how to control multiple 78xx with one potentiometer?

[samna_asadi]

hi
i need to control output of 2 or 3 78xx regulator with one potentiometer there are separate power source and leds and a chip way to dim their lights.
how can i do that?

 

[betwixt]

The simple answer is don't do it. The 78xx series CAN be adjusted to some extent but they are not designed to have an impedance in their ground pin. You might consider the LM317 series instead but even they need a sink current to the adjustment network.

Any attempt to link 78xx regulator grounds in different power supplies is doomed to fail and may cause damage. If you need several programable power sources they need to be independant and only linked by an isolated control signal.

Brian.

 

[Venkadesh_M]

Why do you want to use 78xx series, for power control?

switched regulators will provide better brightness control, with more efficiency.

how can i do that?

As of my opinion, I didnt see the voltage control of 2 or more 7805s difficult or dangerous with a single power supply and same voltage ICs.

But why do you want to use 2 or more 78xx ICs ? How many power sources you are using ?

 

[samna_asadi]

betwixt i will definitely consider your recommendation but its as is situation and it should be simple stable and economic. there are 45 4w leds with their own power source in 3 2 and 1 in rows and they should be able to control them in each row. the single leds are simple but in 2 and 3 there should not be multiple pots! i'v already considered a 555 ic switching schematic but its gonna be more expensive and the problem of multiple pots still exist! and if i wanna have a single power source in each row to control them by single pot its gonna be expensive!

 

[Venkadesh_M]

45 * 4W = 180W when all of them in half brightness Its about 90W power loss !!

Simple more questions,

Why do you want to control the brightness of Each row individually ?

What is the Voltage of one 4W LED ?

Why separate sources for each row ?

Are you using same power supply for all LEDs in same row ?

Why don you connect the all LEDs in one in parallel in the output of 555 circuit you made ?

 

[chuckey]

You could try and replace R1 with a transistor (collector to chip, emitter to earth) and feed the output from your new pot into the base via a high value resistor. Using the same pot output to feed the bases for your other chips. I do not think it will be very stable as the current through the original R1 , I think is in the order of 50µA, which gives a base current off .5µA. Looks like its getting comparable to thermal drift currents?
Frank

 

[samna_asadi]

these leds are light source for tables in a restaurant . the light from outside is different in each table so they should be controlled separately
the voltage of each led is 12v and each led has its own power supply.45 power source. if i wanna use same power supply for each row i have to change power supplies in 2 and 3 rows and that's what i'm trying to avoid because of more expenses!!

 

[betwixt]

I don't think the potentiometer with or without a transistor in the ground pin will work when the power supply grounds are not connected.

The simplest solution I can think of is to use LM317 devices instead of 78xx and use an LDR as the voltage setting resistor of each supply. A chain of LEDs each pointing at one of the LDRs will allow single isolated control using only a potentiometer. It will safely control all the output voltages but as others have pointed out, a linear regulator will be very inefficient in this application.

PWM is the way to go but it gets more complicated. Without doing any calculations to confirm how well it might work, you could try a MOSFET and opto-coupler in series with each supply wired to form a fast operating switch. Driving all the opto-coupler inputs from a single 555 would allow simultaneous control and still be reasonably efficient.

Brian.

 

[Venkadesh_M]

last phase of questions:

Do you mean each row of LEDs are in same table so same brightness ?

what is the voltage and power rating of each each power supply ?

what is the power rating of the circuit you designed using 555 ?

Are you already having one power supply for every LED you are having ?

 

[samna_asadi]

each row is in the same table and should be dim together.
each led has its own power supply in 12v 4w. the power rating for 555 circuit is 12v 1amp but with one power supply that i dont wanna happen!
the leds are working right now with their own power supply that are not dimmable so i have to dim them from the 12v side!

betwixed i'm working on what you said.

 

[Venkadesh_M]

can you post a schematic of your 12V power supply ? Is it based on transformer and rectifier? What rectifier ?

Is it ok for you to use one 555 IC circuit for one row with number of power supplies in parallel ?

 

[samna_asadi]

the power supplies are the ones come with leds and are tiny switching power supplies.
i don't know if i can connect power supplies in parallel and expect to have a stable system!

 

[Venkadesh_M]

Is LEDs and their power supplies are attached ?
LED power supplies how getting power from the mains, what is the voltage input for power supplies ?
At least post a photo or schematic of one LED module.
Also post how did you connect the LED module with 555 IC.

 

[samna_asadi]

the leds and power supplies can be separated! its 220v ac power main with wires!
i dont have eny schematic of PSUs they are in a plastic box and are sold with leds

 

[D.A.(Tony)Stewart]

why not use a simple PWM switch for each table from a central DC source such as battery & charger.

The regulator would cost same or less than one 4W LED.

A Master brightness could then be used with a central power source with voltage from 11 to 14V. Each table should have PTC protection and EMI filter.

Then you can use my design for outdoor glass patio table cloth backlit by LEDs.


What is your concept? And Specs?

 

[Venkadesh_M]

Is it having a mini transformer inside, like the one in mobile charger ? or it is just having one bridge and a capacitors ?

If you cant connect power supplies, then you cant control multible LEDs with single pot. atleast one termiinal should be in common(gnd).

 

[chuckey]

I like your and Sunnyskyguy's idea about back lit restaurant tables but I think for a commercial restaurant the idea of feeding PSU (or mains) and control wires around is dangerous. A better idea would to make each table battery powered, the batteries being charged (or swapped) when the restaurant is closed. The LED light levels should be "radio" controlled. This way the tables can be re-laid out for large parties, moved to cope with spillages etc. The system would be more expensive but would be a very marketable product if done properly. P.S. You could even include a "waiter call" button.
Frank

 

[D.A.(Tony)Stewart]

Anything <=50V is considered low voltage and safe, and does not require additional Safety Certification.

Many LED modules now are 48V to reduce current in cables.

Although 12V is Ok, it requires heavier copper wire. If existing wiring is AWG 16, it can be rewired from breaker panel for centralized DC on that line (only) for distribution using low voltage wire and AC outlets modified to use Molex 10A connectors to use existing wiring.

This is my concept for homes of the future to use distributed DC on selected lines from a PSU centralized near the breaker panel. With battery & charger, you also have emergency illumination when the power often fails in India/Pakistan.

The backlit table design is just mine.

 

[mtwieg]

The solution depends on the characteristics of your existing LED power supply. If they are current regulated then using linear regulators or PWM won't work. If they are voltage regulated then using PWM control is definitely the easiest way.