Large Scale Central

Zener Diode

Electronics
#1

Can a Zener Diode be used to drop voltage?
For example:
I want to power a circuit board at 4 Volts with a 14.4 battery pack.
This pack also will drop voltage as it is used to power the locomotive.

#2

Dear John,

Not sure exactly what your circuit would look like, but it depends on how many volts you are dropping (14.4 - 4 = 10.4 volts in your case), and how many amps you are passing through it.

For example, if you needed to supply 1 amp through a 10 volt zener, you would be dissipating 10 watts. (1 amp X 10 volts). That’s a lot of battery power to waste, and a lot of heat to dissipate, if you can find a 10 watt zener diode.

A pulsing power supply would be much more efficient.

Hope this helps.

Joe Satnik

#3

I would suggest 2 alternatives.

  1. Tap into the pack to get the desired voltage (most efficient solution).

  2. Buy am LM317 adjustable voltage regulator then set it’s output to 4 volts. Like the zener, this will be dissipating the excess voltage as heat but will provide a constant output which the zener won’t do if it is just hooked in series with your load.

#4

A zener is about the lowest level regulator you can get. You can do it, but…

Here’s how:

+14 -------- Resistor ---------- Zener ----------- -
| |
---- Load -----

The zener will conduct enough current so the voltage across the resistor stays at 14 less the zener voltage. You have to figure the resistor carefully, knowing the current the load draws (I).

IR = IL + IZ
R = 10.4 / IR

Zeners are little things and don’t conduct much current or dissipate much power. Usually you use them with resistors of several K Ohms, and an amplifier so the r and z are only conducting a few milliamps.

Go with the LM317. There’s probably a zener inside somewhere, along with noise suppressor, temperature compensation and so on. Yes, zeners are noisy and temperature sensitive.

#5

Hmm. Can’t type a schematic.

(http://www.outsidetrains.com/mls/zener.gif)

#6

Dear All,

The Bachmann Large Scale power pack (wall-wart transformer and controller, included in their newer Big Hauler sets) combo has a power efficient pulsed voltage reducing circuit in the controller.

http://www.bachmanntrains.com/home-usa/products.php?act=viewProd&productId=1301

It’s major flaw as a variable DC power pack is that its minimum output voltage is 1.25 Volts DC @ zero throttle.

http://www.bachmanntrains.com/home-usa/board/index.php/topic,10227.0.html

(Scroll to bottom.)

You could gut the electronics out of a controller, make a few simple mods, adjust it to 4 volts output, and you’re good to go.

(1 amp or so max. current.)

Scott might have one to sell…

Hope this helps.

Sincerely,

Joe Satnik

#7

Oh, and the current the load requires better not vary by much. If, say, you remove the load, the zener will try to conduct enough current to keep the voltage constant. If you’re not very careful, smoke will result.

#8

I solved the prollem.
I determined the exact voltage with a variable transformer and a digital meter.
Tried out a 3.7 volt Nimh pack (3 AA).
Worked like a charm, however only got about an hour and half of usage. It was a little Mah pack. (I don’t know the Mah, it was totally wrapped.
I am going to try a 3.7 2200 Mah pack and see what I get.