# When trying to design a prototype what value of the diode threshold votlage should you use?

#### xso111

Joined Oct 12, 2017
24
im trying to build an AC to DC powersupply using 1n4002 rectifier diode
the datasheet says
forward voltage 1V
but the book says that Si has 0.7V threshold
but when measured by the DMM(using the diode symbol in DMM) its 0.58V

which one should i use in my calculation.?

also what voltage does the DMM measure? Average? RMS? or Peak?
i measured my filter capacitor in my DC supply to be around 15V using my DMM in "auto DC". is that the peak? because my calculated average filtered voltage is 9.98V.

#### AlbertHall

Joined Jun 4, 2014
12,346
Diode voltage drop varies with current. Your DMM will pass a low DC current through the diode so you will read a low DC voltage.
For the 1N4002 the datasheet says:

Notice the conditions at which the forward voltage is specified - If = 1.0A and 25°C. If you are using this diode at less than 1A then the voltage drop will be lower than specified. Also note that it will depend on the diode temperature.

When you measure a DC voltage with the DMM it will show the average value of the DC.
If your measured value differs from the calculated value then the calculation is in error (assuming the meter is accurate).

#### dl324

Joined Mar 30, 2015
16,918
Post a schematic.

Don't use the forward voltage given by your DMM. That's using a low current and isn't useful for more than a DORA (Dead OR Alive) check.

Unless you're doing an aggressive design, it doesn't matter much whether you use 0.7V or 1V. What is more significant is the type of rectifier circuit you're using (one or two diode drops). Personally, I use 0.7V.

FYI, we refer to the voltage drop of a diode as it's forward voltage. We use threshold for devices such as MOSFETs.