Using resistor network instead of regular resistors

Question

This might be a simple question but I'm kind of new to resistor networks so for me it's not so straightforward:

I have a digital electronics project. As part of the project I have 8 pins of an IC connected to 8 LEDs. Each LED has its own 100 ohm resistor (it sits between the IC and LED itself).
In order to simplify the PCB design I'd like to use a single 100 ohm resistor network for those LEDs instead of each LED having its own resistor.

Is it possible? How should I connect the replacement exactly?

This is a simplified part of the schema involved (I drew only 2 LEDs and resistors for simplicity). The LEDs are typical 3 mm Kingbright. All 8 LEDs might be on at the same time but for very short period, less than a second.

Answer 1

It depends on the exact topology of your array, but it's generally possible.

Supposing you have an IC configured as follows:

^{simulate this circuit – Schematic created using CircuitLab}

Assuming that the current requirements (and necessary voltage drop) of each resistor is low enough (so that your resistor network part doesn't overheat), you can directly replace your eight resistors with a resistor network of eight isolated resistors on the same package. These networks are just eight independent resistors sharing a package, as shown in this screenshot:

(all datasheet screenshots are from https://www.ctscorp.com/wp-content/uploads/74x.pdf)

You can also do this with a bussed resistor network, having a topology like this:

However, because all of the resistors are tied together on one end, may need to rearrange your initial circuit a bit first if it isn't already designed with resistors tied together:

^{simulate this circuit}

Swapping the order of series components (resistance and diode) like this won't change their behavior significantly.

In this case, the common pin of the network goes to the supply rail, and each individual resistor pin goes to the LED.

Answer 2

Yes it is possible, and commonly done in commercial equipment.

There are different types of resistor arrays, some have one pin of each resistor available, and the other end of the resistors are all connected to one common pin. These are good for things like pullups or pulldowns where you need a bunch of the same value resistors all connected to Vcc or ground. Others have each resistor independent from the others, both sides of each resistor have pins to connect them.

You can generally use either type for LEDs, to use the ones that have a common pin you would just move the resistors in your circuit to the other side of the LEDs so that you can hook the common to your LED power or ground depending on if you're driving them high side or low side. For the ones without a common connection just wire them like regular resistors.

You do need to be mindful of total power dissipation, I've seen a number of products that used resistor arrays for 7 segment displays have segments go out because the resistors had failed from overheating.

Answer 3

You can do this if you use networks that are appropriately rated (watch the power rating and allow adequate safety margin for your environment extremes and reliability requirements). It's a very common approach. If you have a spare resistor or two you can combine them in series or parallel to make other values.

The most flexible networks to my mind are the 4 resistor networks with both sides of each resistor brought out. A 1206 (Imperial) Yageo YC-164 4-resistor network is rated for 1/16 W for each resistor so if you derate to 50% that's only about 30mW each. Smaller networks such as YC-104 have half the rating (and are not marked).

Note that (depending on your via diameters etc. ) it may not make the PCB all that much smaller or easier to lay out since you've now got some strong constraints on where the traces have to go and SMT layouts are naturally almost like single-sided PCB layouts.

It's also a bit less convenient to deal with since switching resistor positions to ease layout and avoid vias and/or leave a more intact ground plane involves back-and-forth between layout and schematic. This will be easier with some EDA software than with others. That's optional, of course, and there's the intermediate step of just flipping (not rotating, you can do that in layout) the network on the schematic when the traces are all coming out twisted.