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I am trying to find a suitable diode to place on the wire that connects c and e in the circuit, with the cathode on the right hand side. I need to justify why I chose the specific diode as well.

However I have no idea on what to chose in terms of forward current and voltage rating as when I start putting other diodes in the circuit later (which is the next step), the currents and voltages through that wire change.

I suppose my question is, am I suppose to find out what the highest current is in that wire out of all possible future diode configurations and then use that?

And would I be right in using a power (rectifier) diode rather than Zener etc?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Try using a simulator to see what happens. \$\endgroup\$
    – Andy aka
    Commented Jan 16, 2015 at 10:14
  • \$\begingroup\$ Have done already but not sure how to go about finding that max possible current along with wire and proving it mathematically. i.e do I create an equation for the current along that wire somehow in reference to other currents along subsequent wires, then make the derivative of that equal zero, then find its max? Or am I making this more difficult than it is? \$\endgroup\$
    – user88720
    Commented Jan 16, 2015 at 11:18
  • \$\begingroup\$ Work out max V across diodes. Work out max I in diodes. Go to eg thgis page and set parameters using shift-drag across current range of interest then click "set filters" then voltage range of interest then set filters. | Note that higher Vf (voltage drop) = higher losses. JNote pries. Make a choice..... \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 17, 2015 at 10:03
  • \$\begingroup\$ .... | Here is a subset which is NOT set exactly correct but gives some guide. digikey.com/product-search/en/discrete-semiconductor-products/… \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 17, 2015 at 10:05
  • \$\begingroup\$ Also try setting relevant min and max values using this superb selection guide: newark.com/diodes-rectifier \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 17, 2015 at 10:08

1 Answer 1

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(its like deja-vu all over again)

The reason for selecting a diode with certain parameters is as follows:

Reverse Voltage: Select to be above the highest possible reverse voltage that might be seen across the device. (When the diode is off.)

Forward Current: Select to be above the highest possible current that might be seen flowing through device. (When the diode is on.)

Forward Voltage: Select if the small voltage drop in the forward direction needs to be at or below a certain value. (When the diode is on.)

There are several other diode parameters but they are usually less important, especially in simple DC circuits.

For the circuit above:
Reverse Voltage:
Calculate the highest voltage that might be seen if the line (between c & e) had the diode installed. Calculate the voltage in the reverse direction of the diode current. (In other words put the diode in the schematic and do an analysis.) In your last inquiry the question included the possibility of the other supplies failing, so do the analysis with that in mind). The analysis will give an idea of the Reverse voltage needed. You would normally select a diode with a reverse voltage above the calculation including some margin of safety (if that is given). Diodes often come in certain reverse voltage values (Sort of like resistors that come in standard values)

Forward Current: Same as above but calculate the highest possible current that might be seen passing through the diode in the forward direction.

Forward Voltage: If the requirement is to have a certain known value, or a minimum value, for the forward voltage, then select the diode with that parameter. (For example, a silicone diode has about 0.7v forward voltage when on.) Note - this parameter can be current dependent.

A Zener diode would only be used if you wanted the diode to pass current in the reverse direction at a known voltage. These are normally used only if you want a fixed voltage at some point in the circuit. For example, these can be used to create a voltage reference.

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  • \$\begingroup\$ Thank you for that. I think I'm a little stuck on finding the highest currents and voltages though. I feel like I need to create an equation for the current going along that wire somehow, then work out what values make its rate of change zero. I can do a heap of simulations to find the maximums but I need to prove it mathematically that's all \$\endgroup\$
    – user88720
    Commented Jan 16, 2015 at 11:22
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    \$\begingroup\$ ***Update ----- You likely should do the analysis with all the possible diodes installed. In looking at the circuit (and knowing the last question), some diodes will block current from going in certain directions. If you only put in one diode at a time and do the analysis it will not give true values for the complete circuit. \$\endgroup\$
    – Nedd
    Commented Jan 16, 2015 at 11:22
  • \$\begingroup\$ Okay I'll give it a go by placing all the diodes in the circuit and work from there. That makes more sense. Cheers \$\endgroup\$
    – user88720
    Commented Jan 16, 2015 at 11:25
  • \$\begingroup\$ Nedd what type of diodes would you recommend I use for this? \$\endgroup\$
    – user88720
    Commented Jan 17, 2015 at 7:18
  • \$\begingroup\$ Would really help to mention "Operating Temperature" and Rth(j-a) \$\endgroup\$
    – johny why
    Commented Nov 27, 2020 at 23:03

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