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ETO Online Calculators

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ericgibbs

Well-Known Member
Most Helpful Member
hi,
There are a number of Online Calculator programs available on the Tools Section of ETO.

Two recent additions are:

1. Thermistor Resistance Calculator.
Used to determine the approximate resistance of a Thermistor at different operating temperatures.
The user enters the Beta, Resistance at Tref and the Tref temperature from the thermistor datasheet.
https://www.electro-tech-online.com/tools/thermistor-resistance-calculator.php



2. LED Series Resistor Calculator.

The user can enter the parameters from the LED datasheet together with the required current, supply voltage and the number of LED's in a series string.
Also it is possible to enter a preferred value series resistor and see the effect on the LED current.
To give some guidance regarding the effect of 'spread' of the voltage drop about the typical value, currents for +/-10% spread in Vfwd are also calculated.
https://www.electro-tech-online.com/tools/led-series-resistance-calculator.php

E.
 
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Nice. Perhaps that will reduce the number of questions posted re LED series resistors.
 
Most online calculators for LED current-limiting do not have any common sense.
I tried it with a low voltage across the current-limiting resistor and the calculator gave a NEGATIVE current when the LEDs have 10% high forward voltage so they become a GENERATOR!

Then of course when the LEDs have a 10% low forward voltage then they burn out because the current is too high.
 
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Most online calculators for LED current-limiting do not have any common sense.
I tried it with a low voltage across the current-limiting resistor and the calculator gave a NEGATIVE current when the LEDs have 10% high forward voltage so they become a GENERATOR!

Then of course when the LEDs have a 10% low forward voltage then they burn out because the current is too high.

Like most calculators, 'a little common sense' is required when using them, a negative current result from this calculator is an obvious error in the choice of the entered parameters.

If I used my hand held calculator to determine the LED current and keyed in stupid values I would get a stupid answer, which would be obviously wrong.

There is an old saying about computers, ' Rubbish in, Rubbish out!'

I dont see the point of your post unless its just to find fault with some else's work.


EDIT:

I guess you did not bother to read this comment in the header.
For a precise series resistor value, the Vfwd of the LED's you are using should be measured.
This is especially important when connecting LED's in series chains.
as the combined spread in the Vfd can lead to an excessive LED current.
 
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I dont see the point of your post unless its just to find fault with some else's work.
I posted about the problems with the LED current-limiting calculator because many nOObs do not know the details that the calculator should say.
 
I posted about the problems with the LED current-limiting calculator because many nOObs do not know the details that the calculator should say.

OK,
As you seem to be having a problem understanding the obvious erroneous negative current value result from this calculator, what 'useful' additions would you like to see included in the calaculator that would make the output easier to understand???

I would be pleased to upgrade the program.

E.
 
Thanks for this, I get people asking me to calculate a dropping resistor for an led all the time.

what 'useful' additions would you like to see included in the calaculator that would make the output easier to understand???

Might be helpful to show resistor wattage - lots of people want to run one red LED (1.7V) off a car's 12V supply (which can go to 15V) at the full 30ma the led is rated for. This requires a half-watt, not a quarter-watt resistor. Additionally, there's these new high-current white LEDs that can require something even bigger.
 
People seem to forget... Common sense is not common, it is learned through knowledge of a subject, these calculators are useless unless you know how it works in the first place, there is no way to improve upon that, and they will never be useful to someone that has no clue about what they're doing.

I think it would be FAR more useful to show the equations that these calculators were based on and describe what and why each variable in the equation is important so that users can actually eliminate the need for the calculator itself. The Thermistor calcutor throws up a bit of math but no explanation, how is a newbie going to even know what beta is, or how to determien what the reference values for their thermistor are?

I don't think these calculators are a good addition to ETO, simply because there are hundreds if not thousands of them already available via a simple Google search, many of which have a description of their methods much better than the ones I've seen posted here.
 
I am not a teacher and I don't want to be one.
A calculator should be able to be used by an "idiot" who knows nothing about mathematics or electronics.
This current-limiting resistor calculator for an LED or for a few series'd LEDs is a failure. There might be some good ones out there.

Anybody with a small amount of common sense can calculate the simple value of a resistor to limit the current for a few series'd LEDs.

Calculate the current of some low forward voltage LEDs then calculate the current if their forward voltage is high. Then re-calculate so that all of those LEds will be bright enough and/or will not burn out. Simple.
 
This one prints wattage, and has reference schematics, that's helpful -
https://ledz.com/?p=zz.led.resistor.calculator
(I'm not too impressed with their 1-resistor parallel mode)

This one gives a pictorial diagram, with the closest standard color code printed right on the resistor, and tells what standard wattage size resistor to use -


This one gives you a schematic (or a wiring diagram), will tell you the color codes for the resistors using closest standard values, tells you the wattages for everything with closest standard sizes for the resistor, and will automatically make series/parallel chains appropriate for the voltage -
**broken link removed**
 
Thanks for this, I get people asking me to calculate a dropping resistor for an led all the time.



