Getting negative current on Zener Voltage Reg Circuit

[George317]

i've built a bridge rectifier with a Zener Voltage regulator.
12V peak secondary
3300uF Capacitor
1k ohm load
1k ohm series resistor for the zener
and a 1n4728A Zener diode.

i'm getting a 2.8V steady voltage, but when i measured the current going through the load with my DMM it's measuring around -0.06mA, and the value doesn't change despite switching the polarity of the probe(still negative current) is my DMM broken? or i just made a mistake in the circuit.

also i find it weird that the current on my load is so small wasn't the current supposed to be 2.8V/1k ohm which is 2.8mA because the load and zener is in paralell

 

[tech99]

i've built a bridge rectifier with a Zener Voltage regulator.
12V peak secondary
3300uF Capacitor
1k ohm load
1k ohm series resistor for the zener
and a 1n4728A Zener diode.

i'm getting a 2.8V steady voltage, but when i measured the current going through the load with my DMM it's measuring around -0.06mA, and the value doesn't change despite switching the polarity of the probe(still negative current) is my DMM broken? or i just made a mistake in the circuit.

also i find it weird that the current on my load is so small wasn't the current supposed to be 2.8V/1k ohm which is 2.8mA because the load and zener is in paralell

Sounds like a multimeter fault.

 

[Borek]

Schematic of the circuit with (and without) the DMM probably wouldn't hurt (yes, it is rather obvious, but we have seen tons of surprisingly non-obvious mistakes in obvious questions ).

 

[davenn]

Sounds like a multimeter fault.

or user error maybe

i've built a bridge rectifier with a Zener Voltage regulator.
12V peak secondary
3300uF Capacitor
1k ohm load
1k ohm series resistor for the zener
and a 1n4728A Zener diode.

draw out your circuit and post it here in the thread
show us how and where you were measuring the current

 

[George317]

Schematic of the circuit with (and without) the DMM probably wouldn't hurt (yes, it is rather obvious, but we have seen tons of surprisingly non-obvious mistakes in obvious questions ).

or user error maybe

draw out your circuit and post it here in the thread
show us how and where you were measuring the current

thanks very much for the help
https://imgur.com/Jes5gg2

 

[davenn]

thanks very much for the help

you didn't answer the second part of my question

show us how and where you were measuring the current

 

[George317]

you didn't answer the second part of my question

oh, sorry bout that.
i'm measuring by placing the probes of the multimeter in series wiith the load resistor. i remove the jumper wire that was connecting the load to the ground then connect one probe to the jumper wired that's on the ground while the other probe is connected to the load resistor.

something like this

 

[jim hardy]

i'm getting a 2.8V steady voltage, but when i measured the current going through the load with my DMM it's measuring around -0.06mA, and the value doesn't change despite switching the polarity of the probe(still negative current) is my DMM broken? or i just made a mistake in the circuit.

My guess would be the fuse in your DMM's current circuit is blown. Happens a lot to beginners.
Do you have another meter of any sort ? Set your DMM to a medium current scale, measure its resistance with the second meter. Should be quite low, less than an ohm on most scales..

 

[Baluncore]

Either your multimeter is broken, protection fuse blown, or you are using an AC current range.
The constant negative reading suggests a hall effect meter or a bad DC offset adjustment.
Does it also read –0.06 mA when disconnected or with terminals shorted?

The 1N4728A zener is a 3.3 volt regulator diode when reverse biassed with 76 mA.
You can expect a slightly lower voltage with less current. Maybe that is why voltage is closer to 2.8V.

A quick check;
TFMR 12V peak, less 2 volts for a pair of bridge diodes = 10V DC;
3300uF = 10VDC;
1k0 limiter, to 1N4728 zener = 3V3;
Current from cap will be (10V – 3.3V) / 1k = 6.7 mA;
The 1k load will draw 3.3V / 1k = 3.3 mA;
( 6V7 – 3V3 ) = 3.4 mA will flow through the zener.

Check capacitor size is sufficient, C = Q / V; C = i * dt/dv; dv = i * dt / C;
dv = 6m7 * 10ms / 3300 uF; So ripple voltage = 20.3 mV, and will not be a problem.
With an AC current range you would expect an AC current through the 1k0 load of 20.3 mV / 1k = 0.023 mA.

 

[George317]

Either your multimeter is broken, protection fuse blown, or you are using an AC current range.
The constant negative reading suggests a hall effect meter or a bad DC offset adjustment.
Does it also read –0.06 mA when disconnected or with terminals shorted?

The 1N4728A zener is a 3.3 volt regulator diode when reverse biassed with 76 mA.
You can expect a slightly lower voltage with less current. Maybe that is why voltage is closer to 2.8V.

A quick check;
TFMR 12V peak, less 2 volts for a pair of bridge diodes = 10V DC;
3300uF = 10VDC;
1k0 limiter, to 1N4728 zener = 3V3;
Current from cap will be (10V – 3.3V) / 1k = 6.7 mA;
The 1k load will draw 3.3V / 1k = 3.3 mA;
( 6V7 – 3V3 ) = 3.4 mA will flow through the zener.

