Problem with VCC of NAND and NOR circuits

Hello everyone,

I am using 1 NAND Gate and 1 NOR gate, which are:
NAND: SN74LS00N
NOR: SN74LS02

They both require 7V in the VCC, I am using a 12V PSU.

I made a Voltage divider with R1=34k ohm and R2= 47k ohm, and I get the exact VCC I need for the logic gates (7V).

All GND are connected.

The problem is when I connect the 7V to the VCC of the logic gates voltage drops from 7V to 2V, and I do not understand why.
What am I doing wrong here?

Also I still have not connected the 5V input pins because I am afraid I will fry the logic gates by connecting 5V IO pins on a logic gate that has 2V VCC. Can I test this safely?

Thanks.

Hi,

Is there a reason you want to power them with 7volts?
The spec sheet says they are 5V devices.

sn5400.pdf (1.4 MB)

Using a potential divider will not provide a stable and safe supply for your ICs.

Tom... :grinning_face_with_smiling_eyes: :+1: :coffee: :australia:

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No, these devices are designed for 5V. 7V is the maximum VCC they can withstand without breaking You should never operate them with 7V.
And you cannot create VCC be means of a voltage divider. The output of such a devider is highly dependent on the current. And these devices need too much current for that. You need a stabilised voltage from a voltage regulator.

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I was using 7V because I thought it was the opperating voltage.

Thanks for that information, I will now use 5V.

I am a novice in electronics :sweat_smile:

I am not aware of what a voltage regulator is, but I will study what that is and might comeback with more questions :sweat_smile:

Thank you!

Hi,
As @MicroBahner has pointed out , you need a 5V regulated supply, or make a regulated supply to run off your 12V power supply.

Tom... :grinning_face_with_smiling_eyes: :+1: :coffee: :australia:

1 Like

So I got these 2 voltage regulators (just in case something was wrong):
L7805CV
MC7805ACT

I already tried both. I connected as it shows in this image:
imagem_2021-09-07_141240

I am getting 12V on the Output, while I should be getting 5V, right?

@MicroBahner @TomGeorge

note: this picture is wrongly connected (L7805)

This might have been terrible or dangerous, I dont know :sweat_smile:
but I have connected the L7805 both ways, with the input connected to 12V, and Output connected to the 12V (just to see if I was connecting them in the wrong way), the outcome was the same, nothing fried, and I was getting 12V on both INPUT and OUTPUT pins, with the voltage regulator connected in the right way, and in the wrong way.

Your picture doesn' really show how you connected it, but it seems to be wrong. Did you interchange input and output? On your picture the input should be on the left, and the output on the right.

Yes, I interchange them.
Got 12V on both tests

thank you for your advice and sympathy.

I do read datasheets, here is a picture of the legs meaning:
image

You should not do that - maybe its broken now.

My mistake I said ground but it is connected to +

I have more, not a problem, will try with a different one.
My first try it was connected as it says in the datasheet.

From the data sheet, which also has little pictures showing which are which 1, 2 and 3.

Pin 1. Input
Pin 2. Ground
Pin 3. Output

If you are reading the data sheets, why are you confused about the role of input and output?

Input - the voltage to be regulated.
Output - the regulated voltage.

Wildly swapping things around when they don't work first time is a way to destroy your components. Think first, wire and confirm, then apply voltages to test.

a7

Doesn’t seem to match the diagram

@alto777 read the MC7805ACT datasheet, the picture and image is from the L7805

guys I found the mistake, I am to embaressed to show it :sweat_smile: :sweat_smile:


this is the picture from data sheet, do you see numbers on the connections? The one you posted is wrong, numbers don’t match to the pin out

I mistakenly connected GND to VCC