# Positive and Negative definitions.

Hi

I am trying to understand the definition of how an electronic component is
connected in a circuit.

I have a shiftRegister 595N IC.

And when i wire components to the outputs of it, the components must get
from one pin the positive
and the negative pin of the component connects to the shift register.
(maybe not all shift registers, but the ones i have work this way)

Now i also have a 4051 multiplexer, and with this i can connect the positive pin
to the multiplexer and the negative pin to GND.

i think the shift register works as a "DRAIN" whats the definition of the reversed usage?

if i have a 7 segment LED display that has a common cathode, is there a way for me to get it working
using the shiftregister i have?

Thanks.

I didn't understand half of what you said :), but the 595 is not open drain, it's a push-pull output so you can hang components either way.

With a CC display connect the cathode to 0V and the 8 anodes to the 595 through resistors. Job done.

Rob

All ICs take two (or more) "supply rails". Usually these are called ground (GND) and Vcc (or Vdd). They are also
thought of as 0V and +5V (or sometimes +3.3V or something else). You can also say negative and positive
supply rails, but this can be confusing if a chip takes ground, +5V and -5V for instance. Ground is normally
seen as zero volts, so people tend to say "supply and ground".

Most logic ICs use ground and either +5V, +3.3V, +2.5V, +2.1V, +1.8V, etc etc, the latter being only for newer faster
VLSI devices, 5V and 3.3V are by far the most common.

So every circuit requires all supply and ground pins to be connected to suitable power rails at the right
voltages (and with decoupling capacitors), whether explicitly stated or not.

You also seem to be talking about sourcing and sinking current, which is something else to do with
signal connections.

i think the shift register works as a “DRAIN” whats the definition of the reversed usage?

No this is where you mean sink. As MarkT said yes it can act as a sink if one end of the external component is connected to +5 and the other to the output pin.
With a shift register you can connect one end of the component to ground and the other to the output pin and then it acts as a sink.
In each of the two cases the current flows in a different direction through the output pin.

There's lots of terminology in electronics (and every other field ) and different devices use different pin names.

You have to study each device separately. Diodes have an anode & cathode. Transistors have a base, collector, and emitter. FETS & MOSFETs have a gate, source, and drain.

Current flows from positive to negative* (or from higher] voltage to lower voltage). When an current is flowing out of an output/pin, we say it's "sourcing current". When currrent flows into an output, we say it's "sinking current". The Arduino can souce or sink current. You can wire an LED to come-on when the output is low (sinking current). Most LED drivers sink current.

The "arrow" on a diode/transistor schematic shows you the direction of current flow. With transistors you need to know if it's NPN or PNP, and with FETs/MOSFETs you need to know if it's N-channel or P-channel.

Don't confuse current flow with signal flow. The signal always goes into an input and out of an output, but the current can go either direction, or both directions in the case of AC. i.e. The AC audio signal comes out of the preamp and into the power amp... Then out of the power amp and into the speaker.

• This is "conventional current". Electrons actually flow the opposite direction, so it can get confusing if you take a physics class.

Grumpy_Mike:

i think the shift register works as a "DRAIN" whats the definition of the reversed usage?

No this is where you mean sink. As MarkT said yes it can act as a sink if one end of the external component is connected to +5 and the other to the output pin.
With a shift register you can connect one end of the component to ground and the other to the output pin and then it acts as a sink.
In each of the two cases the current flows in a different direction through the output pin.

Thanks.

Is there any chance you have some diagram showing
A shift register working as sourcing?

"This is "conventional current". Electrons actually flow the opposite direction, so it can get confusing if you take a physics class."

Or a class on semiconductor devices in the Electrical Engineering Department.

I know what conventional current stands for.

This is not the case.

i have read that not all shift registers can be used for sourcing current from the outputs.

i am using this shift register “TPIC6B595N”

by reading the datasheet i am unable to understand how it can work as sourcing.

attached is how i used this shift register as sinking current.

what would i change here in order to use it for sourcing current?

Thanks.

Shift_Register_Switching-Model.pdf (11.6 KB)

i am using this shift register “TPIC6B595N”

by reading the datasheet i am unable to understand how it can work as sourcing.

That is because you can’t, it is an open drain output, it will pull down or sink only.

i am using this shift register “TPIC6B595N”

That’s not what you said in the OP.

I have a shiftRegister 595N IC.

You can’t use that chip with a CC display, well you possibly could but it’s not a good match.

Rob

Yeah, if you are going to give part numbers, give the whole number. When I first read this thread, I thought that you meant the 74LS595 shift register, which is a TTL totem pole push-pull output. Not a TPIC6B595N shift register, which is CMOS open drain current sink output.

i am using this shift register “TPIC6B595N”

That’s not what you said in the OP.

I have a shiftRegister 595N IC.

You can’t use that chip with a CC display, well you possibly could but it’s not a good match.

Rob

I assumed that since it ends with 595N
It is a 595 IC
Am i wrong?

Thanks anyway.

I assumed that since it ends with 595N
It is a 595 IC
Am i wrong?

Technically you are probably right, it's very common to just use the last 3-odd digits as you did, but only after making it clear which exact chip is being used. In this case as you found out there are (at least) two flavours of "595" and in this context they have totally different characteristics. Possibly the "N" was a clue but who's going to look up all the data sheets to see which variety (for example of the SN74HC595) has an "N" version.

It's all in the context really. If we were talking about flip flop logic most people might be happy with a '74 reference and know it means a 74xx74 dual D flip flop, but if the discussion is about a FF driving an LED or conversely being driven by a CMOS gate then the "xx" part is important.

So in general spell out the exact chip first, then you can drop to an abbreviation.

Rob