Might be helpful to show resistor wattage - lots of people want to run one red LED (1.7V) off a car's 12V supply (which can go to 15V) at the full 30ma the led is rated for. This requires a half-watt, not a quarter-watt resistor. Additionally, there's these new high-current white LEDs that can require something even bigger.

hi duffy,
This is the sort of positive feedback I would hope for, many thanks.:D

I will include your suggestion in the next upgrade of the Calculator.

If you have any further idea's , please let me know.

E.
 
I think it would be FAR more useful to show the equations that these calculators were based on and describe what and why each variable in the equation is important so that users can actually eliminate the need for the calculator itself. The Thermistor calcutor throws up a bit of math but no explanation, how is a newbie going to even know what beta is, or how to determien what the reference values for their thermistor are?

I don't think these calculators are a good addition to ETO, simply because there are hundreds if not thousands of them already available via a simple Google search, many of which have a description of their methods much better than the ones I've seen posted here.

hi Sc,
Thanks for your comments.

The goal is to have available on ETO a range of 'calculators' to which 'Entry level' members can be linked too, rather than being told to Google.

You say other calculators are much better, so why not suggest improvements to the ETO versions in order to bring them to a higher standard.??

By having in house calculators for ETO, you have the option to contribute your idea's via the Forum.

E.
 
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I am not a teacher and I don't want to be one.
A calculator should be able to be used by an "idiot" who knows nothing about mathematics or electronics.

Anybody with a small amount of common sense can calculate the simple value of a resistor to limit the current for a few series'd LEDs.

Calculate the current of some low forward voltage LEDs then calculate the current if their forward voltage is high. Then re-calculate so that all of those LEds will be bright enough and/or will not burn out. Simple.

hi agu,

First you say a calculator should be able to used by an 'idiot', then you say:
Anybody with a small amount of common sense can calculate the simple value of a resistor to limit the current for a few series'd LEDs.

So which is it.??

Ref your last point.
I guess you did not bother to read this comment in the header.
For a precise series resistor value, the Vfwd of the LED's you are using should be measured.
This is especially important when connecting LED's in series chains.
as the combined spread in the Vfd can lead to an excessive LED current.
 
hi,
The LED series resistance calculator and Thermistor resistance calculator Tools have been upgraded

Recommended series resistor wattage added, also more warning messages added to the LED Tool.

A resistor/thermistor divider diagram and calculation of the divider output voltages for high and low end fixed resistors added to the Thermistor Tool.

https://www.electro-tech-online.com/tools/

E.
 
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The resistor calculator would benefit from showing the nearest standard value resistor that will work within specs rather than just the idealized value, might be a good idea to derate the resistor wattage produced to reflect that most new users won't know that they should derate from maximum component values to avoid burnt out components in non-ideal conditions. 25-50% perhaps?
 
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The resistor calculator would benefit from showing the nearest standard value resistor that will work within specs rather than just the idealized value.

ElectroMaster has already posted a Tool for selecting the nearest preferred value resistor from the 'E' ranges.

https://www.electro-tech-online.com/tools/resistance-values-calculator.php

The lower section of the LED resistor calculator is so the User can Enter nearest standard values, usually called preferred values, to see the effect upon the LED current.
 
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Provide a mention of this on the LED calculator page and provide a link to the second calculator then? That kind of cross referencing will be extremely important for new users.

What about wattage derating of the resistor? The example that pops up when you load the page shows a resistor that's being run at it's fully rated power right from the start, this is bad practice.
 
Provide a mention of this on the LED calculator page and provide a link to the second calculator then? That kind of cross referencing will be extremely important for new users.

What about wattage derating of the resistor? The example that pops up when you load the page shows a resistor that's being run at it's fully rated power right from the start, this is bad practice.

For starters its not bad practice to run resistors at their rated wattage, I don't know where you got that idea from.???

Secondly the example does NOT come up with a resistor rated at full power.

Its a 10V supply minus a 2V led drop , so thats 8v across a 400R resistor which is passing 20mA.

Do the maths.... 8V *.02A = 0.16Watts , the calculator recommends a 0.25Watt resistor.

OK.??

EDIT:
Perhaps I should post an Ohms law and a simple Power calculator.!
 
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When a person suggests operating a resistor at its rated power then that person should try holding the hot resistor (250 degrees C?)on his tongue or on another very sensitive part of his body (you know) for a few minutes. Or watch the hot resistor melt a nearby plastic cased capacitor or wire insulation.
The maximum power in a resistor should be HALF its rating.
 
When a person suggests operating a resistor at its rated power then that person should try holding the hot resistor (250 degrees C?)on his tongue or on another very sensitive part of his body (you know) for a few minutes. Or watch the hot resistor melt a nearby plastic cased capacitor or wire insulation.
The maximum power in a resistor should be HALF its rating.

Why would you want to hold a hot resistor on your tongue or any other part of you anatomy??:confused:

Are you saying a 0.25Watt resistor dissipating 0.16Watt is going to give you 3rd degree burns.??
Resistors are designed to work at their specified wattage rating.!
 
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