# voltage divider with load question and guess

#### Nanren888

Nov 8, 2015
623
Have you tried circuit simulators? There are a range of simulators based on SPICE that tend to work well, in both transient and DC analysis.
Generally if you can't be bothered doing the study to solve circuits, these will give you an answer very close to what you'd expect to happen.
I think if you simulate the circuit you've provided, you'd see that, in the steady state, current flows through the resistors and stable voltages exist.
Might even be quicker than trying it once you get familiar with teh simulator & you can always build it to check the result.
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Some simulators are combined with schematic. circuit capture, meaning they let you draw the circuit neatly.
Oh, while I've been typing, I see Bertus has suggested similar.

#### Nanren888

Nov 8, 2015
623
So, I came into this one late, just checking;
It's DC, right?
What colour is the LED? This changes the voltage across it.
The LED and resistors conduct current, so they fix the voltages at different places in the circuit.
After any initial transient, the capacitor charges up to a voltage determined by the dividers formed by LED and resisitors.
After that for steady-state, the capacitor is irrelevant.
It's just a resistive divider with an LED in there.
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It's not really clear at all where you are going with this.
Do you have some aim, or guiding principle in mind?
Are you interested in transient behaviour, or something?
Is this to get a briefly brighter flash from the LED at switch on, as the capacitor charges, or something?
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#### ratstar

Aug 20, 2018
485
I have a funny feeling about different coloured LEDS being different too, even tho isnt it just a different colour plastic on the outside? But the green ones seem to act different than the orange ones and I dont know why.

I hope you dont kill me for saying the truth.

This strange little circuit, is half of an oscillator I ***FLUKED**** 3.5 years ago, at the back of my friends house running around homeless pretending to be a prepper. The circuit was an oscillator, which only had capacitors as resistors. (and the 2 LEDS.)

Its been 3.5 years and I havent been able to get it working again, I dont know what it was, but it had green led's in it, And I dont know if it was the led's being overvolted by the 9v battery (which is what it was running on.) that caused it to work, or it was just the capacitors and resistors... also I dont know if it goes forever, or it actually runs out of power eventually, or how stable it is. But I remember it did hang, after so long of it running. So I dont know what it was, I can barely remember holding it up to the mirror looking at it pump slowly at about 4 hz.

The circuit was soldered together, but it mysteriously vanished from my bag, and I didnt get to look at it ever again, its only in fading memory about it.

That circuit is a one-shotter, so when the battery is connected it delivers a pulse. Its half of the oscillator, and the two halves invert each others pulses, but I dont know how.

That strange double loop thing on it (which makes it tricky to predict the circuit.) Is to discharge the cap, or release of the power, and the second loop is to make it so when the capacitor is finished filling, is to divert the current away from the output (the led.)

And im supposedly supposed to use it to divert power away to another use. but I dont know what its for. I cant remember, and I probably didnt even know, it was just a fluke.

So far, all ive managed to do, is get one hit out of it, and then it dies off. I do have a theory why it does it, but I just need to get better at looking at circuits in general before I can try and figure it out for real, without just fluking it like an idiot, like I did.

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#### Nanren888

Nov 8, 2015
623
Sure, you can make an oscillator with a storage capacitor and something that conducts when it hits a voltage threshold and stays conducting, for example till the current drops below a threshold. (so it resets to charge again)
It's that second bit that makes for the oscillation rather than just ending up with noise like a zener, sitting on the threshold. You need it to not just start conducting as a zener and limit any further rise, but to go all out and discharge down below the threshold, then stop conducting to allow it to charge again.
Things that act in this way tend to be devices that have avalanche mechanisms: something starts, then continues after the initial threshold no longer remains.
Look up UJT or diac, maybe scr, or PUT; probably your spark gap is of a similar nature, as are neons, but maybe better to stick with the low voltage examples.
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The "sort of" equivalent circuit for an scr is just two transistors, so you can create sort of equivalant devices with normal devices if you don't have the means to get these ones.
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Might be of interest to look for ways to use such mechanisms as the key, the starting point. The specific resistor values and capacitor values to create the right voltages and currents to make it all work in that region where it works, will come with a few calculations after you have selected the device that gives you effect at the core of it all.
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The price of parts has dropped significantly over the years. 555 timers/oscillators & transistors for monostables and flipflops are now easily within the reach of most experimenters.
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I think
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Enjoy

#### Audioguru

Sep 24, 2016
3,650
Ratstar showed one LED but talks about a red one and a blue one. Is the LED a back-to-back complementary red/blue one?
The capacitor does not pass DC and with the 200 ohm total of the resistors in parallel with it, its charged voltage will probably be very low.

