Resistors in Serie/Parallel

Hi,

I'm a curious hobbyist/noob with a little basic knowledge in electronics... and I have a question about resistors in parallel...

I did build a small DC 5 volt circuit to fool the Power bank's Auto Shutdown current limit to stay always on with a load dummy of 39 ohms. So I can run these low current draw applications with it.

5/39= 128mA load dummy... This comes on via an ATTiny85 and an NPN for 1 second every 110 seconds... I added a small 2.2v-20mA LED to blink with it, to see it's on as long the LED blinks.

Now I connected the LED from the VCC with a 150-ohm to the Anode to give the LED the proper current to function at 5 volts. The 39-ohm resistor is in parallel from the VCC to the LEDs Cathode and this goes via the NPN to the common ground. I did paint a crude diagram of the circuit added as an attachment. To give a better picture of the whole I hope. It concerns R3, R4 and D4. The circuit works like in real life on a breadboard and the power bank is already on for over 14 days. Thus so far so good I got it working without any flaws, yet.

I did build a simulation of this in TinkerCad and in it, the dummy shows a current draw of 133mA... Pretty close to the theory.

But what I do not understand is when I take the LED and its resistor out of the circuit the load doesn't change it stays the same? At least in Tinkercad, it does. Is this a TinkerCad flaw? Or is it a matter of chance that they cancel each other perfectly out?

I understand from theory the current draw should change with parallel resistors added? I can find no information about how this works in this case with a LED in the circuit as well. Does the current draw from the LED cancel out that 150-ohm resistor and does the 39-0hm load dummy simply stay the same in this case?. :slight_smile:

I would like to understand how does this really works? And how do I calculate something like this?

Oh, I might add, the NPN switching the 3 x 12-hour LEDS draws only 50mA.

Why shouldn't it stay on ?
It's still drawing the minimum 128mA.

raschemmel:
Why shouldn’t it stay on ?
It’s still drawing the minimum 128mA.

I’m not sure I understand what you are referring to? Do you mean the power bank? This bank has a built-in chip that shuts the bank off if not enough current is drawn and it does this in exactly 1 min 58 sec. The only way to keep this bank on indefinitely I tested it needs a current draw from +/- 120mA, with a minimum of 1 sec. every 110 sec. To reset the shut-off timer to 0 and start counting again.

The circuit without the dummy load draws only 67mA current at High load.

Why this is build in is a mystery to me… Anyway, with a current draw lower than 128mA, it shuts down automatically after 118 sec. Then it requires a manual reboot.
Thus if I want to run 3 x 20mA LEDs for 12 hours and an ATTiny for 24/7 with this particular bank this is the only way to do it… And after all, it’s a cheaper solution than searching and buying a bank that doesn’t have an auto shut-off feature at low currents. And the bank is strong enough to run this circuit for at least 28 days… :slight_smile:

Now I connected the LED from the VCC with a 150-ohm to the Anode to give the LED the proper current to function at 5 volts.

But what I do not understand is when I take the LED and its resistor out of the circuit the load doesn’t change it stays the same?

Again, why should removing R4 and D4 change anything ?
The power bank is still drawing (5V/39 ohms)=128mA is it not ?
Or am I misunderstanding what you have said ?

raschemmel:
Again, why should removing R4 and D4 change anything ?

That was exactly my question... It doesn't change, even tho it is in parallel with R3... And theoretically, resistors in parallel should give a different current draw... In this case, it doesn't seem to do that... And I like to know why?

In the simulation, it seems also the 20mA the LED should draw extra is not registered.

Here the LED and resistor are not in the circuit.

CD1.jpg

Here they are... But it still draws the same current. But the total current draw above the battery goes up 3mA

CD.jpg

CD1.jpg

CD.jpg

Is the green led blinking ?
Why does it matter that the current doesn't change ?
I thought all you cared about was the power bank staying on and it does , right ?

raschemmel:
Is the green led blinking ?
Why does it matter that the current doesn't change ?
I thought all you cared about was the power bank staying on and it does , right ?

Yes, the green led is blinking for 1 sec every 110 sec.
Practically it doesn't matter why it doesn't change... Theoretically, it doesn't make sense to me because all theory can find about it says it actually should change. And I'm a noob trying to learn something and this makes me curious...
In the end, indeed all I care about is that it works... Knowing why and how is just a welcome bonus... :grin:

Yes, the green led is blinking for 1 sec every 110 sec.

You can’t seriously think that a led drawing 20mA blinking on for 1 second every 2 minutes is going to change the current measured with a DMM in a few milliseconds can you ?

Seriously, that is not a logical question, because the average on time of the led is 9ms. The DMM acquires the
average current measurement in ms and can’t detect such a brief or small change in the average current.
(Iave=ton/(ton+toff)=0.020A/110 = 181uA)
(for example, if ton=toff=1 s, tave = 1/2 s)
“Do the math”
energy (J) = power x time
P= I x V
Let V = 5V
I = 0.020A
t = 1 second
P = 5*0.020A
P = 0.1W
tave = 1 second/110 second = 9 ms
E = P x t
= 0.1W x 0.009 s
E = 909 uJ (1 1100th of a Joule)
FYI, 1 Joule is the amount of energy represented by 1 A passing through 1 ohm for 1 second.
The led draws 20mA (20 thousands of 1 A)
The resistor is 150 times larger than 1 ohm
The time is 9 thousands of 1 second.
It is no surprise you can’t measure it.

Hello panyvino,

++Karma; // For doing experiments to try and learn something

For circuits in parrallell just add the current for each branch of the circuit, so if you have one brnach taking 100mA, one taking 15mA then the total will be 115mA.

The current in one branch does not affect the current in another, they are independent, however, the change in current being drawn from the supply might affect the supply voltage if the supply is not able to supply all the current. In your circuit you have a transistor, maybe there is not enough base current for the load. Bipolar transistors function as current sources, so that might be what you are seeing. You need to measure the voltage between the collector of the transistor and +5V (that is, the actual voltage across the load), don't assume the voltage is not changing when you change the load.

BTW,
Neat circuit but upside down: It should be positive voltage at the top, negative at the bottom. AT Tiny is upside down as well. Even with those mistakes the schematic is far more useful than the Fritzy thing.

The current in one branch does not affect the current in another, they are independent, however, the change in current being drawn from the supply might affect the supply voltage if the supply is not able to supply all the current. In your circuit you have a transistor, maybe there is not enough base current for the load. Bipolar transistors function as current sources, so that might be what you are seeing. You need to measure the voltage between the collector of the transistor and +5V (that is, the actual voltage across the load), don't assume the voltage is not changing when you change the load.

If the current in one branch is always there, and the current in the other branch is there on the average of 9ms , it
is not going to be measurable. If you took the current measurent during the 1 second the led is on, it might be
measurable.

Neat circuit but upside down: It should be positive voltage at the top, negative at the bottom. AT Tiny is upside down as well. Even with those mistakes the schematic is far more useful than the Fritzy thing.

This is a reference to the fact that there are standard conventions for schematic representation:

  1. transistors are shown with the emitter point down if npn and may be up with pnp.
  2. ground symbols are always pointing down toward the bottom of the page
  3. ICs are drawn right side up
  4. Positive voltages are shown at the top of the page, negative at the bottom.
  5. leds are shown with cathode pointing down towards the bottom of the page.
    etc., etc. etc,

PerryBebbington:
Hello panyvino,

++Karma; // For doing experiments to try and learn something

For circuits in parrallell just add the current for each branch of the circuit, so if you have one brnach taking 100mA, one taking 15mA then the total will be 115mA.

The current in one branch does not affect the current in another, they are independent, however, the change in current being drawn from the supply might affect the supply voltage if the supply is not able to supply all the current. In your circuit you have a transistor, maybe there is not enough base current for the load. Bipolar transistors function as current sources, so that might be what you are seeing. You need to measure the voltage between the collector of the transistor and +5V (that is, the actual voltage across the load), don't assume the voltage is not changing when you change the load.

BTW,
Neat circuit but upside down: It should be positive voltage at the top, negative at the bottom. AT Tiny is upside down as well. Even with those mistakes the schematic is far more useful than the Fritzy thing.

Thank you!! I guess you are Spot on! I have to review my transistor base resistor calculations, which seem to be sensitive stuff... That's a challenge by itself to figure out correctly...

But indeed when I change NPN base R1 from 110 ohms with 133mA current draw to Base R1 to 90 ohms then it draws 150mA instead.

Thus I'm simply limiting the current draw through the transistor regardless of what is connected to it it stops in this case at 133mA with a Base R of 110 omhs...

Now I see the bell should have rung much sooner... Since the D1, 2, and 3 drawing 49mA that should actually be 60mA, but the transistor base resistor is also pulling this C- E current down.

Thanks again for the explanation... 8)

panyvino,
A follow up for you as I was (still am) tired when I answered and it has been suggested I didn't give you a complete answer, so I'll try some more.

Nothing wrong with my reply #8 but I skipped over the bit that raschemmel mentioned in reply #9. You are not going to see the LED current on a multimeter when it is on for such a short time.

Also, in your schematic you have D1, D2 and D3 without series resistors, this is bad, LEDs must have some means of limiting the current.

