zener diode at low current

[buenos]

could someone suggest a zener diode that works within tolerance at low currents like 0.5-3mA ?
I have tried a small 500mW fairchild one and it is specified at 20mA, measured 3.3V at 3mA and 3.6V at 6mA. I was trying to browse for this parameter at farnell, but the selector and the datasheets dont mention minimum current for regulation. they mention test current and its 20mA for all different brands.
I need a 3.9V zener.

 

[ultrabrains]

i think you should do the convertion amp to watt, it make selection a lot easier

 

[KerimF]

Perhaps you can try using 6 small diodes ( as 1N4148 ) in series in the forward direction.
3.9V/6 = 0.65 V which is likely the value of the forward drop at about 1 mA for each diode. This is not a super solution but it may help.

 

[muthukumar_ece2004]

You may use a series resistor with the zener diode.

 

[umesh49]

You can choose a a zener diode with 3.9V and around 5mA zener current.
Better to search on Mouser; Here is the link;
zener

Umesh.

 

[keith1200rs]

Depending on what you are doing with the voltage, if you want something that provides a voltage reference at low currents you may be better off looking at adjustable voltage references or shunt regulators.

Keith

 

[FvM]

You'll face the fact, that Z-diodes below 6 - 7 volts zener voltage have a rather soft or resistive characteristic. In addition, most manufacturers either don't specify the low current behaviour at all or give only typical data. In any case, you should referer to original datasheets rather than catalog distributors data compilations.

If the problem is in finding the typical characteristic at lower currents, select a suitable device family and the diode voltage based on your own measurements. For exact regulations, a shunt regulator or other higher performance device should be used.

I guess, you realized, that the other suggestions won't help much.

 

[alexan_e]

TL431/TL431A Programmable Shunt Regulator may do what you need

Minimum Cathode Current for Regulation IKA(MIN)
0.45mA Min
1.0mA max

Alex

 

[buenos]

well, thanks for the replies.
what im going to do is: set 20mA on the zener, meaning that i will have 10% efficiency in my regulator, but still at a few mW power loss.

 

[BradtheRad]

Since you need .6 V additional, you might try adding a diode inline with your zener. That will get you closer to your spec. A diode conducts under 5mA at .45 to .55 V.

muthukumar pointed out that you can add a series resistor inline with the diode. This is a way to get the exact voltage you need.

Regulation is not as tight, however.

You have a 3.3V zener. You want 3.9V. Under 3mA.
You want 600-700 ohms. Start by putting a 1K potentiometer above the zener. Then simply dial it to your spec voltage. Tap above the resistance.

 

[Orson Cart]

how about setting the current through your zener as you say - say 1mA, then buffer the output with an opamp and adjust the gain of the opamp to give you the desired voltage.
Or use a 4v7, 5v1 or other voltage zener and step down the output with a resistive divider and buffer with a transistor.
Regards, Orson Cart.

 

[buenos]

I think using it at 1mA would give a very wide voltage tolerance for series production. its not for some home made hobby circuit that is build one piece.
As i said, i will go for the 20mA and live with the 10% efficiency.

 

[FvM]

I don't understand the solution. A lot of industry standard zener diodes has a 5 mA specification, e.g. NXP BZX84.

 

[dick_freebird]

Zeners can vary a lot in their low current behavior, sharpness of knee and
so on. If low current reverse voltage is a concern, then buy one that has
a spec for it.