Author Topic: Current value in Forward Voltage for Diodes  (Read 486 times)

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Offline A_Wild_Noodle

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Current value in Forward Voltage for Diodes
« on: December 06, 2021, 06:40:45 pm »
I did a couple searches on the forum so if I missed a post that answers this I apologize. I was curious about the value of current which is present in the Forward Voltage parameter of a diode. So my question is, Do you need to meet or exceed the current in the forward voltage for a diode to conduct in forward bias or is it a maximum rating to not exceed. obviously you need to meet forward voltage to conduct but must you also consume the amount of current in that parameter as well? For instance in 1N4148 the forward voltage given is 1V @ 10mA. Does this mean downstream of the diode I must have something to consume at least 10mA at 1V with proper paired resistor to limit it? This all came up because I would like to drive an ATTINY85 and an LED matrix with the same 5V source when 'operating' and to program the ATTINY85 I have included an ISP 2x3 header which has its own pin for VCC. So instead of just making the source and ISP power on the same net label id like to include a diode to block current from the side which the LED's are on (with main 5v wall adapter disconnected) so that the LED's aren't consuming current unnecessarily while programming the ATTINY85. Not sure if its even necessary or even best practice because I don't know if VCC from an Arduino programmer would even be able to supply enough current to drive the LED's but I'm not read up on how much current the Arduino programmer would even be able to supply. I'm probably overcomplicating this but I'm new to this so please help! Thanks!
 

Offline magic

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Re: Current value in Forward Voltage for Diodes
« Reply #1 on: December 06, 2021, 06:52:43 pm »
Absolute Maximum Ratings is the limits that are not to be exceeded, such as 300mA continuous current.
Electrical Characteristics describe the behavior of the device. The first line means that voltage drop across the diode will not exceed 1V when 10mA flows through it.
Realistic typical forward voltage vs forward current is plotted below and shows ~0.8V at 10mA, in the datasheet that I have.
 
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Online T3sl4co1l

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Re: Current value in Forward Voltage for Diodes
« Reply #2 on: December 06, 2021, 07:11:56 pm »
Ratings and characteristics are two different things.  I think you've mixed up which section is which!

Characteristics are what you can expect under ordinary conditions.  You might be using a 1N4148 at logic levels with pull-up resistors and such, and so the characteristic at a couple mA might be relevant, and so they give a figure around there.  Notice they might not give full min/typ/max, only the best or worst case, because that's all they'll really guarantee.  These diodes do not make great voltage references, for example -- the Vf might differ by 100mV at a given current, between individual diodes.

You can very reasonably assume diode characteristics follow the nonideal model.  That is, the Shockley equation for the most part (current is exponential with voltage), but with ESR added in.  There's also reverse breakdown (avalanche or failure), and excess leakage (particularly for glass-body types which are slightly photosensitive).  That describes the DC behavior.  At AC, there is also capacitance and reverse recovery.  That's more than a few things in total, but computationally speaking it's not a problem, and this set of models describes everything a full-on SPICE simulator uses -- and does a pretty good job at, all in all.  (The main thing missing is forward recovery, which is only applicable to some types and circumstances, and is harder to simulate anyway so it's fair that they left it out.)

So, that's a lot of flavor, but the takeaway is the DC property, current exponential with voltage.  This is obeyed down to very low currents indeed, say +/-1nA range for voltages in the +/-100mV (or even less, not sure) range.

Likewise, for currents around nominal (say 1-100mA), you can very reasonably assume a fixed voltage, or a voltage plus resistance.  This is where the oft-quoted "0.6V" or "0.7V" comes from.  It's actually a segment of an exponential curve, but because current is exponential with voltage, the inverse of that is voltage logarithmic with current, and so the voltage only varies noticeably over huge ratios of current (like ~60mV/10x), and for ordinary currents in that range, well yeah, the voltage is fairly stable.

So, driving your LED matrix, expect to lose about a volt, maybe slightly more if you're driving it >10mA, maybe a bit less if just a few mA.  It seems unlikely you'd drive an LED matrix much lower than this, so a noticeably smaller Vf probably won't be encountered.

As for the intended application -- I would instead recommend powering the LEDs and MCU from the adaptor, using a schottky diode each, such as 1N5819 or B140.  Don't put diodes around the ISP: the programmer should get exactly the MCU voltage.  Schottky have lower voltage drop so you lose less of the adaptor voltage in normal operation.  I'd also recommend programming with main power applied anyway, so this is all a non-issue.

This [avoiding ISP diodes] is probably most relevant with official programmer adapters, which have voltage interface circuitry for protection and flexibility, and so need to read the target voltage to set their output voltage level, and input voltage range.  They do not power the target (at least, the ones I've used don't; not sure about all of them).  Whereas your Arduino I'm guessing, is just powered straight in parallel with the MCU?

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Offline David Hess

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Re: Current value in Forward Voltage for Diodes
« Reply #3 on: December 06, 2021, 08:42:24 pm »
Do you need to meet or exceed the current in the forward voltage for a diode to conduct in forward bias or is it a maximum rating to not exceed.

The forward voltage drop specification applies when a specified test current is drawn through the diode, and is usually specified as a maximum.  A more typical value at the test current will be 0.6 to 0.8 volts.  The useful forward current through a diode can be very small, on the order of nanoamps or picoamps in some circuits.

Some circuits do impose a voltage across a diode to force a current, like current mirrors and anti-log circuits, but they do so under controlled conditions.
 
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Offline A_Wild_Noodle

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Re: Current value in Forward Voltage for Diodes
« Reply #4 on: December 06, 2021, 08:48:07 pm »
Attached I have included a screen shot of what I have and what I was thinking I should do pre forum post. this may make it a bit easier. Like I said I'm brand new to schematic design so there may be obvious mistakes that I have not seen yet! The way it is currently without diode is probably the best way and I should just keep it the way it is seems to be the best path forward if I am understanding correctly.
 

Offline TimFox

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Re: Current value in Forward Voltage for Diodes
« Reply #5 on: December 06, 2021, 09:18:58 pm »
Very brief reply:
Zener diodes have an abrupt increase in current when the voltage exceeds a reasonably well-defined threshold and very low current at voltage near zero (in reverse bias).
Regular diodes have a smooth variation in current with forward bias voltage (roughly exponential in voltage) that goes through zero current at zero voltage.
 


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