Email: Password: Remember Me | Create Account (Free)

Back to Subject List

Old thread has been locked -- no new posts accepted in this thread
???
06/23/09 08:30
Read: times


 		 #166377 - The ULN works perfectly well - as far as it goes.
Responding to: ???'s previous message
Chico Magalhães said:
The ULN is a nice option, but it didnt worked well as I said.

The ULN works perfectly well according to its specifications.

The problem is that you didn't account for the voltage drop across the device - called "Vce(sat)" - in your calculation of the series resistor value.

Vce(sat) is the voltage across the collector & emitter terminals when the device is switched fully-on; ie, "saturated".
Look at the datasheet - you will see that it can be quite significant...

You obviously need to account for this voltage in your calculation of the series resistor value - as it means that there is less voltage to be "dropped" across the resistor.

And this brings us to the suggestions of Erik & Richard: you can avoid this dependence on voltages by using a Constant-Current Driver for the LEDs.

List of 30 messages in thread
TopicAuthorDate
High power led driver            01/01/70 00:00      
   Have you accounted for Vce(sat) ?            01/01/70 00:00      
   resistors are not a good solution            01/01/70 00:00      
      Have you considered a current mirror?            01/01/70 00:00      
         mirror, mirror on the wall            01/01/70 00:00      
            No question about it ...            01/01/70 00:00      
               But was that the question?            01/01/70 00:00      
                  I focused on the brightness match ...            01/01/70 00:00      
                     Doesn't it show that he's missed an important point?            01/01/70 00:00      
                        He doesn't really have to consider voltage            01/01/70 00:00      
                           for a (short) while            01/01/70 00:00      
   Sorry for the delay            01/01/70 00:00      
      The ULN works perfectly well - as far as it goes.            01/01/70 00:00      
      MOSFET Technical Info            01/01/70 00:00      
      Key term is 'logic level'            01/01/70 00:00      
         commom part            01/01/70 00:00      
            Catalogue?            01/01/70 00:00      
   Mosfet            01/01/70 00:00      
      I believe you'll have a problem ...            01/01/70 00:00      
         IRF530            01/01/70 00:00      
            Dig a little deeper, Chico ...            01/01/70 00:00      
               circuit... will work if I do this way?            01/01/70 00:00      
                  No!            01/01/70 00:00      
                  Look at the specification for IRLD120            01/01/70 00:00      
                     now i get confoosed            01/01/70 00:00      
                        High power            01/01/70 00:00      
                        He has digressed            01/01/70 00:00      
                           I suspect it's because he needs a constant current switch            01/01/70 00:00      
                              And I suspect that we are wildly guessing...            01/01/70 00:00      
                                 a little more specificity, please, Chico            01/01/70 00:00      

Back to Subject List