#### Nanren888

Nov 8, 2015
623
The capacitor does not pass DC and with the 200 ohm total of the resistors in parallel with it, its charged voltage will probably be very low.
Seems he might have been a little hit and miss with the design in this example and experiments as variations from it.
Not sure further anaylsis of this one will help him.
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Seems as if he is striving for something that is not yet present in the circuit.

#### Audioguru

Sep 24, 2016
3,650
Yes, his "oscillator" needs something in it that is active to cause oscillation. Maybe a 555 IC.

#### ratstar

Aug 20, 2018
485
Thanks Nanren888 for the advice. I could probably fix it tomorrow by adding a transistor to it, but I'm not comfortable with my theory of resistors and capacitors yet, I want to explore it further still, because the circuit still doesnt respond how I think it will, my theory is too approximate and I want to study more before I continue.

A transistor would fix it yes, I could get it going fairly quickly, but I will be more ready for the rest of the system (Im making an analogue computer.) If I understand it better, I know its been 4 years, but maybe ill need 1 more year then I'll be ready to continue.

Seems wierd that I'm wasting so much time, but until I'm ready, I don't think im going to continue much past little circuits, because I dont even understand these properly yet, before I want to move on.

That circuit I drew has all its resistances wrong, I'm randomly experimenting cause perhaps im not going so well.

AudioGuru- "Yes, his "oscillator" needs something in it that is active to cause oscillation. Maybe a 555 IC"
Funny, you should joke like that, because I did manage to make a cricket sound by putting a buzzer in instead of a speaker, and I actually thought it was working and was really excited, until I realized it was a bit of a non-result...

Thanks for not kicking me off, Ill try to be good, I can help out here, I know the way I do electronics is wierd, but I can give advice too sometimes, i'll be careful before I do though because I do tend to post a little impetuously I admit.

#### ratstar

Aug 20, 2018
485
Bertus, thanx for the advice. Ill get this osc done soon.
If you overvolt an LED you can get oscillations also. =)

I dont understand voltage dividers properly, when you introduce a load onto a voltage divider, it wastes current doesnt it - because what is the output line, when the output load equals the resistor making the divider? that means it has to be passing current down both lines.

is this right?

#### bertus

Moderator
Nov 8, 2019
2,792
Hello,

No, it will not work with leds.
The circuit with a neon bulb depends on the breakdown in the bulb.

Bertus

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,890
If you overvolt an LED you can get oscillations also. =)
Maybe, but i doubt it. If it should work, that is going to destroy the LED. Contrary to neon bulbs and spark gaps, LEDs are not meant to be driven with overvoltage.

I dont understand voltage dividers properly, when you introduce a load onto a voltage divider, it wastes current
And I don't understand your picture. Try to post clear images where one can see the full circuit and labels are next to where they belong. In your post #30 there re a few numbers like 9, 3.4, 2.2( (?) with no obvious association as to where they belong. Mark these voltages next to where they are connected. If necessary, you'll have to place the labels more than once. You've been shown enough examples of well drawn schematics to do better. It doesn't have to be a "perfect" schematic from a high end CAE program, but your schematics are simply unintelligible.

#### ratstar

Aug 20, 2018
485
Maybe, but i doubt it. If it should work, that is going to destroy the LED.

Yes the LED doesnt last very long before it burns out.

So if I have R1, R2 and RL. When computing the current I compute for IQ -> volts/(R1+R2)=current IL -> volts/(R1+RL)=current?

But what about the voltage, does it stay the same as without the load? (Given both resistances (R2 and RL) are the same resistances.)

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Moderator
Nov 8, 2019
2,792

#### ratstar

Aug 20, 2018
485
heres the next attempt

Heres 3 oscillators, an 8 cap, and 2 4 cap oscs, none of them work fully, but they all get one hit to happen.

