Got it!
Does anyone use zener diodes under forward bias?

Generally, no.

You might use them in a circuit in which the primary use is when they are reverse-biased, but that the nature of the circuit means that they can get forward biased under some operating conditions. It's usually not so much that you WANT them to be forward biased, but rather you determine that you are willing to LIVE with them being forward biased.

 

Yeah, thanks! does it matter which value of Pull-up resistor I use?

A resistor can have a resistance value between 0 ohms and infinity.
Certainly there must be a sweet spot (optimal value) somewhere in that range that works best for that specific application.
So think about how much current has to flow to present a particular output voltage.

The circuit being driven has a certain input resistance, either its own pull-up or pull-down.
Hence your driving circuit has to source or sink current in order to get the voltage within acceptable logic voltages.
Think voltage divider circuits.

 

Yeah, thanks! does it matter which value of Pull-up resistor I use?

Also, all the other values I used here can stay as is? I mean the voltage supplies I used to operate the 741 and the voltage inputs, it's important for my circuit's goal.

If you really are driving 50-year-old original 74-series TTL, then 1k to 4.7k is usual. If you have something more modern such as 74HC then anything up to 1MΩ and beyond.
All the input voltages can stay the same, but 13.5V inputs on a 15V supply is pushing your luck (but more likely to work with an LM393 than with a 741)

 

If you really are driving 50-year-old original 74-series TTL, then 1k to 4.7k is usual. If you have something more modern such as 74HC then anything up to 1MΩ and beyond.
All the input voltages can stay the same, but 13.5V inputs on a 15V supply is pushing your luck (but more likely to work with an LM393 than with a 741)

Haha, yeah I'm using 74HC32N.
Ty again, I hope 0V & 15V supplies will be find with the LM393, otherwise ill just change the +VCC to at least 20V and then ill change the peripheral resistors as well.
BTW, so if I got you right, If I use the 393 with the 5V+Pull up res so I can remove the Zener diode from the circuit or I can leave it as is?

 

Yeah, thanks! does it matter which value of Pull-up resistor I use?
Also, all the other values I used here can stay as is? I mean the voltage supplies I used to operate the 741 and the voltage inputs, it's important for my circuit's goal.

Yes, it matters. There are a few factors at play, but most of them probably don't matter for you. If there are no speed issues, then you want to use a resistor that will limit the current that the comparator has to sink to just a couple milliamps. The 393 is spec'ed to pull the output to within about half a volt of ground while sinking less than 4 mA, so shoot for 1 mA to 2 mA.

The input common mode range still only goes to within 1.5 V of the rail, which you are pushing with a 13.5 V input. Would be best to put attenuators on your input signals to get all those down. Just cut everything in half.

 

Haha, yeah I'm using 74HC32N.
Ty again, I hope 0V & 15V supplies will be find with the LM393, otherwise ill just change the +VCC to at least 20V and then ill change the peripheral resistors as well.
BTW, so if I got you right, If I use the 393 with the 5V+Pull up res so I can remove the Zener diode from the circuit or I can leave it as is?

You can remove it. No need for the 1 kΩ series resistor a the output, either.

One thing to ask yourself is whether your circuit will still work properly if the supply voltage changes. Let's say that, for whatever reason, your supply drifted up to 17 V when you had designed everything around a 15 V supply. Or if it dropped to 13 V.

If the answer is no (and the fact that you talk about changing resistor values if you change the supply to 20 V is a good indication that the answer is no), then you should redesign your circuit so that it doesn't matter. This is where a voltage regulator or a Zener reference could be very handy.

 

Yes, it matters. There are a few factors at play, but most of them probably don't matter for you. If there are no speed issues, then you want to use a resistor that will limit the current that the comparator has to sink to just a couple milliamps. The 393 is spec'ed to pull the output to within about half a volt of ground while sinking less than 4 mA, so shoot for 1 mA to 2 mA.

The input common mode range still only goes to within 1.5 V of the rail, which you are pushing with a 13.5 V input. Would be best to put attenuators on your input signals to get all those down. Just cut everything in half.

