Very weird, no DMM able to measure zener current? Why? - Page 1
Forum section: electronics / beginners

author: hurricanehenry
author_rank: Regular Contributor
timestamp: January 11, 2023, 07:29:52 pm
content: Hi - I have a very simple circuit.Voltage of 9 V feeds into a resistor of 56 Ohms.From the resistor, it goes out into parallel paths.One goes to a 6V lamp, and the other path goes to a zener diode 1N4735 (reversed) which is a nominal 6V zener.I am able to measure all the currents and voltages in the circuit EXCEPT for current from the zener.Every DMM I use and every lead wire I use is the same. So it's not the DMMs or the leads.All the DMMs including cheap DMMs and expensive Fluke show zero current. But that can't be because the circuit works. The lamp lights up, etc.I am only able to deduce the zener current from subtracting the lamp current from the circuit main current (9V divide by 56 ohms = 160 mA approx).What is going on???(The current range is properly set on all the DMMs)(This circuit is actually in the book calledComplete Electronics Self-Teaching Guide with Projects 4th Edition, figure 2.33 page 83)

author: bdunham7
author_rank: Super Contributor
timestamp: January 11, 2023, 07:43:47 pm
content: Can you draw (or photograph) exactly how everything is set up? Can you measure the voltage across the zener? One possibility is that your lamp is drawing enough current to drop the voltage across the zener (and lamp) below the zener voltage, thus no current. And you can't calculate your overall circuit current the way you did--it's a lot lower because you can only drop ~3V across the resistor if you want to have 6V for the lamp.

author: wraper
author_rank: Unknown
timestamp: January 11, 2023, 07:47:27 pm
content: DMM show correct current. Voltage on the lamp is lower than zener clamping voltage. Measure the voltage across lamp/zener.QuoteWrong, zener diode does nothing and is not needed for circuit to work.QuoteWrong. It would be 160mA only if resistor was connected directly across the 9V voltage source with nothing in series.

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2023, 07:48:01 pm
content: Quote from: hurricanehenry on January 11, 2023, 07:29:52 pmI think this means you measure a current of zero?QuoteWhy can't the current be zero? If you measure zero, it probably is zero. Especially if you know the meters are working for other other current measurements.

author: Bud
author_rank: Super Contributor
timestamp: January 11, 2023, 08:40:09 pm
content: Lets suppose that everything is fine and there is 6V across the zenner and lamp, and the lamp lights up. In this case, given the power supply of 9V and series resistor 56 Ohm, the voltage drop across the resistor is 9v-6v=3v. Then current through the resistor is 3v/56=53 mA. This current is a sum of currents through the zenner and through the lamp. Zenners require minimum current to operate, lets assume 10 mA flows through the zenner and it provides 6V regulation. Then current through the lamp is 53-10=43 mA. The wattage of the lamp then is 43*6v=0.26 W. So if you use a 0.25W 6V lamp in this circuit you will have approximately 10mA through the zenner. If your lamp is more than 0.3W it will suck all the resistor current leaving nothing for the zenner, the zenner will not perform its voltage regulation and you will measure less than 6v across it.

author: hurricanehenry
author_rank: Regular Contributor
timestamp: January 12, 2023, 09:58:44 pm
content: OK many thanks for everyone's help. You guys are correct. The lamp is drawing way more current than even available and from what I understand, it is the lower resistance path and so no current is even left available for the zener.I had so many DMMs measuring everything that I wonder if it has injected measurement issues into the system.Thanks for helping me understand this perplexing situation!!