Diode drops

[mhuss]

All my life a standard silicon forward-biased junction has dropped .6 to .7 volts when conducting. The Vishay 1N400x data sheet (https://www.vishay.com/docs/88503/1n4001.pdf) says Vf varies between ~.6v (at low current) and 1.1v at one amp.

We have a batch of 1N4007s that are forward-dropping 0.45v at meter test levels (very low current), and a similar amount when in circuit conducting ~16mA.

Has anyone else seen this and can anyone explain it?

This came up because we have an application that uses three series Si diodes in parallel with an LED to protect the LED from over-current, but this recently stopped working (3 diodes dropping a combined 1.4v end up stealing current from the LED!)

[martin manning]

Miss-marked Schottky's?

[wpaulvogel]

All my 1N4007 and UF4007 diodes test at about 0.400- 0.450 with my meter when I’m checking before install. I’ve never had a situation where I could or would actually check them once in circuit to see if they really take 0.7 to forward bias. My conditions are usually rectifiers and the readings get confusing between the A/C and D/C components of measurement. In what you’re doing I’d say you can measure the drop. I usually use a resistor for current limit with LED’s but I’m just using them as an indicator.

[pompeiisneaks]

All my 1N4007 and UF4007 diodes test at about 0.400- 0.450 with my meter when I’m checking before install. I’ve never had a situation where I could or would actually check them once in circuit to see if they really take 0.7 to forward bias. My conditions are usually rectifiers and the readings get confusing between the A/C and D/C components of measurement. In what you’re doing I’d say you can measure the drop. I usually use a resistor for current limit with LED’s but I’m just using them as an indicator.

That's weird, almost every 1N4007 I've ever used was about .6 to .7 as well... did they recently change something? Most of mine were bought in bulk when Radio Shack was going out of business they had a ton of diodes for dirt cheap.

~Phil

[R.G.]

Bottom line is - diodes vary. A silicon diode drop is not actually a single number. It's an almost-constant.
In reality, a diode's forward voltage is an exponential function of the voltage across it. At low voltages, the current through the diode is hard-to-measure small. But it increases and the current is an exponential function, so at some point it rises to a tickle of current, then a really small current, then a noticeable current, then a hoo-boy-at's-a-lotta-current. The exponential nature means that the rise up from noticeable zero to the hoo-boy level happens over a small amount of forward voltage.

The profs in the EE circuits class had a hard time with some of us getting across the idea of an approximately-constant constant, like a diode drop. You actually have to pick what the current threshold you want "off" and "on" to be. If milliamperes are significant to you, 0.5V to 0.7V will do nicely. If microamps matter, you're in for some hunting around 0.4V to 0.5V.

Each diode is different in detail, not only from type number to type number, but from wafer to wafer in the same part number.

It's kinda constant, fuzzily. Not really constant.

[wpaulvogel]

All my 1N4007 and UF4007 diodes test at about 0.400- 0.450 with my meter when I’m checking before install. I’ve never had a situation where I could or would actually check them once in circuit to see if they really take 0.7 to forward bias. My conditions are usually rectifiers and the readings get confusing between the A/C and D/C components of measurement. In what you’re doing I’d say you can measure the drop. I usually use a resistor for current limit with LED’s but I’m just using them as an indicator.

That's weird, almost every 1N4007 I've ever used was about .6 to .7 as well... did they recently change something? Most of mine were bought in bulk when Radio Shack was going out of business they had a ton of diodes for dirt cheap.

~Phil

This is what I find with my meter set for diode check. Might just be my meter.

[mhuss]

It's kinda constant, fuzzily. Not really constant.

An inconstant.

I went back and measured a bunch with my meter diode check function (probably very low current) and the Vf readings were all over the place. The UF400Xs were really low, ~0.41 Vf.

So much for leftover schooldays assumptions...