Have I missed anything?

At 5V, 39Ω will pass 128.2mA, the parallel LED and 150Ω resistor will add 19mA, total, 147.2mA, 5V / 147.2mA = 34Ω equivalent resistance. The 110Ω base resistor is too low, will draw 24mA from the Tiny's output pin, I would go with 270Ω, about 9.6mA. + what @PerryBebbington said:

Also, in your schematic you have D1, D2 and D3 without series resistors, this is bad, LEDs must have some means of limiting the current.

What is the green LEDs Vf? Probably more than 2.2V, let's assume 3V, 5V - 3 = 2V, 2V / 150Ω = 0.013333A, 13.333mA.

@JCA34F,
I think you missed the part about the parallel led only being on for 1 second every 110 seconds...which gives an
average current of 181uA. (0.020mA/111)

Yes, I saw it, OP asked how to calculate a parallel circuit, I think. :confused:
BTW: I've done several of those circuits.

@PerryBebbington Thanks again for explaining... And sorry for the clumsy schematics, as said I'm still a noob, this is my first tiny project, and I'm far from familiar with the conventions of how to properly draw something like that. I just used MSPaint to quickly put something together to the best of my knowledge. Though it's still easier than words only. But I look into it before I draw one again...

Regarding D1,2, and 3 I indeed do read all over the place that all LEDs need a current limiting resistor. In this case, I went with the vendors' webshop product description that states at 3.3v supply voltage this LED with an Fv of 3.3v does not need a resistor. Then if I still use a calculator this also comes up with 0 and the closest rounded standard E-12 of 10 ohms 1/8W. But I just went with what the shop said it doesn't need. And since with the measurements, it's only drawing 50mA, so I thought that can't be bad. But it still is?

JCA34F:
What is the green LEDs Vf? Probably more than 2.2V, let's assume 3V, 5V - 3 = 2V, 2V / 150Ω = 0.013333A, 13.333mA.

Thank you too for explaining the calculation... I think I get it now. The green LED is this one.
https://cpc.farnell.com/kingbright/l-424gdt/led-flat-top-3mm-green/dp/SC11540?ost=led%2C+flat+top%2C+3mm%2C+green
Regarding the NPN Base R, I indeed calculated 10% of the total 200mA the transistor can pull, thus it came close to 110 ohms for a 20mA base current. But since it doesn't need to draw 200mA I get me 270 ohms and see how it runs with that. For now, I went with the closest values I have at hand and used 2x 220ohms. Obviously, I need a larger and better-equiped supply box. In these crazy Lockdown times, the shipping costs are driving me nuts on 5 cents resistors.

@raschemmel
Thanks a bunch for explaining, I get it now that it is simply too little and too short to be measurable... That was what threw me off my rockers... Theoretically, it didn't add up... But it all makes more sense now...

"BTW: I've done several of those circuits."

I'm sure 'several' is a gross understatement.

My approach to this question would immediately have been "put more current meters on the LED
and dummy load" - in otherwords instrument everything to see whats really going on (for instance
if the LED was backwards this would have immediately showed up). You'd also be able to see
if your simulator was obeying KCL (Kirchoff's current law)...

raschemmel:
“BTW: I’ve done several of those circuits.”

I’m sure ‘several’ is a gross understatement.

“Several” = 5, for 5 different power banks, here’s a test program from way back when.
OOPS, wrong un, try this un:

// To set interval to 15 sec type i15000, pulse length to 100 millis, p100
unsigned long iStart; // interval start
unsigned long iEnd = 30000; // interval end
unsigned int pLen = 100; // pulse length
const byte pPin = 8, // pulse pin
           ledPin = 13; // led pin
void setup()
{
  Serial.begin(9600); //set serial monitor to match
  pinMode(pPin,OUTPUT);
  pinMode(ledPin,OUTPUT);
  prntIt();
}
void loop()
{
  digitalWrite(pPin,millis() - iStart < pLen);
  digitalWrite(ledPin,digitalRead(pPin));
  if(millis() - iStart > iEnd)
    iStart += iEnd;
  while (Serial.available() > 0) {
    char inChar = Serial.read();
    if(inChar == 'i')
      iEnd = Serial.parseInt();
    else if(inChar == 'p')
      pLen = Serial.parseInt();
    else
    {  
       Serial.println("invalid input");
       while(Serial.available() > 0)
         Serial.read();
    }
    iStart = millis();
    prntIt();
  }  
}
void prntIt()
{
  Serial.print("interval = ");
  Serial.print(iEnd);
  Serial.print("  pulse length = ");
  Serial.print(pLen);
  Serial.println("\n");
}