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I am trying to control RGB LEDs using 74HC595s. Three of them determine the input voltage and one of them determines the ground.

Here is what I found while seeing the answers to a slightly similar question

The 74LS04 have internal resistors to reduce the shoot-thru current. When that pin is driven high, the Vbe and Resistor limit the current. Is that resistor designed for permanent heating? Those pins are short-circuit-proof

I want to connect the two 74595 together with a LED in between without a current limiting resistor since I believe the buffers have an internal resistor connected to the upper MOSFET in the totem pole

schematic

simulate this circuit – Schematic created using CircuitLab

LED ratings:

  • Max current = 25mA
  • Typ. Vforward R = 1.95V
  • Typ. Vforward G = 3.3V
  • Typ. Vforward B = 3.3V

I have searched through the documentation but none of them shows the internal structure. What is your opinion about not using the current limiting resistor?

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  • \$\begingroup\$ Welcome to Electrical Engineering Stack Exchange. Could you please add a schematic / circuit diagram to your question? There is a tool for creating circuit diagrams above your edit box. Also, what is the current limit of your LEDs? Their typical operating voltage? (These can be found on the manufacturers datasheets for the LEDs) \$\endgroup\$
    – Math Keeps Me Busy
    Commented Dec 12, 2020 at 3:51
  • \$\begingroup\$ Is Buf2 backward? Or did you mean to connect the LED to an input? \$\endgroup\$
    – Math Keeps Me Busy
    Commented Dec 12, 2020 at 4:54
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    \$\begingroup\$ Are you aware that the only way for the led to emit light is for the anode side buffer to be high (a 1), and for the cathode side buffer to be low (a 0)? All other combinations will result in the led being off. Are you aware that the 3 LEDs have a common cathode pin, so if there were ever an occasion where the three anodes buffers were high, and the cathode buffer low, the cathode buffer would carry the current of all three LEDs? Are you aware that you only need to drive the anode side to control the LEDs, the cathode side could be tied permanently to ground? \$\endgroup\$
    – Math Keeps Me Busy
    Commented Dec 12, 2020 at 5:21
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    \$\begingroup\$ I believe the buffers have an internal resistor connected to the upper MOSFET in the totem pole assuming (as there's no internal schematics available) that the CMOS tri-state inverter at the output is a minimalistic design (i.e. consisting of only 4 transistors), 2 of them are always in series with the outputs. These can be thought of as series resistors but they may not act as current limiters. Plus, the max output current per pin under abs. max ratings section may tell you something. I'd put series resistors anyways. \$\endgroup\$
    – Rohat Kılıç
    Commented Dec 12, 2020 at 5:33
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    \$\begingroup\$ Another question. I am guessing that you wish to create an LED matrix or array, and that is why you want two drivers controlling each LED. Are you constrained to using 74HC595's on both sides, perhaps because you already have a board with those chips? Or are you free to substitute a TPIC6B596 on the cathode side? \$\endgroup\$
    – Math Keeps Me Busy
    Commented Dec 12, 2020 at 5:50

2 Answers 2

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My opinion? Bad idea. Whilst the internal mosfets of the logic device have a finite on resistance, relying on this is not wise as they will be thermally limited - the more current you flow through them, the hotter they get. The HC595 datasheet will outline these limitations. Sure, it will 'work', but it is still bad practice. There's WS2812 (neopixel) leds and their variants that have an internal controller and driver. You can run a number of them off one port pin.

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There is a mixup of logic families going on.

The text you quote uses a LS type chip as an example, which is a bioolar TTL chip. TTL type chips do have an internal structure that is compatable with a current limiting resistance.

But the component you want to use is a HC type chip which uses CMOS technology, and CMOS chips have no such structure in them that would act as a current limiting resistance.

So the answer is, no, you can't use a 74HC595 to drive LEDs directly without a separate current limiting resistor.

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  • \$\begingroup\$ How about the LS type then? \$\endgroup\$
    – InputBlackBoxOutput
    Commented Dec 12, 2020 at 8:45
  • \$\begingroup\$ TTL parts usually have better sink than source capabilities. Still, it is bad juju to do what you are proposing. Use a specific device that ensures the required current or design your circuit (ie use resistors) to ensure the correct current. Or simply ignore our advice and it may work...for a while. But I wouldn't make a number of these devices. \$\endgroup\$
    – Kartman
    Commented Dec 12, 2020 at 9:05
  • \$\begingroup\$ Most likely you would not be happy with the results, as the LS595 can source only 2.6mA according to the recommended operating conditions. Even the HC595 can't reach 10mA output current to be within the recommended operating conditions. The same with screws and nails, you should not use hammer with screws and screwdriver with nails. Use the correct chip for the job. LED matrix driver chips do exist and can connect directly to LEDs without external resistors. The HC595 and LS595 are not meant to drive LEDs directly without resistance. \$\endgroup\$
    – Justme
    Commented Dec 12, 2020 at 9:22

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