as non-linear as an ordinary transistor that doesn't have negative feedback like this one: