Voltage drop monitor across load

[dimoune]

Hello,

First of all, I want to mention that I am a complete novice in electronics. Having said that, I have a 28v dc power supply and it goes through a current regulator. The output of the load is normally 12v or more. I am looking for a circuit to monitor voltage drop below 10v in the load. I want to have a led light up when load voltage is above 10v and another led to light up if voltage drops below 10v. I also want the led to draw power from the power supply instead of the load. How can I achieve this please? Thank you.

 

[MikeMl]

I cobbled up this circuit using a TL431. D1 is a Red LED and D2 is Green LED. They could actually be a Red/Green Bi-Lateral LED like this. Green is on when the voltage is above 10V; Red is on when the voltage is below 10V.

V1, R4, and V2 is how I modeled your 28V supply, the current limiter, and your existing load, respectively. The circuit on the right connects to the existing stuff at 28V, load, and Gnd.

The circuit has a >100KΩ input impedance, so shouldn't perturb your load... Current draw from the 28V supply is less than 20mA.

I sweep the voltage across the load V(load) from 11V to 9V along the X-Axis as the independent variable, while I(D1) Red trace and I(D2) Green trace shows what happens in the two LEDs. Note that there is a small region around V(load)=10.06V where neither LED is on.

 

[dimoune]

Thanks for your reply and schematics. I am newbie in electronics. I have to refer to the internet to make sure I read the diagram correctly. Please let me know if I got it right in the diagram annotation below.

 

[MikeMl]

That is what I assumed...

To correctly simulate your stated conditions, I had to create a circuit that emulates your supply, regulator, and load...

Refering to my posted circuit, electrical schematics can show connections explicitly (with wires), or implicitly (by assuming that electrical nodes with the same names are connected together), so "load" connects to "load", "28V" connects to "28V", and all three Gnd symbols connect together.

 

[dimoune]

I have one more question please. Since the DC voltage can go upto 28V, what will happen if voltage goes above 11v? Thanks.

 

[MikeMl]

I have one more question please. Since the DC voltage can go upto 28V, what will happen if voltage goes above 11v? Thanks.

Not much. The LEDs, R1, R5, R6 and the TL431 cathode are fed from the 28V, which doesn't change. The only thing that changes much as V(load) goes to 28V is the current in R2. There will be only an imperceptable effect on the operation of the circuit. Later this morning, when I get to another computer where I did the sim, I will rerun the LTSpice simulation with V(load) sweeping from 0V to 28V, and post the results...

ps. I did, and the current in R2 reaches a max of ~200uA when V(load)=28V.

 

[MikeMl]

A discussion of how I used LTSpice to come up with this circuit is in post #10
of this contemporary forum thread:

https://www.electro-tech-online.com/threads/are-pspice-ltspice-worthwhile-or-just-trouble.147547/

 

[dimoune]

Hi MikeMi,

Sorry to bother you again with my newbie question. How can I add a led indicator for the 28V power supply please to indicate power is on? Thanks.

 

[MikeMl]

...How can I add a led indicator for the 28V power supply please to indicate power is on?

Add an LED and a resistor. LED cathode to Gnd. LED anode to the resistor. Other end of the resistor to 28V.

**broken link removed** have a forward voltage drop Vf of ~2V. Most LEDs make a good indicator when If = 10mA = 0.01A

Resistor value: R = E/I = (28-Vf)/If = 26V/0.01A = 2600Ω.

Resistor power dissipation: P=IE = 0.01A*25V = 0.25W, so use a 2.49KΩ resistor rated for 1/2W or more, or use two 1.3KΩ, 1/4W resistors in series...

 

[dimoune]

Thanks again for your simple and clear explanation.

 

[MikeMl]

Occurred to me later: Since you can buy "ultrabright" or high-efficiency LEDs, you can reduce the power dissipation in the resistor by operating an ultrabright LED at much lower currents, say 1-2mA. So R = 26/0.001 = 26KΩ. 1/4W is fine.

 

[dimoune]

Noted. Thanks.

 

[dimoune]

My shop has 3 leg bi-color led with the following characteristics. Is this the same as in your circuit? Thanks.

both short legs are positive [ + ], the long leg is [ - ]

WLD : 465-470 NM
IV: 8000-10000 MCD
VF: 3.4 -3.6V

 

[Mikebits]

Your bi-color LED has the Cathodes (- negative terminals) connected together. In Mike's example the LED's are configured back to back. In other words:
LED1 Cathode ---> LED2 Anode, and LED1 Anode ---> LED2 Cathode.

You want LED 3 (same as Mike's example) shown in image below.

**broken link removed**

 

[dimoune]

You mean I need to choose the led wich has 2 pins? Is forward voltage important? I found one on the net which says Fwd Volt 3.2-3.6V. Thanks.

 

[MikeMl]

You mean I need to choose the led wich has 2 pins? Is forward voltage important? I found one on the net which says Fwd Volt 3.2-3.6V. Thanks.

That will be a single white LED.

Single Red and Green ones will have a Vf of ~2V.

I gave you a link to where you can buy a Red-Green inverse-parallel two-pin LED assembly back in post #2. Did you click on the link? That provides some industry part numbers you might be able to procure.

If you just want to use a single Red and a single Green LED, then just hook them up in inverse-parallel.

If you had put your country in when you registered for this web site, we might have a clue as to where you could buy stuff...

 

[dimoune]

I am from Mauritius. I did mentioned this in my profile. We have limited electronics items available here. I need to order, preferably from ebay.

Here are 2 links I came across. Please tell me if they are ok for this porject. Thanks.

**broken link removed**

**broken link removed**

 

[MikeMl]

I am from Mauritius. I did mentioned this in my profile. We have limited electronics items available here. I need to order, preferably from ebay.

Here are 2 links I came across. Please tell me if they are ok for this porject. Thanks.

**broken link removed**

**broken link removed**

The first link is to a "flashing" LED; it has an electronic flasher chip inside it. This one is not suitable for your project.
The second link would work. You are buying 100 of them...

You might also consider buying an assortment of different colors like this, and then using the Red and Green ones..

I see no "location" in your postings; look at mine, click on it...

 

[dimoune]

Thanks for the tip. These assorted led might become handy in the future.

 

[dimoune]

While electronic is fascinating, I know nada about it. By building small project like this, I become acquainted with the basics. Well, I have another question please. I have a 12v 16A switch. Can I use this switch for the 28v led indicator? Thanks.