Faking a lambda diode.

[digi2t]

Just going over the Guyatone PS-021, and I came across a potion of the circuit that serves as a bypass and voltage indicator using a lambda diode. My question is....

If one wished to fake it with a couple of jfets, what characteristics would be key to getting a valley voltage (Vv) close to the MA522-R in the Guyatone? The datasheet lists the Vv for this diode as 3.5v to 4.3v. So, if I'm looking to fake this with a P and N jfet, is there a particular parameter(s) that I should be looking for in the datasheets?

Here's the Guyatone schematic for reference. The indicator LED portion is the lower right section. I've marked in the transition voltages for the LED.

[PRR]

https://industrial.panasonic.com/content/data/SC/ds/ds4/MA522_E_discon.pdf

[digi2t]

Yeah, got that, and read through it, along with some other documents. My question is if there are some basic jfet characteristics that one could look for that would give you a ball park idea of how two jfets would interact to give you particular lambda diode characteristics. You know, a sort of "Complimentary jfets as a lambda diode for dummies".

[PRR]

I never even heard of the darn thing. Apparently it was used in a few products for a year or so, because they are sought-after repair parts.

Almost looks like a 2-BJT SCR except no base current(?).

What does it do for you that you can't do with an opamp or a cleverer battery monitor? (I still have not wrapped my head around what it does.)

[Rob Strand]

I ain't heard of no Faking Lamba Diodes either.

It kind of tunes the middle point like a divider and finds the balance of gate source voltages for equal drain current.

For equal JFETs
Vpk  = VP / 1.5
Ipk = 0.3 * IDSS

If the VP's are different (but IDSS same),
  Vpk = (VP1+VP2) / 3

Once you exceed VP1 + VP2 it has to turn off.
So Vv  ~ VP1 + VP2

Here's an interesting way to replace one JFET (the Pchannel) with a PNP transistor.
http://www.zen22142.zen.co.uk/Theory/neg_resistance/negres.htm

Not 100% the same as it has about a 0.6V dead-spot on the left side of the hill.
The sloped part to the right is caused by the resistor divider.  You can flatten it by increasing
the resistors but if you go too high there won't be enough base current.

The other option - get rid of it!

[antonis]

I second Rob's other option.. 

[rankot]

There are some circuits regarding lambda diode faking here: http://www.zen22142.zen.co.uk/Theory/neg_resistance/negres.htm

[R.G.]

I'm with the "don't do that" crowd. I suspect someone got fancy and decided to think about how to use this fancy new kind of part that was really whizzy, and designed it in for fun. It would make more sense from today to design it back out and do the same LED switching stuff with more prosaic components.