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I'm trying to control RGB strip used in 7 segment display with an arduino via transistors. the following image illustrates the schematics even though i didn't put the exact resistors values the configuration is a common anode when i control the segments with pwm it works but when i try to PWM the positive side (Transistors Q4, Q8 and Q10 in the image) all i get is fully off or fully on

PWM Led strip control

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  • \$\begingroup\$ What's the connection to ground from each display for? Where are your current limiting resistors? \$\endgroup\$ – Finbarr Apr 3 '18 at 19:01
  • \$\begingroup\$ Your anodes are controlled by high side "power" switches driven by low side "driver" switches. There's no current limiting resistors at the base of the driver BJTs. Also, I have to assume that your "wiring" regarding the segments is driven in some way? Or are you, for now, just attempting one segment each as a test? (I assume you ALSO have current limit resistors for this test, yes?) Finally, are you supplying the same PWM signal to all three at the same time? And really finally, do you want to support the idea of dimming the displays, too? \$\endgroup\$ – jonk Apr 3 '18 at 19:05
  • \$\begingroup\$ led strips have current limiting resistors, and yes i used resistors to protect the transistors too, the low side of the segments is controlled with NPNs no problem with them the only problem is that i have two digits with RGB so i added two transistors Q1 and another one to control each digit when multiplexing, and transistors Q4,Q8 and q10 to control color \$\endgroup\$ – you Apr 3 '18 at 19:13
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    \$\begingroup\$ Maybe someone else can help. You still haven't come close to providing an accurate schematic. I am not familiar with using led strips as 7 segment displays, so you'd need to educate me on that, too. Basically, this is for someone else better equipped to "read between all the lines" and fill in the huge (to me) gaps. If I were doing this myself, there would be no ambiguity and I could write up the diagram quite exactly when asking such a question. If I see that kind of improvement here, I may add something. Otherwise, other things to do for me, anyway. This just isn't clear to me. \$\endgroup\$ – jonk Apr 3 '18 at 19:19
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    \$\begingroup\$ "i didn't complete the schematic because i don't have enough time". But you think it's OK to waste our time with a wrong schematic? \$\endgroup\$ – Transistor Apr 3 '18 at 20:15
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My crystal ball says your 7 segment display needs 4 to 5V with voltage drop on the cathode segment side and you show 12V.

So when you enable Q1 they are burning up bright and something must be getting hot.

Make sure the segment bits have 100~200 in series (depending on color) or so and then R2,5,7= 600 Ohms
- 10% of 7 segments @10mA?20mA? so 70 (or 140mA if huge) * 10% or 4.3V/7mA = 600 Ohms or so.

oh-oh my crystal ball is getting fuzzy like your logic diagram.... over

I see you have some unknown LEDstrip Current 2.4A per 5meter?? or ?

If a Ledstrip draws 2A at 12V equiv to 6 Ohms then you need a FET or the base current must be 1/20th or more so R2,R7,R8 must be 20x bigger or 120 Ohms which draws too much power for a 1/4 W resistor. So get a 50 mOhm or less Nfet for low side switch or Pch FET for high side switch.

tons of examples on this forum... search LEDstrip driver

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  • \$\begingroup\$ your crystal ball should have told you that i used Led strips the 7 segements on the picture are for illustration sake thank you sir. \$\endgroup\$ – you Apr 3 '18 at 20:57
  • \$\begingroup\$ You should have put a schematic that shows what you have actually connected, not wasting peoples free time by supplying something different/unfinished \$\endgroup\$ – MrGerber Apr 3 '18 at 21:01
  • \$\begingroup\$ @MrGerber i wrote it in the post, i don't have a led strip in the software i used to draw the schematic \$\endgroup\$ – you Apr 3 '18 at 21:35
  • \$\begingroup\$ @you you have no idea on the LED voltage or load current, no wonder it fails \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 4 '18 at 2:08
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it turns out that the problem was me trying to use hardware PWM in arduino analog pins which is not supported. thank you all for your time

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    \$\begingroup\$ That's why folks were looking for the schematics. How could anyone have guessed that you were using an analog pin? \$\endgroup\$ – Transistor Apr 3 '18 at 22:05
  • \$\begingroup\$ what a useless question \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 4 '18 at 2:51

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