# LED indicators in variable V and I circuits

#### Jzarux

Joined Dec 4, 2020
5
After hours of combing through the thousands of irrelevant results Google came up with, I give up- How would I add indication LEDs to two parallel circuits (referred to as A and B, respectively, hereafter), of which either A is live B is off, A is off B is live, or A and B are off. The idea is to indicate the live status of either A or B via an LED for each. Where I run into issues is the need to use the single supplied switch- which is a latching ON/OFF/ON. For added complexity, this is a circuit in which the voltage varies from 1V-30V and current varies from 1mA-5A. Also potentially useful info: A and B are jacks, so the circuits are open until leads are connected with a load.

#### BobaMosfet

Joined Jul 1, 2009
2,082

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Or even this nifty little area at the top of the All About Circuits website (where you are at now):

#### dl324

Joined Mar 30, 2015
15,849
Welcome to AAC!

How about posting a block diagram of what you want and describing what A and B are and why you can't just use LEDs that are powered from whatever is powering A and B.

#### Jzarux

Joined Dec 4, 2020
5
Welcome to AAC!

How about posting a block diagram of what you want and describing what A and B are and why you can't just use LEDs that are powered from whatever is powering A and B.
Thank you for the warm welcome! This was my first block diagram, hopefully I did it correctly... Also, the LEDs cannot be powered by the source for A and B due to its varying nature. At any point it can range from 0-30V @ 1mA-5A

#### dl324

Joined Mar 30, 2015
15,849
Is using SW1 to indicate which is selected acceptable? Does SW1 connect power to set A and B?

#### Delta Prime

Joined Nov 15, 2019
1,311
Hello there
Also, the LEDs cannot be powered by the source for A and B due to its varying nature. At any point it can range from 0-30V @ 1mA-5A

#### MisterBill2

Joined Jan 23, 2018
15,713
Is the only reason that the indicators can not be powered by the source that the voltage is variable? There is a work-around for that, actually several work-around solutions. If the voltage does not need to be indicated when it is less than 3 volts then the solution is simple, if the current draw is not a problem. One option would be a shunt voltage clamp circuit, the other would be a series current regulator circuit.

#### BobTPH

Joined Jun 5, 2013
7,564
If I understand you, A and B are two connector jacks on which there is either no connection or connection to a power source that, when activated, will result in a current between 1mA and 5A at a voltage of between 1 and 30V.

If this is correct, a separately powered circuit should easily be able to detect 1V or more and light the LED.

If you require that no other power source is used, it becomes difficult to impossible when the output is at the low end, i.e. 1mA at 1V.

You have not specified what happens if nothing is connected to an output.

Bob

#### MisterBill2

Joined Jan 23, 2018
15,713
If I understand you, A and B are two connector jacks on which there is either no connection or connection to a power source that, when activated, will result in a current between 1mA and 5A at a voltage of between 1 and 30V.

If this is correct, a separately powered circuit should easily be able to detect 1V or more and light the LED.

If you require that no other power source is used, it becomes difficult to impossible when the output is at the low end, i.e. 1mA at 1V.

You have not specified what happens if nothing is connected to an output.

Bob

#### Jzarux

Joined Dec 4, 2020
5
There is still voltage being output to the connectors when nothing is connected. The output is a variable 1-30V and 1mA-5A. Also, I have no issues with using another power source, such as the 5v regulated rail already included, but I don't have a clue how it would work with the switch being single pole.

#### MisterBill2

Joined Jan 23, 2018
15,713
Is this power source a regulated voltage or a regulated current? it is a serious challenge to regulate both at the same time. There are many power supplies that have an automatic change from constant voltage to constant current at some point, but not both at once.
A simple scheme for indicating that a voltage of some value is present is to use a comparator IC to detect a level above zero, and illuminate an indicator. It would use a shunt diode and resistor to avoid an excessive voltage to the input.
To do it without any external power would take an LED with a series resistor to limit the current through the LED, and a shunt string of diodes to limit the maximum voltage across the LED. But probably it would not indicate voltages below about 3 volts. Also, it would add to the current load at all voltages.
So another question comes up, which is just what purpose is this indication supposed to provide? If it is for safety then it would be very much worth while to change to a multi-pole switch and power the indicator from a separate contact.

#### dl324

Joined Mar 30, 2015
15,849
I don't have a clue how it would work with the switch being single pole.
Why don't you just use a double pole switch?

#### AnalogKid

Joined Aug 1, 2013
10,591
I don't see the problem. A SPDT switch powers one of two loads (circuits), but never both, with 33 Vdc. An LED and series resistor from each throw to GND will indicate which circuit is powered, whether or not something is connected to the A or B output connectors. Actually, you can do it with two LEDs and only one resistor.

If you want an indication when something is connected to the A or B output connectors and drawing current theough the A or B circuits, that is a different, but not difficult issue.

ak

#### MisterBill2

Joined Jan 23, 2018
15,713
The issue is the very wide voltage rang- zero to 33 volts. It will need some means of limiting the current of the LED at the high voltage end while still allowing enough current to light the LED ar the low voltage and of the range. If the constant current LEDs are still available that would work, if they could stand the higher voltage. Adding a constant-current diode could also work, if those are still available.. But some parts are now hard to find, after 35 years.

#### AnalogKid

Joined Aug 1, 2013
10,591
A 2-transistor constant-current sink circuit saturates at less than 1 V. However, if set for 20 mA that's over 0.5 W into a TO-92 package, more than enough to fry it. I think a TIP29 and 2N4401 will do the job if that's not too many parts - still no information of physical limits, ambient air, etc.

ak

#### MisterBill2

Joined Jan 23, 2018
15,713
The simple work-around to the problem of excess dissipation mentioned by AK in post#15 would be to use a "high brightness" led and set the current lower, perhaps 10mA or less.

#### Jzarux

Joined Dec 4, 2020
5
Everyone, thank you for your help. It's just not worth it to keep the switch. I'm going to ditch it, and opt for a much easier DPDT setup. Correct me if I am wrong, but this setup is as simple as 1 set of poles interrupt the + to each connector pair. The second set is interrupting the 5v CV supply in parallel circuits, each with a 220 Ohm series resistor before the LED on its way to GND.

#### MisterBill2

Joined Jan 23, 2018
15,713
Everyone, thank you for your help. It's just not worth it to keep the switch. I'm going to ditch it, and opt for a much easier DPDT setup. Correct me if I am wrong, but this setup is as simple as 1 set of poles interrupt the + to each connector pair. The second set is interrupting the 5v CV supply in parallel circuits, each with a 220 Ohm series resistor before the LED on its way to GND.
That is certainly a reliable and effective way to do it. And since only one LED at a time would be lighted, you can simply put the resistor before the switch and only need to use one resistor.

#### Jzarux

Joined Dec 4, 2020
5
That is certainly a reliable and effective way to do it. And since only one LED at a time would be lighted, you can simply put the resistor before the switch and only need to use one resistor.
Well... Aren't you just "Mister Efficient" . Joking, obviously, I genuinely appreciate the advice and will put it to use. The only problem is that the original question continues occupy the vast majority of my pondering capacity... I must know. Isn't there a transistor of sorts or a MOSFET or something suited to the task.

#### MisterBill2

Joined Jan 23, 2018
15,713
Certainly a constant current regulator would work, BUT there is still the low voltage end of the challenge,. And any such arrangement would get complex in a hurry.