Is it ok if I just cut every voltage input in half by using a secondary voltage divider and then connect the 393 input to that result?

 

Is it ok if I just cut every voltage input in half by using a secondary voltage divider and then connect the 393 input to that result?

Depending on exactly how you do it, yes. But keep in mind that just cutting the voltages in half will still leave you sensitive to changes in the supply voltage.

Do you have any voltage regulators in your kit?

 

Depending on exactly how you do it, yes. But keep in mind that just cutting the voltages in half will still leave you sensitive to changes in the supply voltage.

Do you have any voltage regulators in your kit?

Well, to be honest I don't know one and I'm only allowed to use families 74 and/or 40, do you know any voltage regulator from that families?

 

Depending on exactly how you do it, yes. But keep in mind that just cutting the voltages in half will still leave you sensitive to changes in the supply voltage.

Do you have any voltage regulators in your kit?

I bet the 15V is from a lab power supply.

 

I bet the 15V is from a lab power supply.

You are right dear friend.

 

Tell your instructor that an LM741 opamp design is 54 years old but the LM358 is newer, better and costs less than half.
An LM358 dual opamp has outputs that go very close to ground and do not need output pullup resistors.

The inputs of some LM741 opamps will not work when the supply is 15V and an input is 12V or higher.
The inputs of some LM358 opamps will not work when the supply is 15V and an input is 13.5V or higher.
The inputs of some LM393 comparators will not work when the supply is 15V and an input is 13.5V or higher.
Then reduce the voltage of PR2 and PR3.

 

Well, to be honest I don't know one and I'm only allowed to use families 74 and/or 40, do you know any voltage regulator from that families?

Those are digital logic families. A voltage regulator is designed to supply power to a circuit while holding that supply voltage at a constant value.

If you don't have any voltage regulators, then you could use Zener diodes to create reasonably good voltage references. Or you can ignore the issue of power supply rejection.

 

Those are digital logic families. A voltage regulator is designed to supply power to a circuit while holding that supply voltage at a constant value.

If you don't have any voltage regulators, then you could use Zener diodes to create reasonably good voltage references. Or you can ignore the issue of power supply rejection.

I see that there is LM317 in the college's electronics warehouse, is it suitable component? if so, I will check if It is allowed for me to use it in the project.

 

If you are running it off a lab power supply, a regulator is pointless.

(Why has AAC decided that I am ignoring comment from @raziell122 ?)

 

Hi Ian,
No record of a Mod or Admin having set any ignores.?

Mod

 

Hi Ian,
No record of a Mod or Admin having set any ignores.?

Mod

Thanks. I have just discovered that I can click on his name under the thread title and unignore him. Until now, I didn’t know how to do that, so I’m puzzled how he became ignored!

 

If you are running it off a lab power supply, a regulator is pointless.

(Why has AAC decided that I am ignoring comment from @raziell122 ?)

I'm not talking about powering the circuit from the regulator, I'm talking about setting a reference for the input conditioning. The ICs have decent power supply rejection off-the-shelf, but the reference signals he is generating via voltage dividers don't. Good design practice includes addressing that. I noted that he can choose to ignore it, since it is a lab exercise. But if he chooses not to ignore it, then not only does he learn something valuable, he also sets himself up to get that much better of a grade on the assignment.

 

I see that there is LM317 in the college's electronics warehouse, is it suitable component? if so, I will check if It is allowed for me to use it in the project.

Yes. Not the best, but much better than ignoring the issue. But don't worry about this until the end. You can get the circuit functioning by deriving your reference voltages directly from the supply you are powering the circuit from. That should be good enough for full marks if everything is done correctly. But addressing power supply rejection in the generation of your reference voltages is something that you should generally address and doing so should (can't guarantee that it will) either earn you extra credit or offset some of the points lost due to little things. It also tells your instructor that you are serious about learning and applying this stuff, which will almost certainly bias them in your favor, even if only subconsciously.

 

Got it!
Does anyone use zener diodes under forward bias?

I don't think so. Why use a 5¢ Zener when a 1¢ 1N4148 equivalent is available?