Blocking Diode / Rectifier Diode




 75 Amp

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  Fully enclosed 15Amp Blocking Rectifier Diode Mounted in Enclosure


Education Video-  How  A Blocking Diode Works





Blocking Diode Simulation on Solar Power Panel / Pedal Power Bicycle Generator Applicationss


You will need to use a blocking diode in your alternative energy - battery charging system to keep current from flowing in reverse direction back into your power source. For example, you could hook a solar panel straight up to a lead acid battery to charge it.  But when the sun goes down, the solar panel would stop producing electricity and start behaving as if it were a regular old resistor.   This means that the battery will start to give up its stored energy as current flows from the battery back into the solar panel.  A diode solves this problem because it only allows current (Amps) to flow in one direction.  From the Anode to the Cathode. (See simulation diagram above).
Likewise, the blocking diode installed on a pedal power bicycle generator will prevent current from leaving from the battery and flowing back into the generator assembly causing the rear tire to spin even when you are not riding on it.
You may ask, "Why does a blocking diode get hot" .   or "Why does a blocking diode need a heat sink".  The answer is that for every Amp of current flowing through the diode there is about 0.7 Watts of energy being dissipated through heat.   The diode has a P/N junction that allows current to flow one way and not the other.  This  P/N junction has a typical voltage loss of about .4 to .9 volts.  So the power being dissipated is  = to Amps X .7 Volts.    
Perhaps the biggest challenge for people new to blocking diodes is what size to get, how to figure out meax operating temperature, how to pick out a proper heat sink for your diode, whether or not to electrically isolate the heat sink, and how to attach the heat sink.  
Below you can see a typical thermal response from a LabVIEW test showing diode temperature (YELLOW), top of heat sink temperature(RED), ambient (room) temperature (GREEN), and voltage drop across the diode(BLUE).  Note on the graph below that the diode ends up with a 0.85 Volt DC drop across it while 15 Amps of current is going through.  The power being dissipated from the diode is calculated as follows:
Diode Power = Voltage Drop Across Diode x Current (Amps)    0.85 Volts x 15Amps = 12.75 Watts
So from the data below you can see that just 12 Watts of power got the diode case temperature up to 60 Degrees C.  (60 Degrees is about the point where it will feel very uncomfortable touching your skin).  


      Blocking 50A diode thermal image solar wind pedal power generator


To pick out your diode & heat sink correctly, you need to first determine the max temperature you want to run your system at.  If a diode runs too hot, it can either self destruct and stop working, or it can smoke and cause stinky electronics smell, or even it could start a fire by igniting something next to it.  
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