High Side Switch - Resistor Values

Discussion in 'General Electronics Chat' started by Sloucher, Feb 8, 2014.

  1. Sloucher

    Thread Starter New Member

    Feb 7, 2012
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    0
    Hi Guys

    Can anyone help me with my calculations for the resistor values associated with the high side switching circuit in the attached diagram? The values on the diagram are pretty arbitrary at the moment, but I'm happy to use them if I can be convinced that they'd work ok. The switching circuit is just a simple pulsed ON-OFF cycle controlled from an Arduino.

    I understand the theory of what each resistor is for, but get confused when trying to combine two sets of transistor data. Are the transistors I've selected to use here overkill? Can anyone suggest a suitable pair in SOT-23 form that would be better?

    I've used this configuration because I believe the voltage at the PNP base would be more negative than Vcc, which although is the same supply as the Arduino, there would be sufficient voltage drop that the PNP wouldn't switch off - does that sound sensible?

    Thanks in advance
    Bernie
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    Look at this image, if any queries please ask.
    E
     
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  3. Sloucher

    Thread Starter New Member

    Feb 7, 2012
    23
    0
    Eric you're a star, many thanks!

    Can you tell me the process at which you arrived at the values?

    Bernie
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    The BC807 current gain is specified in the datasheet at approx 50.
    So this means that the Base current must be at least 125mA [your load current] /50
    But most d/s specs are typical so I assume a gain of 20.

    So the requires Base current is 125/20 = ~ 6mA

    When the BC817 is hard ON, the 680R is connected to 0V via the BC817.
    Assuming a Vsat of ~ 0.3V and Vbe of 0.7V for the BC807 means that the 680R has ~4V across it. So the 680R Base current into the BC807 is ~ 6mA

    The 1K base resistor from the +5V MCU pin will give a Base current into the BC817 of 4.3/1000 = 4.3mA.. so the BC817 will switch hard ON and saturate.

    Do you follow OK.?
    E
    BTW: What is the Load type.? eg: motor etc.???
     
    Last edited: Feb 8, 2014
  5. Sloucher

    Thread Starter New Member

    Feb 7, 2012
    23
    0
    Followed that perfectly thanks Eric - thanks again.

    The load is simply a parallel array of leds. They have to be high side switched because of a combination of circumstances that dictate that only one ground return can be routed from the led pcb. Ie the led pcb is a pre-existing design.

    Bernie
     
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