How to switch multiple resistors without rotary switch

Thread Starter

alexv

Joined Jul 7, 2009
25
I use 3 LED drivers to drive 3 strings of LEDs. LED driver current can be adjusted via an external resistor. I want to be able to set three levels of current which means that I need to switch among 3 resistors per LED driver.
I will use following resistors 10 ohm, 50 ohm and 100 ohm. Considering the value of resistors switching device should not add any additional resistance above 10 ohm.
Since I’ve got 3 drivers I need to switch among 9 resistors. I don’t want to use rotary switches. Are there multriplexers or SS switches that don’t add additional resistance ?
 

ElectricSpidey

Joined Dec 2, 2017
1,232
Sure, there are options but first need to know how do you want to make the selections.

Example:

If these resistors connect to ground you could select a series of NPN transistors using a 4017 clocked with a pushbutton.
 

dl324

Joined Mar 30, 2015
11,465
Are there multriplexers or SS switches that don’t add additional resistance ?
Yes, for the less than the 10 ohms max you specified.

Post a schematic so we can see how these resistors are wired with respect to the drivers.
 

AnalogKid

Joined Aug 1, 2013
8,578
Your question is missing important details.

What voltage(s) is/are available to run the control circuit?

What is the voltage powering the LED strings?

What are the max and min current values for each LED string? giving us the three resistor values is not enough information.

Do you want to select any current value at any time, or is stepping through them in sequence to get to the one you want OK?

What is your skill set for assembling a small circuit on perf board?

ak
 

Thread Starter

alexv

Joined Jul 7, 2009
25
Sure, there are options but first need to know how do you want to make the selections.

Example:

If these resistors connect to ground you could select a series of NPN transistors using a 4017 clocked with a pushbutton.
Yes, the resistors connect to
You can dim an LED using PWM (pulse-width modulation). There is no need to switch different resistors.
I can't use PWM for my application.
 

Thread Starter

alexv

Joined Jul 7, 2009
25
Your question is missing important details.

What voltage(s) is/are available to run the control circuit?

What is the voltage powering the LED strings?

What are the max and min current values for each LED string? giving us the three resistor values is not enough information.

Do you want to select any current value at any time, or is stepping through them in sequence to get to the one you want OK?

What is your skill set for assembling a small circuit on perf board?

ak
AK, here is the circuit. Powering voltage is 24V. Current is regulated in 100 - 300 ma range. I can add a step down voltage regulator for the control circuit. .Stepping the sequence is fine. I make my own PCBs and can build very simple circuits (AVR processor, few transistors...).
1584942747298.png
Resistors R1=10 ohm, R2=50ohm, R3 = 100 ohm.
I'm using three strings connected in parallel.

Alex
 
Last edited:

AnalogKid

Joined Aug 1, 2013
8,578
Two options come to mind.

1. One button, sequential selection. CD4017 contigured to cycle through three stages in a repeating loop, one stage per button press. Each output drives a small MOSFET (2N7000/7002) that switches one resistor to GND. The 7000 adds about 2 ohms. The circuit needs a regulated source voltage below 15 V.

2. Three buttons, random selection. This is called a radio button circuit. Pressing any button activates an output and deactivates any other outputs. One flipflop per stage, plus some gating. Same 2N7000 output.

ak

See what happens when you give up some details?
 

Thread Starter

alexv

Joined Jul 7, 2009
25
Two options come to mind.

1. One button, sequential selection. CD4017 contigured to cycle through three stages in a repeating loop, one stage per button press. Each output drives a small MOSFET (2N7000/7002) that switches one resistor to GND. The 7000 adds about 2 ohms. The circuit needs a regulated source voltage below 15 V.

2. Three buttons, random selection. This is called a radio button circuit. Pressing any button activates an output and deactivates any other outputs. One flipflop per stage, plus some gating. Same 2N7000 output.

ak

See what happens when you give up some details?
I can do it but I was hoping to find a single chip solution.
 

dendad

Joined Feb 20, 2016
3,602
Do you have a reason for saying you cannot use PWM? That is what I would use. It is by far the best way to go.
If your fix is to add a processor of some kind to control the switching, it can generate the PWM signal.
Otherwise, a center off DPST switch can work. This will give you 100R, 50R and 10R, but no off position.

3posLED.jpg
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
1,232
Anything with chips is likely to require more than one with that 24 volts.

Even if you could get a micro to sink the current, you would still need a regulator.
 

AnalogKid

Joined Aug 1, 2013
8,578
The micro would not be sinking the LED current. It sinks only the regulator programming resistor current.

Maybe. Depends on the regulator chip/circuit in post #8, which is a secret.

ak
 

ElectricSpidey

Joined Dec 2, 2017
1,232
Yes, I’m well aware of that fact, but we don’t know how much of the load the resistors share.

But my point was you still need at least 2 chips.
 

MrChips

Joined Oct 2, 2009
21,820
One resistor, one transistor, PWM (555-timer for example) and you can have continuous control on brightness.
At 200mA sink current you would not need the transistor.
 

ElectricSpidey

Joined Dec 2, 2017
1,232
Seriously, we are going to debate semantics, I was referring to the regulator to step down the voltage to something a “chip” can handle.

I have always referred to regulators as “chips” but I’m sure others use different terms…

Oy vey
 
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