LED reverse polarity

Thread Starter

Johnnowen1

Joined Jun 12, 2021
6
Hi, is it possible to light a LED even in reverse polarity? Like passing through a circuit. Im applying on automotive. Thanks
 

DickCappels

Joined Aug 21, 2008
7,555
I had to look that up to be sure.

When light emitting diode is reverse biased, the free electrons (majority carriers) from n-side and holes (majority carriers) from p-side moves away from the junction. As a result, the width of depletion region increases and no recombination of charge carriers occur. Thus, no light is produced.
Source: https://www.physics-and-radio-elect...lightemittingdiodeledconstructionworking.html

Besides, most LED manufacturers recommend against reverse biasing LEDs more than a couple of volts.
 

Thread Starter

Johnnowen1

Joined Jun 12, 2021
6
Thanks a lot for those who help. I was wondering if it can make the LedScreenshot_2021-03-02-11-34-04-77.jpg stays lit up. This circuit makes Led flashing mode. Im planning to install it with our local rescue vehicle for more visibility. The problem is that the leds cathode and anode are connected. If I put 12v dc directly on the Led it will fry the circuit and the Led. Is there a way to make the Led stays lit up and not flashing? Like a flashing mode and steady mode? I want to DIY it since our budget is tight
Thanks
 

MisterBill2

Joined Jan 23, 2018
9,028
Indeed, speeding up the flash rate would do it, and it would be simpler, as well. But really, constant illumination is not as attention getting as flashing. perhaps just speeding the flash rate to 10Hz would work.
But if the display is not a lot more leds than shown, it is really not much of an attention getter. It may show up in the dark but be invisible around noon.
 

MisterBill2

Joined Jan 23, 2018
9,028
The TS already has an existing product that they wish to modify.
Reference the circuit in post #7. Change C1 to 100nF, changeR1 to 100K and the flash rate will be much faster. Ifr that gives results as desired, then it is just a matter of adding a relay to make it remotely selectable.
 

crutschow

Joined Mar 14, 2008
27,445
If you want full DC on the LEDs for maximum brightness, here's a way to do it with a 4PDT switch or relay.
You have to break the LED connections in the circuit and connect them to the contacts as shown:

The four switch/relay contacts are shown in the normal flashing mode.

When the contacts are moved to the upper position, the LEDs are disconnected from the 555s, and Vcc and ground are applied in the forward direction across all LEDs (D1 and D2 each represent the 4 LEDs in the actual circuit).

I assume the LEDs have some resistance in series to limit their current and allow them to operate from 12V(?).
If not that would need to be added.

1623650463199.png
 

Thread Starter

Johnnowen1

Joined Jun 12, 2021
6
Isn't there standards and certifications for this type of use?
As long as we dont use the flashing mode in major roads its okay
If you want full DC on the LEDs for maximum brightness, here's a way to do it with a 4PDT switch or relay.
You have to break the LED connections in the circuit and connect them to the contacts as shown:

The four switch/relay contacts are shown in the normal flashing mode.

When the contacts are moved to the upper position, the LEDs are disconnected from the 555s, and Vcc and ground are applied in the forward direction across all LEDs (D1 and D2 each represent the 4 LEDs in the actual circuit).

I assume the LEDs have some resistance in series to limit their current and allow them to operate from 12V(?).
If not that would need to be added.

View attachment 241215
Thanks
 

Thread Starter

Johnnowen1

Joined Jun 12, 2021
6
Isn't there standards and certifications for this type of use?
As long as we dont use the flashing mode in major roads its okay
If you want full DC on the LEDs for maximum brightness, here's a way to do it with a 4PDT switch or relay.
You have to break the LED connections in the circuit and connect them to the contacts as shown:

The four switch/relay contacts are shown in the normal flashing mode.

When the contacts are moved to the upper position, the LEDs are disconnected from the 555s, and Vcc and ground are applied in the forward direction across all LEDs (D1 and D2 each represent the 4 LEDs in the actual circuit).

I assume the LEDs have some resistance in series to limit their current and allow them to operate from 12V(?).
If not that would need to be added.

View attachment 241215
Thanks
I must be missing something, Why are you installing a flasher if you want the LEDs to stay lit?

Bob
Poor visibility because of fogs but when on major roads mostly the condition is clear up
 

MisterBill2

Joined Jan 23, 2018
9,028
Not recommended. That would put the flash rate into, or very close to, the range which can trigger epileptic seizures in certain susceptible people.
There are a lot of different flash rates that could be used, and while I have heardthat claim before I wonder about it's validity. AND, is that really a problem in this application?
 
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