switches, leds , inline resistors

Based on the above threefold subject heading I’ll present a few very basic 12-volt questions. Your kind assistance to help me better understand these concepts (in layman’s terms) would be most appreciated. Due to the hodgepodge of these beginner-level questions, I’ve decided to post in this general section of the forum. 1.) Is it ever acceptable to use an AC-rated toggle switch for a DC application, if voltage and current limitations are not exceeded? For example, could a 3A/125-volt AC toggle switch be used in a 12-volt DC circuit where the current draw of the load (e.g., LED) does not exceed 1 ampere? For practical purposes, let us assume that the following scenarios involve strictly a red LED (light-emitting diode)… 2.) An LED can be damaged by the discharge of static electricity. Therefore, consider a toggle switch which is placed between a 12-volt DC power source and the anode (+) lead wire of an LED (which is limited by an inline resistor according to specifications). If static electricity is discharged by an operator’s contact with the toggle switch, will the resistor and/or LED most likely get fried? If the switch itself is grounded, would this help preserve the functional integrity of the circuit and its components? If so, is it therefore essential to ground any switch which controls the operation of LEDs within a 12-volt DC circuit? 3.) A current limiting inline resistor is required whenever an LED is attached to a 12-volt power supply.   Here is a simple online calculator which will determine the ideal resistor value in such applications: CLICK HERE. It is known that the lifespan of an LED can be adversely affected by temperature, current, and voltage. Based on Ohm’s Law, if resistance (R) remains constant, any increase in voltage (E) will involve an increase in current (I). Therefore, it would seem logical that a higher resistor value will most likely (barring other considerations) increase the lifespan of an LED. As the resistor value increases (i.e., forward current decreases), the luminous intensity of the LED would also decrease. Of course, there is a certain threshold which must be exceeded for the LED to light at all.   Here is my question: Will increasing the inline resistor value have absolutely no effect on the “blink rate” (i.e., unchanged frequency) of a flashing LED which has an internal IC chip to make it blink? No mention is made of this matter in the following PDF: CLICK HERE. Once again, I apologize for my obvious incompetence regarding these matters. Thanks for your help!

The AC rated switch will be fine.As long as you are not purposely charging your body by rubbing your feet on the carpet, don't worry about the static.  Yes a higher value resistor will lower the voltage across the device.  The resistor value is not so critical to require a calculator.  If the calculator suggests a 470 ohm resisotor, tripling that value will show no visible difference.  And it will reduce the voltage across the LED by a minimal amount.  Use the suggested value and check the voltage across the LED, triple the resistance and check the voltage again.  Reducing the voltage across it will make it last longer.  It may take more resistance than you think to drop the voltage by enough to get any extra life out of the devices.

i am an idiot wrote: The AC rated switch will be fine. As long as you are not purposely charging your body by rubbing your feet on the carpet, don't worry about the static.  Yes a higher value resistor will lower the voltage across the device.  The resistor value is not so critical to require a calculator.  If the calculator suggests a 470 ohm resisotor, tripling that value will show no visible difference.  And it will reduce the voltage across the LED by a minimal amount.  Use the suggested value and check the voltage across the LED, triple the resistance and check the voltage again.  Reducing the voltage across it will make it last longer.  It may take more resistance than you think to drop the voltage by enough to get any extra life out of the devices.

Thanks for the info. That helps a lot!!! Will reducing the current (and voltage) by introducing a very high-ohm resistor result in a slowed "blink rate" of a flashing LED? Or, is there absolutely no direct relationship between current (or voltage) and pulse frequency? (Sorry if this is a stupid question...) Thanks again!

Thanks for your very informative and helpful advice, Kevin... and for suggesting Parts Express. Coincidentally, this website actually addressed my earlier question pertaining to the blink rate: “Bright blinking LEDs add emphasis to a project and grab the attention of a passerby. Each LED may be speed controlled by increasing or decreasing the supply voltage from 3 VDC to 10 VDC for a speed variance of 2 blinks per second to 2.8 blinks per second.” >>> SOURCE So, it would seem that that voltage does in fact affect pulse frequency in the case of flashing LEDs which have an integrated multivibrator circuit. The most cost effective place for me to buy suitable LEDs (once shipping is taken into consideration) seems to be this small online store which is based in Colorado: CLICK HERE These LEDs are pre-wired with inline resistors to limit current according to load specifications within the 9-12 volt DC power supply range. They have a current draw of 20 mA and a relative luminous intensity of approx. 3000 mcd. The viewing angel is also sufficient. However, a 12-volt DC power supply is pretty much ‘maxing out’ the load capability and doesn’t provide much of a buffer for spikes, as you’ve already mentioned. It would be my preference to add a second resistor in series with the first to protect the diode from spikes and to lower the current draw to roughly 10 mA in an attempt to preserve battery life and reduce lumen depreciation over time. Although the 7805 fixed voltage regulator sounds like a great idea, I’m not too sure that I want the supply voltage reduced to a paltry 5-volt input to load. Even if this component can be purchased with a higher output rating (e.g., 9 volts), can I not achieve a comparable effect by adding a kilo-resistor as you've suggested? Would the best way to approach this problem be to buy a selection of 1000-5000 ohm resistors and see what happens? Or, is there a way to approximate the ideal size of the second resistor in advance? Although I certainly don’t want to illuminate the interior of the whole vehicle, it would be great if the LED would be highly noticeable when looking inward at the dash from the outside. Thanks for the hyperlink, Tommy. I’ll check it out…

The pre-wired resistor which is included with this flashing red LED must be limiting the current draw to 20 mA in accordance with load specifications. Let’s assume that my target current for this installation is 8 mA. Therefore, if a second resistor (1K5) is added in series with the already pre-wired resistor, wouldn’t this suffice (12-volts/.008 ampere = 1500 ohms)? Would the resultant voltage drop provide a sufficient buffer to accommodate any potential spikes in this 12-volt DC (car battery) circuit?   Can the voltage reduction actually be calculated? What about a safe power rating (watts) for the added resistor? Thanks again…