I have two 4 digit seven segment displays that work perfectly fine when I use the LM7805 however, when I use the switching power supply, some segments sometimes flicker on their own.
Can anyone please explain to me why this happens and how can I fix it and still use the switching power supply, I can't really use the LM7805 because of the heat issue.
7 segment display flickering when using switching power supply
7segmentdisplay
Related Solutions
My first assumption from the symptom is that you have a single cathode resistor, which would explain everything. However, you say this is not the case and that you have individual 100 Ω resistors on each anode line.
The answer then must be that whatever is driving the cathode can't handle the combined current. Apparently you are driving the cathode with just a microcontroller output, which has insufficient current sink capability. Do the math. You want 10 mA thru each of 7 segments. That means whatever is holding the cathodes low has to be able to handle 70 mA. A microcontroller pin is very very unlikely to do that. See the datasheet.
The solution is to add a transistor that can sink the current from each of the cathode connections:
70 mA is easy for any discrete transistor you will be able to get. Let's say the transistor has a minimum guaranteed gain of 50. That means the base current needs to be 70mA / 50 = 1.4 mA. Figure the B-E junction drops 700 mV. With 3.3 V drive from the micro, that leaves 3.3 V - 700 mV = 2.6 V accross the base resistor. 2.6 V / 1.4 mA = 1.9 kΩ, which is the absolute maximum allowed base resistor. Given that, I'd use 1 kΩ between the digital output and the base.
Note that this will invert the common cathode logic from what you had. The microcontroller output now has to go high to enable a digit.
The LS248 has an output that looks like this:
It effectively has a (nominal) 2K resistor in series with the output when it is high, so your 150 ohm resistor adds to that, making it effectively 2.15K.
To make this work with a minimum of muss (as well as a minimum of efficiency), add a pullup resistor to each output (Vcc to output) and omit the series resistor. Try about 470 ohms to stay within the ratings of the output, but you could go lower if you feel like abusing that antique chip to get a bit more light. It would be unwise to go below about 220 ohms, as that will make the 'off' current exceed 24mA per output.
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Best Answer
You may be having some noise problems in your circuit due to the use of a switching power supply. Switchers are notorious for injecting noise spikes into the PWR and GND rails and can be coupling noise into other nearby conductors. Only with the most careful design and layout of the circuit board along with the proper selection of bypass and filtering components can keep the noise contained and under control.
You provided few details of how your circuit is configured so it is hard to provide specific suggestions of how to correct the noise problem in your specific case. Even with that said I suspect that with two 4-digit 7-segment displays you may be using an MCU with some external logic to multiplex the display. Or you are using a long shift register approach to drive all the display segments with a typical periodic display update. In any case the flickering you are seeing is maybe being caused by the switcher noise disrupting the currently stored data driving the display until the next MCU display update comes along.
It is also possible that the design of the switcher is not up to par for supplying the amount of surge current required by the display. If this is the case there will be high ripple on the supply voltage which is then modulating the intensity of the display and causing the flicker that you see.
Come back with a photo of your circuit setup, a schematic and maybe we can offer more specific suggestions.