Voltage to calculate required resistance - Please Help Me!

[benpaddlejones]

Hi
The following is an excert from a IR-LED datasheet.
**broken link removed**

Could you please tell me which of these voltages I use as the "Diode Forward Voltage" when calculating required resitance.

I'm using to calculate, I already know my "Source Voltage" and "Forward Current", but I'm confused in which of these 4 voltages to use as the DFV?

My setup is a simple curcuit:
| -Battery+ > Switch > Resitistor > IR-LED |
\__________________________________/

Cheers
benpaddlejones

 

[Pommie]

Neither, those are values for pulsed currents. Check the data sheet for the continuous current and forward voltage.

Mike.

 

[benpaddlejones]

Mike

**broken link removed**

This is all the data from the data sheet (the whole datasheet is at: https://www.vishay.com/docs/81011/tsal6400.pdf)

I assumed:
Source voltage: 1.2V (Rechargeable AAA)
Dioda forward current: 100ma (IF)
Diode forward voltage: ???

If you could direct me to the right place that would be awesome. I need to be able to calculate varrying resistors with different Source Voltages.

benpaddlejones

 

[ericgibbs]

hi,
Whats the IR type, part number.?

 

[benpaddlejones]

hi,
Whats the IR type, part number.?

Vishay TSAL6400
Manufacturers Datasheet: https://www.vishay.com/docs/81011/tsal6400.pdf

I need to know where to find the "Diode Forward Voltage" as I may also need to use different IR-LED's.

benpaddlejones

 

[ericgibbs]

assumed:
Source voltage: 1.2V (Rechargeable AAA)
Dioda forward current: 100ma (IF)
Diode forward voltage: ???

hi,
Its Vfwd = 1.2V at 50mA,,,1.35V at 100mA

The AAA battery is NOT suitable.

 

[benpaddlejones]

So is the Typical VF the "diode forward voltage"???

Or did you get the "Its Vfwd = 1.2V at 50mA,,,1.35V at 100mA" from somewhere else?

(I only listed the 1.2v AAA for arbitory purposes) What I need to know is which of the voltages is the "Diode Forward Voltage" so i can work out a suitable source and resistor.

benpaddlejones

 

[Hero999]

According to the datasheet you posted, the continious forward current is 100mA.

The foward voltage drop of LEDs always varies consideriably, for the LEDs you're using it's 1.35V to 1.6V.

The wizard oversimplifies the issue a bit too much.

You need to know the maximum and minimum power supply voltage.

For example, if you have two LEDs and three batteries:

Suppose the battery voltage is 1.6V, straight off the charger and drop down to 0.9V when they need to be recharged. This gives a maximum supply voltage of 4.8V and a minimum voltage of 2.7V.

If we calculate the resistor values for 1.2V per cell giving a total of 3.6V, the current will be much higher at the 4.8V given by 1.6V per cell which might damage the LEDs.

If we assumed an LED voltage of 1.6V and both LEDs were only 1.35V more current would flow which could damage the LEDs.

The combined maximum LED voltage is 2*1.6 = 3.2V which means they'll get very dim as the battery voltage approaches this voltage so using four AA cells is a better idea.

As a general rule, use the minimum LED voltage and maximum supply when calculating the resistor value. In this case we need to use 1.35V per LED and 1.6V per cell.

When looking at the circuit when the power supply is at its lowest voltage, use the maximum LED voltage. In this case it's 1.6V per LED and 0.9V per cell.

This avoids burning out the LEDs or only using a quarter of the power in the batteries.

 

[benpaddlejones]

Your my Hero999

Fantastic, thank you so much for your help it answered my question.

I will admit however, it took me 4 reads to understand, I certainly don't have a future in electronics

benpaddlejones

 

[ericgibbs]

So is the Typical VF the "diode forward voltage"???

Or did you get the "Its Vfwd = 1.2V at 50mA,,,1.35V at 100mA" from somewhere else?

(I only listed the 1.2v AAA for arbitory purposes) What I need to know is which of the voltages is the "Diode Forward Voltage" so i can work out a suitable source and resistor.

benpaddlejones

Hi,
Look at the graphs on the datasheet, thats where I got the forward voltage drops for the IR diodes, 1.2V at 50 and 1.35V at 100mA

The suitable resistor will depend upon the battery voltage and IR current.

Make you mind up which battery voltage and type you are going to use and we will advise on the resistor value..OK.