Resistor In Supply Line

Discussion in 'General Electronics Chat' started by sjgallagher2, Feb 20, 2015.

Feb 6, 2013
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I've been analyzing a lot of circuits lately, and I've noticed that they sometimes have resistors in the supply line in between stages, like in this metal detector circuit from talkingelectronics.com:

the 330R resistor on top is what I mean. I've seen it in transistor amplifiers as well. What effect does this have on the circuit? I feel as though it would make the analysis much more complex, with voltage drops and all. Can somebody shed some light for me? It would be much appreciated. Thanks in advance

Apr 5, 2008
16,967
2,960
Hello,

The resistor works in combination with the 5.6 Volts zener on the far left of the schematic.
It makes the voltage for the oscillator more stable, so the oscillator will not drift from voltage changes on the battery (to a certain level).

Bertus

3. wayneh Expert

Sep 9, 2010
13,435
4,272
Just to add a small point, the resistor limits the current passing through the zener, to protect it. You choose the value to compromise between competing factors; you want to minimize current wasted through the zener if the circuit is not using any - meaning the zener must absorb it all, while allowing for the maximum current needed by the circuit to pass through the resistor without dropping the voltage below the zener voltage. That would cause the loss of voltage regulation.

In the example above, the maximum current would be (9V-5.6V)/330Ω=10mA. The zener will keep regulating the circuit voltage as long as the current it needs stays in the range of 0-10mA, and the zener+circuit will draw 10mA all the time.

4. MrChips Moderator

Oct 2, 2009
14,281
4,195
In the above usage, the zener is used to provide a regulated DC voltage. The resistor is to limit the current through the zener diode and protect the diode from overheating.

In other usage, the resistor R (or sometimes an inductor) is used in conjunction with a filter capacitor C to create an RC low pass filter to clean up the power supply to preamp stages of an amplifier.

5. crutschow Expert

Mar 14, 2008
16,206
4,330
From a battery life point-of-view it would be better to use a series regulator to give a stable voltage, since that would use only the current required by the oscillator stage, but a resistor and zener diode is a cheaper solution.

Feb 6, 2013
121
8
Okay I understand a little better now. But my next question is obviously why not put the resistor directly in series with the zener? I see that it is in series with the zener, but it also has an effect on all parallel loads in between the zener and the resistor. The only reason for the resistor seems to be there is to limit current in the zener, but it also has some small effects on voltage, because of where it is placed. I just need to understand why they put it where they did, as opposed to further left or right, is what I'm getting at haha. Thanks again for all the replies.

Last edited: Feb 20, 2015
7. WBahn Moderator

Mar 31, 2012
20,064
5,665
The purpose of the zener is to get a stable voltage for the oscillator. Thus the top of the zener is the power supply for the oscillator circuit. To the right of the resistor you have a 9V unregulated DC supply that will vary in voltage as the battery dies. To the left of it you have a 5.6V regulated supply that will maintain a nearly constant voltage, provided the oscillator circuit doesn't draw too much current, until the battery is well past anything resembling "dead".