LED working with SCB68

[alieas]

Hello Sir.... i created a VI in Project Explorer Methode to test the working condition of DAQ card PCI 7830R which i installed in my PC , in both Sub VI and the Host VI the programme is working , now i connect the SCB 68 hardware with DAQ card thro RMIO cable , i wired the DIgital Pins of SCB68 (respectievely with DAQ card Pins) in a Bread Board an place a LED corresponding to pins to show the real world output...

Now my prob is after i connecting the wire i run the VI, in breadboard the LED's are not getting ON same status is displayed in Labview Front panel too , when i remove the wire the LED in front panel of both sub and Host VI is getting ON..... y this prob occuring? to solve this what i have to do? i want to show realworld output (LED On in Bread board) thro SCB68... how to solve this? ..

[Coq_Rouge]

Do you use a series resistor on each LED to limit the current

[alieas]

No I didnt add any resistor on the breadboard...if i want to connect resistor means how many resistor i want to use for my VI..... resistor calcultion is (R=Vs-VL / I) right?.... if it true means .... i want to use 3.75 ohm resistor?

[Ashu__]

Hi Alieas

Could you please check the Voltage across corresponding terminal on SCB 68 block, which are connected to LEDs on the bread board?

Lets say if this voltage is V(s).

Also please measure the Voltage which comes across the LED. Lets say this voltage is V(L). Now generally an LED is able to take up 20mA of current.

Let this value of current be I(s).

Finally, the value of resistance to be connected across each LED would be: [V(s)-V(L)]/I(s) , where I(s) is 20mA.

Please revert in case of any doubts or clarifications.

Regards,

Ashutosh Singh 

[alieas]

so you r trying to say that, If  supply voltage VS = 9V, and have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A, R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).

In my case (5-2) /0.8  =3.75ohm (Violet, orange, green)... so if i place this kind of resistor in each LED front.... the LED will on right?

[_Bryan]

Sorry to jump in on this....  But where are you getting 9 volts from?  I thought you were using digital channels, which only put out 5 volts.  I have played with at least 2 led's on a scb68 with no resistors and it seemed to work fine.  Like Ashu said, take voltage readings directly from the scb68 to make sure you are getting 5 volts from your digital channels.

[AnalogKid2DigitalMan]

Your LED consumes 0.8A? there is no way to illuminate it directly using the digital outputs- you would need and intermediate transistor and separate power supply to turn the LED on/off. Outputs on the card are limited to a few mA's.

-AK2DM

[_Bryan]

I don't have any specs on the LED's, they were just some small digeykey (sp?) colored LED's.  I found a pack of them one day and wondered if I could make them blink like police lights.  I might have used a SCC-68, but I am sure they were just wired into digital channels with no extra hardware.  I could light them up and blink them with no problem.

I don't know if it was good for them or not, but it worked.  I found some specs:

Category	Optoelectronics
Family	LEDs - <75mA, Discrete
Series	-
Color	Red
Luminous Flux @ Current - Test	-
Millicandela Rating	3mcd
Current - Test	15mA
Wavelength - Dominant	-
Wavelength - Peak	700nm
Voltage - Forward (Vf) Typ	2.2V
Lens Type	Diffused, Red Tinted
Lens Style/Size	Round, 5mm, T-1 3/4
Package / Case	Radial
Size / Dimension	-
Height	8.65mm
Viewing Angle	-
Mounting Type	Through Hole
Packaging	Bulk
Other Names	LN21RP.HL P300

[Broken_Arrow]

---

@_Bryan wrote:

{...} make sure you are getting 5 volts from your digital channels.

---

Or look up the spec. The  DAQ outputs 3.3V for a logic high, and can sink or source 4mA. The rest is ohm's law.

[_Bryan]

Must be something wrong with my setup then?  I just measured 4.966 volts from my digital output.  We also use some relays that have a control voltage of 4-28 volts, with the digital output on I read 4.9xx volts. 

Under the specs for the card I am using (PCIe-6259) it lists the digital IO

Maximum Output Range

0 V , 5 V

Current Drive Single	24 mA
Current Drive All	400 mA

So I am not really sure where the 3.3v and 4mA is from.

I did find this:

Almost all of National Instruments digital DAQ products support only 5 V TTL logic, as this is the most prevalent. If you need to read 3.3 V signals as an input, you may generally directly connect the signals to the DAQ board because 5 V TTL considers anything above 2.2 V to be a logical high. However, if you are outputting signals to a device that requires 3.3 V signals, you will need to reduce the voltage.

http://digital.ni.com/public.nsf/allkb/2708A51F9C861E6C8625681000648C4A