# JFET as a constant current source

#### Drmario5237

Joined Oct 14, 2018
65
Hello. I was wondering if a JFET can be used as a constant current source and if so how do you use it as one.

#### dl324

Joined Mar 30, 2015
11,250
A JFET is on with Vgs=0V. This is the data for 2N3818 N channel JFET:

With Vgs=0V, drain current will be 2-20mA. Vgs=8V will turn the device off.

EDIT: That datasheet seems to have a polarity problem. Should be Vgs=-8V for cutoff.

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#### crutschow

Joined Mar 14, 2008
25,256
Hello. I was wondering if a JFET can be used as a constant current source and if so how do you use it as one.
Depends upon the current required and the precision/accuracy needed for the current.
More accurate constant current circuits can be made using a couple transistors, or a transistor and an op amp.
What's the purpose of the constant current?

#### Drmario5237

Joined Oct 14, 2018
65
Depends upon the current required and the precision/accuracy needed for the current.
More accurate constant current circuits can be made using a couple transistors, or a transistor and an op amp.
What's the purpose of the constant current?
You take a antenna, lead it to a tank circuit, lead that to a transformer, the diodes to rectify it to DC then to capsitors and a resistor to build up charge then to a non transistor to turn off once the voltage from the capsitor drops to low due to discharging. Then to zenner diodes that are reverse bias to have constant voltage then to something the makes a constant current then to a load

#### crutschow

Joined Mar 14, 2008
25,256
You take a antenna, lead it to a tank circuit, lead that to a transformer, the diodes to rectify it to DC then to capsitors and a resistor to build up charge then to a non transistor to turn off once the voltage from the capsitor drops to low due to discharging. Then to zenner diodes that are reverse bias to have constant voltage then to something the makes a constant current then to a load
All that to capture a few microwatts of power?

#### DickCappels

Joined Aug 21, 2008
6,533
Maybe if you provide a clear and simple idea of what you want to do somebody can suggest a circuit for doing that.

Let me take a guess: Do you want to flash an LED with energy picked up by an antenna and tuned circuit?

#### Drmario5237

Joined Oct 14, 2018
65
Maybe if you provide a clear and simple idea of what you want to do somebody can suggest a circuit for doing that.

Let me take a guess: Do you want to flash an LED with energy picked up by an antenna and tuned circuit?
The idea is to capture a radio wave at a certain frequency using the antenna and tank circuit then use a transformer to turn the millivolts from the radio wave into volts, and then rectify the current and voltage to DC using diodes then feed that current into supercapsitors in series, so you can handle higher voltage than just one supercapsitor and have a high enough farad, so when the capsitors discharge though a resistor in series with the capsitors, you can still get a higher voltage with at least a amp or two current then from there use a reverse bias zenner diode to keep the voltage the same, because when the capsitor discharges, it decreases little by little in voltage same as it does with current, so after the zenner diode have something like a FET or a l200c regulator keep the current constant before it goes to the load.

I might put two sets of capsitors in series with transistors as switches, so when one drains past a certain point it stops discharging and charges while the other set starts to discharge.

If I can get it to work right I might be able to get 5 volts at 2amps output enough to charge a cell phone. Once I figure out what to use as as a constant current source and how to hook it up I'll come back here and post my schematic

Joined Mar 10, 2018
4,057
Your primary problem is how much charge you can capture over time.

Basic computations with losses accounted for should lead you to some conclusions as to weather
the goal can be reached sensibly.

2 Amps is a lot of current....Question is how long do you have to harvest to get enough charge to establish 2A for any length of time. How much do super caps leak.....

Regards, Dana.

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#### Sensacell

Joined Jun 19, 2012
2,587
I would ask the simple question:

If this could be done, why aren't others doing it?
The answer is because it's not workable.

The amount of energy you can harvest is astonishingly small, many orders of magnitude too small to charge a phone. None of the techno-complications you mention increase the actual energy collected.

Unless you live right next to a 100 KW radio station, then it might be feasible.