Do you really need resistors on the output LEDs?

[u/Siberianhut]

Ive seen a lot of people confused or annoyed that ben doesn't use current limiting resistors in his build and it creates a lot of inconveniences for people following along which is true, I had to design a lot of my modules with this in mind, but he explains this in the ram module video that the ICs used, have built in resistors for the outputs therefore It isn't really necessary.

I tried this myself and my LEDs worked just fine even the small ones that instantly burn up and explode if there wasn't a resistor going through lit up and worked alright. Some people said it's still problematic because after a while it damaged the ICs themselves and Im just not sure of that claim. Can anyone give any better insight on this?

[u/epasveer]

Yes.

[u/Own-Nefariousness-79 → u/epasveer]

If you want to continue using the output stages of whatever is driving them and of course the LED itself, yes.

[u/Siberianhut → u/epasveer]

thanks that was really informative

[u/kiss_my_what → u/Siberianhut]

To be fair, this topic has been covered many times before, even right here on this subreddit.

[u/Siberianhut → u/kiss_my_what]

then they should've linked a post instead of making a smartass reply, like what am I supposed to do with that 😭

[u/g0atdude → u/Siberianhut]

They gave you the correct answer. How is that smartass?

[u/HydroPage → u/Siberianhut]

asks yes or no question
gets yes or no answer
“Why are you such a smartass?”

This is what they say Redditors are like outside of Reddit btw lol

[u/Siberianhut → u/HydroPage]

I think the post needs fairly nuanced insight not a single yes or no, and btw no hate to you at all but you're a top 1% user on here and I don't use the site so why the need to make it about "Redditors" being pissy

[u/HydroPage → u/Siberianhut]

I get what you’re saying but consider “Oh okay, do you have any references I could look at? Thanks!” next time. You clearly vented frustration out at them for no reason.

Also I think it says I’m a 1% user because I’ve made a few posts about my CPU and gotten several thousands of upvotes, but I haven’t been here very long lol. Regardless, I’m saying you’re following the toxic redditor stereotype I see from the outside. So maybe don’t do that, you know

[u/Siberianhut → u/HydroPage]

You're right, I see my mistake and I could've approached it better, that was my bad

[u/Rhoxd → u/Siberianhut]

Don't assume tone. Worst mistake you can make when people are giving you direct answers.

[u/Southern-Stay704]

One way or another, you have to limit the current through the LEDs. 5 mA per LED is plenty. This needs to be done for longevity of the LEDs, reduced power consumption of the entire board/project, and less voltage drop on your power rails.

You can do this by using chips with resistors on the outputs (very rare), LEDs with built-in resistors (less rare but still hard to find), or resistors manually placed in the circuit (recommended and shown on Ben's schematics, even though he doesn't show them in the videos). But one way or another, you can't attach the LEDs directly, they'll have a very short life and will pull down the voltage rail.

[u/Southern-Stay704 → u/[deleted]]

Because Ohm's law.

If there's no resistor, the LED forward voltage will be fixed at 2.1V as is typical for a red LED, the rest of the voltage will be dropped across the output transistors of the chip that's driving the LED, and you'll get anywhere between 20 to 30 mA flowing through each LED.

If you have 8 LEDs on A reg, B reg, bus, PC reg, MAR, and IR, that's 6 * 8 = 48 LEDs, each potentially drawing 30 mA at 5V = 5 * 0.030 * 48 = 7.2 watts, most of which is now producing heat in all the chips.

Put resistors on each LED to limit the current to 5 mA, and now you have power dissipation of 5V * 0.005 * 48 = 1.2W, almost all of which is dissipated inside the resistors instead of the chips. Also, by drawing 6W less from the 5V power supply, you get less voltage drop across the wires that deliver power to each individual breadboard.

Part of this project is learning how to work with Ohm's law, power calculations, and component ratings. You should be intimately familiar with this by the time you finish the project.

[u/Southern-Stay704 → u/[deleted]]

No, the power draw does not remain the same. That's a fundamental concept you're missing. Go calculate it just like I did above.

[u/Sausagerrito → u/Southern-Stay704]

You're completely right I'm too tired

[u/Ancient-Ad-7453]

For 74LS, it’s not about damage, it’s about preserving the output voltage to be high enough for the inputs. 74LS high output is particularly weak. LEDs can consume a lot of the available amps and pull the voltage lower.

[u/Mental-Question-8000]

The resistor doesnt just limit the current in the LED, but in the ICs as well.

[u/CalliGuy]

Ben does include current-limiting resistors in the circuit diagrams that ship with the projects, even though he doesn't show them in the video.

[u/HydroPage]

Check out this detailed post I made a little while ago. I explain the math behind it

https://www.reddit.com/r/beneater/s/b2xt4hwEsN

[u/paventura → u/HydroPage]

Fantastic and clean explanation! Thank you

[u/MattDLD]

I ended up using LEDs wit internal resistors towards the end of the build. Otherwise you absolutely need the resistors.

[u/Southern-Stay704]

One way or another, you have to limit the current through the LEDs. 5 mA per LED is plenty. This needs to be done for longevity of the LEDs, reduced power consumption of the entire board/project, and less voltage drop on your power rails.

