Voltage drop monitor across load

[MikeMl]

... I have a 12v 16A switch. Can I use this switch for the 28v led indicator?...

What kind of switch? Are you talking about one that looks like this?

A bit of overkill in the current rating. I would use one that looks like this


.

 

[dimoune]

The one I bought is like this:

https://i.ebayimg.com/images/g/kJoAAOSwYaFWfRYX/s-l1600.jpg
Description:
LED Dot Light Car Boat Round Rocker ON-OFF SPST Switch
Condition: 100% Brand New
Light color: Yellow/Blue/Red/Green LED light
Volt: DC 12V
Type: 3 Pins
SPST: ON-OFF
Size: 2.4 cm x 2.4 cm x 3 cm(LxWxH)

Functions:
It can be applied as the switches for the additional electronical equipment inside&outside the car.
(such as flashing lights,chassis lights,and dome lights)

 

[MikeMl]

Are you trying to hook up the LED internal to the switch? You will need to determine if it needs an external current-limiting resistor for the LED or not. I cannot tell from what you posted...

 

[audioguru]

Spec's on ebay have such bad Engrish that nobody knows if the LED flashes by itself or it flashes if you cause its power to flash. The voltage rating of an LED on ebay is just a guess.

A switch with an overkill current rating has silver contacts that corrode then do not work anymore unless it has a high voltage and high power to spark the contacts clean. I use gold plated contacts (costs the same since the gold is thin and the silver is thick) for low power switching.

 

[dimoune]

Are you trying to hook up the LED internal to the switch? You will need to determine if it needs an external current-limiting resistor for the LED or not. I cannot tell from what you posted...

The switch has a small led indicator. I want it to switch on when I turn my 28v dc power supply. The switch is rated 12V dc. Is it ok to use with my 28v power supply? I guess this is why you said it might need an external current limiting resistor for the led because of my power supply being more than 12v?

 

[MikeMl]

Since the switch you posted is designed for automotive use, I am guessing that it is wired like this (you will have to verify):

 

[dimoune]

I will check it out and play with it a little it does not work but I'm guessing it should be ok. Thanks for putting me on the track.

 

[MikeMl]

I will check it out and play with it a little it does not work but I'm guessing it should be ok. Thanks for putting me on the track.

You know how to figure out which terminal on the switch is which using an Ohmmeter?

 

[dimoune]

hmmm.....no...I was planning to hook it up and see if it works, and if not, I would change the terminal. Now that you mentioned it, I would be interested to know how to test the terminal with an ohmmeter please.

 

[MikeMl]

Step 1. Put the Ohmmeter in the OhmsX1 mode (low Ohms). See what it reads with the leads open (my Fluke says "OL"). See what is says with the leads shorted (my Fluke says 00.4Ω).

Step 2. Find the two terminals (three possible pairs) that have near zeroΩ with the switch closed. Try both positions of the switch handle, so six total measurements. Note which two terminals appear to be shorted with the switch closed. One is the "IN" terminal (in my diagram), the other is the "load" terminal, but up to now, we don't know which is which.

Step 3. Put the Ohmmeter in the diode test mode (->|-). See what it reads with the leads open (my Fluke says ".OL"). See what is says with the leads shorted (my Fluke says .000).

Step 4. To find which of the terminals found in step 2 is "load", first make sure the switch is turned off. Connect the Red Ohmmeter lead to either of the terminals found in step 2. Connect the Black lead to the third terminal "GND" (which is neither "IN" or "load"). If no conduction ( .OL), move the Red lead to the second of the two found in step 2. If you see some conduction (about 1.700 on my Fluke, that is approximately Vf of the LED), then the second one of the original pair is "load", and by default, the other one is "IN".

 

[dimoune]

I saved your instructions and will follow the steps. I'm definitely learning a lot on this forum as a newbie. Thank you for your detailed explanation.

 

[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.
View attachment 98149

I noticed that when the voltage is above 3V and below 10v, I have the red light turns on fine but the green one flickers. Do you know what is wrong please?

 

[MikeMl]

What is the nature of the 28V supply? Is it well filtered and regulated? Can you post a schematic or a link?
What is the "current limiter"? Can you post a schematic or a link?

 

[dimoune]

What is the nature of the 28V supply? Is it well filtered and regulated? Can you post a schematic or a link?
What is the "current limiter"? Can you post a schematic or a link?

I'm using a led driver for the 28V DC. I can't find the exact driver I'm using but it looks something like this.
**broken link removed**
The flickering arises whether I user the current limiter or not.

 

[MikeMl]

A constant-current Led driver is not a DC supply... Not even close. It would have been helpful if you had disclosed what is supplying the 28V? in your first post (or if I had been smart enough to ask).

The Voltage detector circuit I posted expects pure DC at the "28V" terminal, and slowly varying DC at the "load" terminal. Likely, you are feeding it Pulse Width Modulation which is switching from 0V to ~33V.

 

[dimoune]

I never thought there was a difference. I wanted 28V dc and the led driver provided it. Is there an easy way to turn the led driver into a dc power supply?

 

[MikeMl]

I never thought there was a difference. I wanted 28V dc and the led driver provided it.

How did you measure the 28V? Using a DC meter? What load did the LED driver have on it while you were measuring it?

Is there an easy way to turn the led driver into a dc power supply?

No. By its very nature, a constant-current LED driver does not put out a constant output voltage. As the load changes, the LED driver adjusts its average output voltage to compensate for the changing load. It is the exact opposite of a constant DC power supply.

You should tell us what you are trying to power. It makes no sense to hang a "current limiter" behind a LED driver which intrinsically regulates current (not voltage)????

 

[dimoune]

Ok I have figured out the flickering was because of the led driver. I substituted it with a printer adaptor with 30V dc and the flickering no longer occurs!

Does using 30v instead of 28v entail any change in your schematics? Also which resistor values I need to change if I want the red light to trigger say at 10.5v instead of 10v? Thanks.

 

[MikeMl]

...Does using 30v instead of 28v entail any change in your schematics? Also which resistor values I need to change if I want the red light to trigger say at 10.5v instead of 10v? Thanks.

Circuit will work fine with 30V.

Use this formula:

R3=R2*2.49/(Vin-2.49)

In your case, leave R2=100KΩ, and make R3 adjustable (using a pot, or a series/parallel combination of standard value resistors).

Solving for Vin=10.5V

R3 = 100000*2.49/(10.5 - 2.49) = 100000*2.49/8.01 = 31086Ω.

Obviously, R3 depends on the tolerance of R2. I would just replace R3 with a 27KΩ fixed resistor in series with a 10KΩ trim pot set to near mid range.

 

[dimoune]

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...

Do two 1.3KΩ 1/4W in series add to 2.6KΩ 1/2W? The power and resistance both add up?