Join Barron Stone for an in-depth discussion in this video Diode characteristics, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] There are three primary characteristics that I consider when choosing which diode to use in my circuit. Forward voltage, which is the voltage at which the diode turns on and allows forward current to flow through it, the breakdown voltage, which is the negative voltage at which the diode fails to stop current from flowing backwards through it, and the maximum forward current, which is the amount of forward current the diode can handle before it will overheat and burn up. To show you how to buy in and interpret these diode characteristics, I'll look at the data sheet for a common 1N4148 small signal diode.
The 1N4148 diode is designed for general-purpose usage. It has a moderate forward voltage drop, but it's not designed to handle much forward current. To find the data sheet for that diode, I'll go to my search engine and type in the part number 1N4148 and the word datasheet. I see that there are several datasheets available for the 1N4148 diode from different manufacturers. For this video, I'll look at the datasheet from Vishay.
The top part of the first page lists some general information about this diode, what it's typically used for and the types of packages it comes in. Below that, is a table labeled absolute maximum ratings. Like all other electronic components, diodes have physical limitations. And these are the values you need to pay attention to to avoid destroying your components. The most important of these to worry about is the maximum forward continuous current. Abbreviated as IF, which describes the average amount of current the diode can safely conduct in the forward biased mode.
Ideally, a diode will be able to conduct an infinite amount of current when it's forward biased, but in reality as the amount of forward current through this diode increases, the diode produces more heat, which it will need to dissipate. So this maximum forward continuous current rating is based on the thermal limitation of how much heat the diode can handle. If I try to maintain a forward current of more than 300 milliamps through this diode for too long, that will cause it to overheat and it'll burn up.
However, this diode can handle more than 300 milliamps if it's only for a short period of time. The peak forward surge current is the maximum amount of current the diode can handle during a surge. I like to relate the maximum continuous current and surge current ratings to how long I can hold my hand over a hot stove. If I pass my hand over the stove quickly, that represents a surge of current. Even if the stove is really hot, it's not going to burn me because of the short exposure time.
However, if I hold my hand over the stove, representing a continuous current, even if the stove is only moderately hot, it will eventually burn my hand due to the long exposure. According to the datasheet, this diode can survive up to two amps of current for a surge lasting up to one microsecond. If I ever exceed that two-amp peak forward surge current even momentarily, then there's a pretty good chance my diode will overheat. Moving on down the page to the electrical characteristics section, there are two main parameters to pay attention to, the forward voltage and the breakdown voltage.
The forward voltage is the amount of positive voltage between the anode and cathode that will cause the diode to turn on, allowing forward current to easily flow through it. For silicon diodes the forward voltage is typically somewhere between 0.6 to one volt. The breakdown voltage is the amount of reverse voltage the diode can withstand before it fails to stop reverse current from flowing backwards. And for silicon diodes, the breakdown voltage is typically anywhere from 50 to a hundred volts or even more.
For this particular 1N4148 diode from Vishay, the forward voltage is listed as having a maximum value of one volt, meaning that one of these diodes should never require more than one volt to become forward biased. A little bit lower in the table, the breakdown voltage for this diode is listed as being a minimum of 100 volts. So, I can expect that one of these diodes will withstand at least 100 volts in the reverse biased direction before it allows reverse current to flow freely.
These minimum and maximum breakdown in forward voltage values in the datasheet are extremes that you can expect from this part. But, like all electronic components, that will vary from part to part due to variations in the specific design and manufacturing process. When I'm building a circuit with one of these diodes, sometimes I'd like to know the actual forward voltage that I can expect the diode to turn on at, rather than just the worst case maximum that's listed in the datasheet. And I can find that out using my digital multimeter.
To do that, I'll turn the knob on my DMM to the diode test mode, which is indicated by the diode symbol. This is the same DMM setting that you can use to check for electrical continuity to find short circuits. So, if I touch the probe tips together, the DMM will beep at me to let me know that there's a short circuit between them. To measure the diodes forward voltage, I'll touch the black probe to the cathode terminal, which is the side of the diode with the little black stripe, and I'll connect the red probe to the anode terminal.
I can see that the forward voltage of this diode is actually closer to 0.6 volts than the one-volt maximum that was listed in the datasheet. It's a good thing because I generally want my diode to switch into the forward biased on mode at the lowest positive voltage it can, so that the diode acts as close to a real short circuit as possible to forward current.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals