I heard of that before, that they repack the leftover Easter bunnies as Santa clauses. Now I see it also happens with Junction FETs.

This is news to me. Great work.

This is the sort of stuff you can only pay for in Gold ! why wasn't I taught this ? lol Great chip of the day ! in case you didn't know I love chip of the day !......cheers.

And here are some notes on using a JFET for a current regulator: 1. Select a transistor with a minimum Vth for low supply voltage operation. 2. Select a transistor with a Idss slightly greater than, or equal to the desired output current. 3. The transfer characteristics of the JFET have a thermally stable point. If you establish the current at this point, it will not depend on temperature. 4. Be mindful of power dissipation and thermal effects on drain current. CRDs are very useful parts, but they are rare and cost like a wing of Jumbo Jet! For this reason they never got popular or mass produced. Also it is much cheaper to take two(or more) BJTs and make the same thing.

Cool 👍 I like the fact that a simple pot makes a variable current source. Also the fact that it is a 2 wire device which should be able to go almost anywhere in the circuit where you need a current source.

The 78xx devices can also be used as constant current sinks, or the 79xx constant current sources. Basically you put a load across the regulated voltage output, and the constant current is what flows from the "unregulated" supply through the regulator. Since the breakdown voltage can be as high as 40 V and the current can be from milliamps to an amp, they could be used in vacuum tube circuits.

Diotec in europe still makes these, such as the CL40M45. Lovely little parts, like zener diodes they exploit the reverse bias properties of diodes, but instead of a specific reverse breakdown they have a specific somewhat constant reverse leakage current. Forward characteristic is basically identical to a normal diode.

Thanks for doing this! (It was my request). As interesting as I expected. The variable version is a bonus! PS Would be interested in your take on the varactor diode too🥸

This is exactly why I keep a bunch of J310 on hand, they make nice constant current sources which you can tailor to your needs with one resistor. Specifically the J310 is good for that because it has a high Idss and that gives you a wide range of adjustment. In fact, the Idss of these parts is so high (30-60mA in the datasheet) that it confuses those cheap component testers, doesn't read it as a JFET but rather either as a resistor, or as two diodes back to back. :) Try it, it's fun.

The LM134, LM234 and LM334 are very good current sources too.

One of my favourite tricks with a popular condenser microphone circuit that uses an emitter follower to drive the output transformer was to figure out the current in the emitter resistor, then substitute a current regulator diode of appropriate value.

Using multiple JFETs in series with all the bases tied together and a source resistor can provide a much better current source. A J111 feeding a J112 feeding a J113 string gives really good results with sufficient burden voltage.

Recently, I was looking at what jfets digikey has. I was surprised to see the power ratings - 63A at 1.2KV, for example. I guess the process parameters for SiC favor making JFETs rather than bipolar or MOS.

OHHHhM curve tracer of loveliness design me a constant current circuit that operates variably to two amps ohhhhhmmmmm!

'Acrossed' you giving away your Upper Midwest upbringing ;)

Current is very variable, depending on doping, so thus the huge number of binnings as to current.

❤ Jfet s they Sound good to

Awe another idea for the old differential amplifier. Your thoughts on this?

Interesting

cute - never heard of that B4

JFET you connected is current sink , NOT current source .

These are great little devices, but are becoming kinda absolute these days unfortunately.

I was watching one of your videos and you said at one time that you worked at HP. I have acquired an HP 8601A sweep generator for a rather low cost for an unknown problem. It turns out that the -75V regulator does not work. I have found a replacement for one of the transistors but not the other one. Its HP part number is 1854-0037 in is a PNP and used a current source for the error amp. Any help would most appreciated.

Those expensive constant current diodes don't even have a good stability or accuracy. I can't imagine why anyone would use them when there are better and cheaper alternatives.

Why aren't these more common for LED drivers? Cost?

I think you either drew the potentiometer wrong in the schematic, or it's not doing what you said it's doing. The JFET is still 100% on, since the gate is still grounded. The current only changes because it's going through the resistor. I think you meant to have the pot change the current to the base to change how turned on the transistor is to vary the current.