About MOSFET switch problem

[maniac84]

Yes for four display you need four IC.
The IC are just power shift registers and the schematic shows that they are connected in chain
You can use common anode display

Can the TPIC6C595 drive a common cathode 7 segment?

 

[alexan_e]

No, the outputs are open drain N-mosfets so they can only be connected as low side switch to sink current

 

[maniac84]

If I were to build something like above picture 7 segment which each segment contain 14 led, how do I design the current limit resistor for the led? Is it each led one resistor? Then if each segment is 14 led then I need 14 resistor. Then one character have 14led x 7 segment = 98 led = 98 resistors. Aren't that too much?
Or can design such that two led share one resistor?
I don't think one segment (14 led) can share just one resistor, won't have enough voltage. Or will it?

 

[maniac84]

From the picture above, I've design two circuits. Each represent one segment which contain 14 led.

Which design can work? The left one or right one?

 

[alexan_e]

View attachment 74012
If I were to build something like above picture 7 segment which each segment contain 14 led, how do I design the current limit resistor for the led? Is it each led one resistor? Then if each segment is 14 led then I need 14 resistor. Then one character have 14led x 7 segment = 98 led = 98 resistors. Aren't that too much?
Or can design such that two led share one resistor?
I don't think one segment (14 led) can share just one resistor, won't have enough voltage. Or will it?

You will use one resistor for each segment, it doesn't matter if the segment has 1,2 or 3 leds.
See post #96

---------- Post added at 09:18 ---------- Previous post was at 09:09 ----------

From the picture above, I've design two circuits. Each represent one segment which contain 14 led.
View attachment 74014
Which design can work? The left one or right one?

I think there will not be enough voltage to light the seven leds in series, in addition the led coupled may not share the current equally.

 

[maniac84]

You will use one resistor for each segment, it doesn't matter if the segment has 1,2 or 3 leds.
See post #96

I've tried connecting all 14 led in series and just one resistor to 12V. The led won't turn on.
But when I put two by two led in parallel and series together like the left side design on #104 thread, it work.
What is the best design?

---------- Post added at 14:26 ---------- Previous post was at 14:19 ----------

I think there will not be enough voltage to light the seven leds in series, in addition the led coupled may not share the current equally.

Then what is the best design? Is it the right side one?

 

[alexan_e]

Depending on the led type used you will need about 2v for each led so for 7 leds you need 14v.
The 14 led version can work too but not with 12v, you need about 24v.
Both designs can work , usually display with many leds in each segment use the leds in series.
The in series connection will requite a lower total current too.

 

[maniac84]

Depending on the led type used you will need about 2v for each led so for 7 leds you need 14v.
The 14 led version can work too but not with 12v, you need about 24v.
Both designs can work , usually display with many leds in each segment use the leds in series.
The in series connection will requite a lower total current too.

Which design do u mean in series connection? Is it below?

 

[alexan_e]

Yes this one which has all the leds in series

 

[maniac84]

Yes this one which has all the leds in series

Then how bout this design? Please take note that this is for one segment only. Still using 14 leds.

 

[alexan_e]

This one needs seven times the current of the previous one where everything was in series, if you prefer it then use this.

 

[maniac84]

This one needs seven times the current of the previous one where everything was in series, if you prefer it then use this.

Nope. Still searching for a suitable design. I think I'll choose the in series one like what you said.
May I know is there any current limit for each segment? For the in series design, if the resistor I use 110ohm then the current will be around 50mA. Of course, the more current the brighter it is. I have no datasheet for my led so, what is the normal current value from your experience?

 

[alexan_e]

Usually leds operate at about 20mA , 50mA seems high for a display but I don't know if the leds have a bigger size that normal and operate with more current.

The display in the link that you posted earlier was for 20mA

 

[maniac84]

Usually leds operate at about 20mA , 50mA seems high for a display but I don't know if the leds have a bigger size that normal and operate with more current.

The display in the link that you posted earlier was for 20mA

How do we calculate the total amount of current for a segment (14 led)? Is it still 20mA?

 

[alexan_e]

If you have 14 leds in series they they all share the same current, the 20mA flow through all of them.
If each two are in parallel and then the seven sets in series like in post #108 then the current will be shared for each couple that is in parallel so you may need 40mA but you don't have control of how is the current shared.
It may be 20+20mA or 15+25mA etc.

 

[maniac84]

If you have 14 leds in series they they all share the same current, the 20mA flow through all of them.
If each two are in parallel and then the seven sets in series like in post #108 then the current will be shared for each couple that is in parallel so you may need 40mA but you don't have control of how is the current shared.
It may be 20+20mA or 15+25mA etc.

Let me get it right first. Where do we measure the current for each segment to know that it is 40mA? Is it like below?

 

[alexan_e]

Your ammeter placement is correct, or before the resistor, it is the same

 

[maniac84]

When I place the ammeter like that, I measure that my current is around 80mA. Its way over 40mA like you recommended already. But it is still not bright enough. Do you know why?

 

[alexan_e]

I don't know the max current of your leds so I can't give you a random suggestion.
This is parameter you should check in the datasheet but you don't have one so...

 

[maniac84]

I don't know the max current of your leds so I can't give you a random suggestion.
This is parameter you should check in the datasheet but you don't have one so...

I've test just using one led, I put in 80mA, it is very much brighter than when 80mA is on 14 led. Do you know why?