zener diodes regulating to lower voltage in joule thief

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
I have been working on a joule thief to act as a small 5 volt power supply and I have been using zener diodes clamp the voltage. My 5.1 volt zeners are regulating from 4.4 to 4.8 volts instead of 5.1. I tried other voltage zeners (9.1 volts, 6.8 volts, 4.2 volts) and they worked just fine. I have tried 3 different zeners from different sources (1 store bought and 2 salvaged from boards) and they all regulated to voltages lower than they should. Here is a schematic of the circuit.

20170930_161957_HDR_uraniumhexoflorite_800x363.jpg
 

crutschow

Joined Mar 14, 2008
25,247
For better regulation you could use a TL431 2.5v reference/programmable zener as shown below:
upload_2017-9-30_14-57-31.png
A better solution is to modify the joule thief so that it regulates the voltage as shown below, which would improve efficiency, as opposed to just shunting away the excess voltage with a zener.
 

DickCappels

Joined Aug 21, 2008
6,531
It may be that you are not supplying emough current to that zener for it
to get up to its rated voltage.

Higher voltage zeners and lower power zeners require lower bias current.

You can increase the current out of your oscillator or find a 5V zener with a lower Izt specification.
 

DickCappels

Joined Aug 21, 2008
6,531
Izt is the test current specified on the datasheet, for example 5.1V ±0.25V @ 20 ma.

If you are using a 500 mW zener you can look for a lower power version,not doubt the Izt will be lower.

upload_2017-10-1_23-7-20.png

The objective is to get the current to be sufficient to reach the knee (left side of curve) where the voltage is nearly flat as current changes.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
For better regulation you could use a TL431 2.5v reference/programmable zener as shown below:
View attachment 136204
A better solution is to modify the joule thief so that it regulates the voltage as shown below, which would improve efficiency, as opposed to just shunting away the excess voltage with a zener.
I replicated this circuit (except I used a wire instead of a 10 ohm resistor because all of my 10 ohm resistors are hiding in the bottom of my resistor box) and it put out 4 to 4.1 volts with no load.
 

crutschow

Joined Mar 14, 2008
25,247
I replicated this circuit (except I used a wire instead of a 10 ohm resistor because all of my 10 ohm resistors are hiding in the bottom of my resistor box) and it put out 4 to 4.1 volts with no load.
Then there is something else wrong with your circuit.
What are you using for a transformer?
just realized that the 2N3904, which is what I used for the transistors, has a hfe of 150. Is this why the voltage is lower?
No.
That hfe is a typical value for a small BJT and should be fine.
 

GuzelSans4

Joined Sep 27, 2017
2
A zener current requires minimum knee current for proper operation. Th current value should be more than knee current and less than max operating current. As this post says, you can use the formula IZmax = PDmax/VZ to calculate the knee current from voltages. Check if the current is enough to drive the circuit.
 

ian field

Joined Oct 27, 2012
6,539
I have been working on a joule thief to act as a small 5 volt power supply and I have been using zener diodes clamp the voltage. My 5.1 volt zeners are regulating from 4.4 to 4.8 volts instead of 5.1. I tried other voltage zeners (9.1 volts, 6.8 volts, 4.2 volts) and they worked just fine. I have tried 3 different zeners from different sources (1 store bought and 2 salvaged from boards) and they all regulated to voltages lower than they should. Here is a schematic of the circuit.

View attachment 136206
The text book way is to put a shunt transistor across B/E of the BO transistor and feed its base via a Zener from the rectifier-reservoir on the output. You have to allow for Vbe, as the output rises to the combined Vz + Vbe, it switches the shunt transistor on and stops the BO transistor.

You need at least 10R in series with the Zener so ripple can't damage B/E.
 
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