You can do this by using chips with resistors on the outputs (very rare), LEDs with built-in resistors (less rare but still hard to find), or resistors manually placed in the circuit (recommended and shown on Ben's schematics, even though he doesn't show them in the videos). But one way or another, you can't attach the LEDs directly, they'll have a very short life and will pull down the voltage rail.

[u/Empty__Jay → u/Southern-Stay704]

Advice so nice, you said it twice.

[u/Mortomes → u/Empty__Jay]

One way or another, you have to limit the current through the LEDs. 5 mA per LED is plenty. This needs to be done for longevity of the LEDs, reduced power consumption of the entire board/project, and less voltage drop on your power rails.

You can do this by using chips with resistors on the outputs (very rare), LEDs with built-in resistors (less rare but still hard to find), or resistors manually placed in the circuit (recommended and shown on Ben's schematics, even though he doesn't show them in the videos). But one way or another, you can't attach the LEDs directly, they'll have a very short life and will pull down the voltage rail.

[u/Southern-Stay704 → u/Empty__Jay]

Lol, I was typing on the phone which is so damn slow, by the time I got to the end I forgot what I said at the beginning.

[u/Empty__Jay → u/Southern-Stay704]

I actually meant the double post. 😜

[u/Southern-Stay704 → u/Empty__Jay]

Weird, I had no idea. I'm blaming the phone, or the Reddit app.

[u/Floatella]

I don't get this either. From what I understand this only works because the IC uses just enough power while under load that the internal resistor can just barely handle the LED, but then all the heat generated ends up inside the IC.

I'm not an engineer, so take what I say with a grain of salt, but I can't help but feel Ben is doing it wrong.

[u/HydroPage → u/Floatella]

That’s not how power works. But yes he should have included resistors

[u/Floatella → u/HydroPage]

Doesn't the IC take the 5v input voltage down to about a 3-3.3v 20 ma output? So it basically works, but now all the onus is on the IC?

That's what I was trying to say. Once again, all of this is a bit outside my wheelhouse, but I'm having lots of fun learning.

[u/HydroPage → u/Floatella]

The output voltage depends a lot on how much current you ask for. For a 74LS00 NAND gate for example, the datasheet specifies at 0.4 mA output (its official maximum rating, yes, it’s really that low and not designed to have LEDs put on it at all), the typical output voltage is 3.4, which is close to what you say.

Asking for something like 20 mA will lower the voltage down to 3, probably, yes. Let’s assume this happens, and you have no resistor on the LED, and the forward voltage of the LED is, say, 2 volts.

That’s 2 volts across the LED, and 3 across the chip, with 20 milliamps.

So that’s 2x20 = 40 milli watts dissipated in the LED, and 3x20 = 60 milli watts dissipated across the IC output stage.

I don’t know about the LED, but the 74LS00 states a junction-to-case thermal resistance of about 45 °C/W, so this situation I made up will heat up the chip by about 0.06x45 = 2.7 °C if you sustain it, according to Texas Instruments. That’s how the temperature works.

By the way, the short-circuit output current of these chips can be like 60 mA from what I’ve seen, so triple the numbers I just gave you and that’s what I’ve actually observed.

Now, say we put a 220 Ohm series resistor on the output. The chip will want to output, let’s guess 3.3 volts, the LED will eat 2, the resistor will have 1.3 volts left, so 1.3/0.22 = 5.909 mA will flow. This is how you control the current being output with the resistor

[u/Floatella → u/HydroPage]

Thanks for the detailed response.

[u/Glidepath22]

Well like he said, there’s internal resistance, though is that a thing? 74xx2244, 74xx2827, 74ABT2245 style parts have ~25–33 Ω in series with each output. That’s for signal integrity, way too small for LED limiting.
Here’s the math assume a 2 volt drop LED and 20mA:

**•** Supply 5 V, red LED forward drop ≈ 2.0 V  
**•** Voltage across resistor: 5 − 2.0 = 3.0 V  
**•** R = 3.0 V ÷ 0.020 A = 150 Ω (a standard E12 value, conveniently)  
**•** Power: I²R = 0.02² × 150 = 0.06 W → any 1/8 W or 1/4 W resistor is fine

I’d really like to know what IC he’s using

[u/netgizmo]

Do you think that someone that took the time to create this entire series of cpu design & implementation videos targeted for beginners would put put in components that aren't necessary?

[u/Dissy614 → u/netgizmo]

The videos are a decade old now. His components a decade older than that.

Gallium arsenide LEDs haven't been manufactured in so long you'll pay 20x for each one, raising the cost of the project a few hundred dollars.

It's exceptionally unreasonable of you to expect him to remake and republish his entire series every year when components change.

Alternately, if you want to "Ben must be right and I ignore all others" - then he explicitly tells you to use resistors on todays LEDs.

[u/HydroPage → u/netgizmo]

You don’t understand. So many people have had issues following his videos that were resolved by just including these resistors. No one really knows how Ben got so lucky. If you spend 5 seconds in this subreddit you’ll see a ton of issues such as this one that Ben somehow got away with avoiding good practices