[SOLVED] Zener resistor when load is op amp input

[d123]

Hi,

How would I calculate load current for the Zener resistor in this circuit, please? Is it negligible because it's an op amp input or is there a current to add to the Zener regulation current, if so, how to calculate?

Thanks.

 

[c_mitra]

Depends on the operating point. You can assume that the 2nd op-amp input is high impedance and draws no current.

 

[Audioguru]

It seems like too many parts to simply limit the current in an LED. A single resistor will do it.

 

[schmitt trigger]

Are you attempting to clamp Vin to a certain voltage?

Or are you attempting to change the Vin range to a fraction of the original range?

Or are you attempting to generate a reference voltage?

Sorry for all the questions, but like AG mentions, you might be over-thinking the circuit.

 

[d123]

Hi,

Depends on the operating point. You can assume that the 2nd op-amp input is high impedance and draws no current.

Thanks. Embarrassed to admit am unclear about "operating point" meaning, even after quick online search. Op amp #2 should see ~150 mV to ~1.8 V at non-inverting input (and inverting input) but no higher, theoretically.

Wanted to check current is negligible, so thanks.

It seems like too many parts to simply limit the current in an LED. A single resistor will do it.

, very true... That's just one part of the circuit I'm thinking about, I'll post the full schematic tomorrow so it makes sense.

Are you attempting to clamp Vin to a certain voltage?

Or are you attempting to change the Vin range to a fraction of the original range?

Or are you attempting to generate a reference voltage?

Sorry for all the questions, but like AG mentions, you might be over-thinking the circuit.

Correct, thank you, the intention is to clamp Vin to the maximum voltage that should appear across the current-limiting resistor so as to avoid driving the second op amp crazy/driving NMOS harder and harder for a voltage the supply can't provide.

Quick summary: thinking of sloppy, short-duration up/down reference voltage for the op amp current source by charging or discharging a capacitor via pushbuttons and transistor-based current source and sink. Think of a cheapo sample/hold as a reference with not much hold. Picture tomorrow later in day will show what I mean, silly circuit of mine as usual but a way of learning things.

Thanks.

Apologies for not posting full schematic.

 

[FvM]

Op amp #2 should see ~150 mV to ~1.8 V

The zener diode circuit can't precisely clamp the voltage, it will cause nonlinear behavior far below the zener voltage. You already spending two OPs for your circuit, a third OP plus a diode can precisely clamp the control voltage to an external reference.

 

[Audioguru]

The minimum voltage for most 5mA zener diodes is 2.4V and at such a low voltage its voltage regulation (affected by temperature and current changes) is pretty bad.
Use an LM10 adjustable voltage reference instead which has excellent voltage regulation down to a very low voltage.

 

[Relayer]

Isn't the RI Limit resistor limiting the current through the LED?
Wouldn't the zener be trying to clamp the voltage going into the + of
the opamp and causing the - side to compensate?
By the way, I'm not too clued up on opamp operations.
Regards,
Relayer

 

[Audioguru]

Isn't the RI Limit resistor limiting the current through the LED?
Wouldn't the zener be trying to clamp the voltage going into the + of
the opamp and causing the - side to compensate?
By the way, I'm not too clued up on opamp operations.
Regards,
Relayer

No. RI senses the LED current and the opamp #2 adjusts the current to be the zener voltage divided by the resistance of RI.

 

[c_mitra]

Embarrassed to admit am unclear about "operating point" meaning, even after quick online search. Op amp #2 should see ~150 mV to ~1.8 V at non-inverting input (and inverting input) but no higher, theoretically.

Operating point of the zener (that was the original intention).

First op amp clamps Vin to V; also acts like a buffer. So Vin (<V) is applied to the Rs. All the current through Rs must go through the zener. As long as the Vin (<V) is less than the zener voltage, the zener acts high impedance. The voltage at the voltage divider junction should be calculated from the data sheet of the zener.

The non-inverting input is going to see the same voltage. if the zener voltage is greater than V, only leakage current flows. Here a datasheet can help.

I do not understand how the op amp 2 can see a neg voltage at the non-inverting input. Roughly speaking, it will see a voltage of Zener leakage current *Rs and a max voltage of the Zener. The inverting input will see the same voltage (but opposite sign).

--- Updated Sep 16, 2020 ---

Sorry, I mistook the ~ sign as the negative symbol. Please ignore the relevant part of that in the above comment.

--- Updated Sep 16, 2020 ---

Isn't the RI Limit resistor limiting the current through the LED?
Wouldn't the zener be trying to clamp the voltage going into the + of
the opamp and causing the - side to compensate?

Broadly correct.

Note that the mosfet is in the feedback loop. In this case, it is used a rheostat.

When the mosfet is fully turned on, RI limit is intended to save the LED.

 

[d123]

Hi,

Many thanks for answers (Not allowed to click on any more "helped me" today, sorry guys...).

The Zener I found is/was 1.8V, 50uA Iz (1.71 to 1.89V min and max), but after FvM and Audioguru's much neater suggestions, I have a SOT 1.25V reference that could fit the bill.

Just to explain premise: I have a cheapo headtorch I need to use frequently every day but mostly only for a few seconds at a time, it has one pushbutton so you are forced to cycle through on-flashing-off and I am sick of the "flashing" button press step to get to "off". I sometimes need to see what's rustling in the bushes at night - fox, boar, hateful feral male cat chasing my furry girls, or if it's actually my cats making the noises - the brightness must be blinding the poor little things, so adjustable brightness would be fun to design. Red or green LEDs would be good for that, but white scares away wild animals which is a big plus in my opinion.
I don't have an up/down counter IC. Using a 4017 for only cycling through up and reset is by now something I'm sick of repeating, even though it makes the stepped current level design really easy to implement.

I came up with this idea:



For a typical online supermarket/European penny store style piece of rubbish I wondered how workable the idea is.

I've only been working on this idea and the number-crunching for a few hours over a couple of days, some values are presumably very provisional until the calculator and reality coincide in a workable way, which is why I didn't post the whole schematic.

The source and sink are supposed to charge and discharge C1 by pressing either pushbutton to make a short-duration variable voltage reference for the op amp constant current source so as to adjust LED brightness.

I know it's a stupid and bad (and maybe pointless) circuit, it's my way of learning a bit more (hobbyists, eh...) and using mainly components I do have at home.

 

[KlausST]

Hi
I`d use a microcontroller instead of the analog UP/DOWN circuit. It is most flexible.

A more simple solution is to use a digital POT with UP/DOWN interface.

Klaus

 

[d123]

Hi,

As the main question was very well answered (OA as Zener load), and practical improvements suggested (precision clamp/LM10 version), I consider this solved. Many, many thanks.