Check capacitor size is sufficient, C = Q / V; C = i * dt/dv; dv = i * dt / C;
dv = 6m7 * 10ms / 3300 uF; So ripple voltage = 20.3 mV, and will not be a problem.
With an AC current range you would expect an AC current through the 1k0 load of 20.3 mV / 1k = 0.023 mA.

thanks.

i bought a new DMM, and I'm not getting the getting the negative current from before anymore. I'm getting 2.6V with 0.26mA current in a 1k Ohm Load, and a new problem comes to mind, shouldn't the current be 2.6mA via 2.6V/1000 ohm? and yet I'm getting 0.26mA even when i changed the load resistor to a 100 Ohm the current in the load is still 0.26mA

 

[George317]

trying to build a DC supply using a zener regulator with
1k ohm load
1k ohm series resistor
3300uF cap
1n4728a zener diode
x4 1n4002g rectifier diode(0.58V threshold)
12V secondary

schematic

in the load with 1k ohm resistor I'm getting 2.6V, and 0.26mA current(is this current too low?) then i changed the load resistor to 100ohm and the current didn't even buldge it is still 0.26mA.

isn't the load current supposed to be I = V / R? so my load current with 1k ohm resistor should be 2.6/ 1000 = 2.6mA, not 0.26mA like what I'm measuring.

 

[QuantumQuest]

in the load with 1k ohm resistor I'm getting 2.6V, and 0.26mA current(is this current too low?) then i changed the load resistor to 100ohm and the current didn't even buldge it is still 0.26mA.

$I_L = \frac{V_Z}{R_L}$ and according to the specific circuit and the info you give $I_L = \frac{2.6}{1000} \frac{V}{\Omega} = 0.0026 A = 2.6 mA$. Now, the only thing to ask is how do you measure this current if you do it on simulation software or if you have implemented the circuit with real components, if it is some read error.

 

[Baluncore]

Maybe your zener current limiter resistor is 10k not 1k0 ? Is that a red or an orange multiplier band?

i bought a new DMM

What is the make and model of your new meter?
Can you give us a web link to the product?

 

[George317]

What is the make and model of your new meter?
Can you give us a web link to the product?

its wheeler model: WDM-528, but i can't find any website leading to the product

 

[NTL2009]

Where are you measuring, and how (be explicit)?

I can't read your diagram, too low resolution. Please post a clearer image. But if that is a 3.3 V zener, you should expect 3.3V across the 1K load (the R in parallel with the Zener), so ~ 3.3 mA.

But if you put a 100 Ohm R there, that gives you a 1:11 voltage divider with the 1K seriesR. If your 12V secondary rectifies to ~ 17 V, that gives only ~ 1.55 V (below the Zener Voltage) across 100 Ohms, so ~ 15.5mA

 

[jim hardy]

The 1N4728A zener is a 3.3 volt regulator diode when reverse biassed with 76 mA.
You can expect a slightly lower voltage with less current. Maybe that is why voltage is closer to 2.8V.

I think you're right. 10K not 1K.

 

[jim hardy]

Is this same question as this other thread ?
https://www.physicsforums.com/threads/getting-negative-current-on-zener-voltage-reg-circuit.931285/

 

[Baluncore]

Thinking about it, R1 zener current limiter and R2, the 1k0 load, form a voltage divider across reservoir C1.

If V(C1) is 10V and Vout is 2.6V then R1 might be about 2.7k, not 10k as I suggested.
If R1 was 10k, then a 1k0 load would pull the output voltage down to 0.9V, the zener would not be conducting.
If R1 was 1k, then a 100 ohm load would pull the output voltage down to 0.9V, the zener would not be conducting.

Please measure DC voltages across R2 load and across C1, with different R2 load values of 10k, 1k and 100R.
Those 6 numbers will tell if the resistor values and zener voltage are reasonable.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

Is this same question as this other thread ?
https://www.physicsforums.com/threads/getting-negative-current-on-zener-voltage-reg-circuit.931285/

Threads merged and re-opened.

 

[jim hardy]

Could even be they're both 10K

 

[Tom.G]

If this schematic is correct, the transformer secondary voltage is 4.3Vrms or 6.1Vpeak. Subtract the 1.4V bridge drop and you are left with 4.7V across C1.
4.7V minus the 3.3v Zener voltage leaves 1.4V across R1. I can't read any of the numbers in the first schematic you posted (even magnified to 3-times size), so if they are different then the above calculations will change.

For troubleshooting, disconnect the resistor between C1 and the Zener diode; then measure the voltage across C1.
For further troubleshooting, re-connect the above resistor and disconnect the Zener diode. Connect the load resistor. Now measure the voltage across C1 and the voltages across each of the two resistors.

Tell us the results. That "should" be enough most of the information needed to find the fault.

something like this

 

[Baluncore]

If this schematic is correct,

The OP specified secondary AC as 12V peak. We need the voltage measurements I requested in post 18.

Please measure DC voltages across R2 load and across C1, with different R2 load values of 10k, 1k and 100R.
Those 6 numbers will tell if the resistor values and zener voltage are reasonable.

 

[FrankMlinar]

What's the resistance of the DMM when measuring current?

 

[rude man]

Just FYI, like all low-voltage zeners, a 3.3V zener is notoriously "soft", meaning that the voltage is strongly dependent on current. As pointed out in post #9, "The 1N4728A zener is a 3.3 volt regulator diode when reverse biassed with 76 mA. You can expect a slightly lower voltage with less current. Maybe that is why voltage is closer to 2.8V."

In view of the fact that the diode is spec'ed at 3.3V at 76mA and you're putting in only about 6 mA that is an entirely plausible explanation.