#### Nanren888

Nov 8, 2015
623
Perseverence is good.
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I'm intrigued with the analgoue computer concept/aim/goal.
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When people say analogue and digital, or even analog and digital, they usually mean something like
digital: discrete time, (sampled) and discrete values (quantised, and represented as bits)
analogue: continuous time, (not sampled) and continuous values (not quantized).
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It seems with your interest in oscilators, you may be after neither of these configurations.
One such, sometimes used is discrete time, (that is sampled, not quantized, where analog samples are processed without quantization.
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In a traditional analog computer, continuous time, continuous values, there's little need for oscilators. Analog computers usually fetaured a lot of opamps that could be connected as gain stages, differentiators, integrators, adders and such allowing allowing direct implementation of equations, usually differential equations.
A few of us have used true "hybrid computers" which interconnected digital and analog computers, but most of these were tossed out years ago,
There are also mechanical elements that perform various processing tasks, such as add, subtract, differentiate, integrate, so in theory analog comouetrs could be mechanical.
.
Just intrigued whether you are going to share any more of the rationale of how you version might work.
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BTW: diacs or Bertus's
It uses the reverse breakdown voltage of a transistor.
reverse breakdown device are effectively two-terminal devices: that is, circcuit-wise they fill a slot rather like your LEDs. Just as simple and if you work out the resistance values into the right ranges, will run for a long time without damage.
Might be worth getting to grips with those Ohms Law calculations so things don't break and you can experiment long without needing replacement parts.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,890
I compute for IQ -> volts/(R1+R2)=current IL -> volts/(R1+RL)=current?
No. Assuming (since as usual you do not supply a schematic that explains what's going on) R1 is the resistor connected to "+" and R2 2 is the resistor to "-" and Rl is parallel to R2:

Then you calculate an equivalent resistor Rpar = R2 || RL:

You can now apply the voltage divider equation using R1 and Rpar to arrive at the correct output voltage of a loaded voltage divider. This is all explained in the text I linked

Moderator
Nov 17, 2011
12,890

#### ratstar

Aug 20, 2018
485
Perseverence is good.
A few of us have used true "hybrid computers" which interconnected digital and analog computers, but most of these were tossed out years ago,

The computer I'm planning on making is a hybrid, with non continuous time, but continuous values. Continuous time is more elegant in my opinion, and I'd love to see that in action, and even make it myself, but I've got a strategy for an oscillated logic chip that is actually potentially more hi-performance, but they are both hi-performance, but I get a little extra for putting the oscillator in, so I'm doing it that way I think at the moment, nothing is decided, until I've actually got the thing tho for real..

Why do we need oscillation? (or continous time.)

If your logic doesnt feedback the output back to the input, you dont need an oscillator... but if it does then, you can implement continuous time, or you can oscillate it. I had an idea for a quasi-continuous time one, that waits for the output caps to fill before they spark across a gap back to the input, and thats an idea for getting continuous time, but that actually isnt truly continuous, because it waits to spark.

Getting continuous time to work, theres a definite trick for it that you have to work out.

I dont believe that you just "throw these hybrid computers in the garbage" tho, If you have a one cycle whole frame advancement, thats a whole frame per cycle, not just a floating point operation per cycle, which they use to judge super computers these days, I dont want just one floating point operation, I want the whole program to be updated per cycle, and you can do that with continuous time, or hybrid oscillated, and you can get a "whole update" done, of all the instructions of the program in one hit.

If you do that, you get a squared performance! Then your kicking butt.

Heres an example of the whole frame of logic of ping pong put in physically. (In a virtual logic Sim.)
This thing actually gets the whole program done at ~4 HZ! so just imagine if it was a megahert, you would pump out the frames like mad in fast forwards! And as it is, you could probably make the logic out of wood, and it wouldnt go too fast for just hand cranked logic! =)

But putting in the 20 or so operation stages in (each stage being about 25 operations maybe) in physically, might get a bit big, so then continuous values comes in, and reduces the bus size to a single wire, and getting the operations in is more possible without a digital bus!

More down to earth -> even if your oscillating bog standard TTL and gate chips with a 555 timer, boring usual style with average transistors, if you get all the operations in physically, if its oscillating at only a megahert, u r still more hiperformance than a gpu, for the physical task you put in, because its a megahert per frame. not per operation!
FPGAs also grant this huge performance hit, I think.

And what im doing isnt much different than that, but I want a faster oscillation, and a smaller bus size to fit more operations in.

And thats the big deal with what I want to achieve... I wonder if I get there...
But the untold secret is, what is the purpose of my logic? what am I going to implement??

HAROLD - Ill go check out the maths on that - then Ill get back to you. thanx for helping me out, Ive got a feeling if I get past these slow formative stages Ill be flying fast like the flash very soon!!!

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