Delaying LED bulb switch on - Page 1
Forum section: electronics / beginners

author: Goran
author_rank: Contributor
timestamp: May 11, 2015, 11:44:18 pm
content: Hi,I have replaced a halogen bulb with a LED bulb (12V 12W) (that comes with a driver). There is a control unit which monitors the current in order to signal error when bulb is dead. Due to a difference in power between the halogen and LED (and current consumption), I needed to add a (load) resistor (load = 3W) in parallel to the LED, which is in total equal to power of the halogen bulb (15W). This is preventing the control unit to display error that something is wrong with bulb.However, when the bulb is off, a control unit will periodically (every 30s), for a very small amount of time (ex milliseconds), feed the bulb just to see if it is in working state or not. When halogen bulb was fitted, this action did not turn the bulb on, probably due to a very small time interval during which the bulb is powered for check. However, when LED bulb is fitted, it seems that it needs much less time in order to turn on, so the result is that LED bulb blinks (every 30 seconds).Question: is there a way I can somehow prevent the LED from blinking?

author: GodOfVolts
author_rank: Newbie
timestamp: May 12, 2015, 01:27:52 am
content: Maybe a time delay

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: May 12, 2015, 04:37:43 am
content: Reasonably large capacitor in parallel + current limiting resistor? Large enough to absorb the pulse before the voltage across the LED passes it's forward voltage drop.Of course my first suggestion would be to just remove this monitoring thing, but there isn't any context given. Because with the above it's starting to get a bit ridiculous.

author: Delta
author_rank: Super Contributor
timestamp: May 12, 2015, 07:57:32 am
content: A relay might suffice. The test pulse probably won't be long enough to pull it in, thus no flashes of the LED.KISS.

author: rolycat
author_rank: Super Contributor
timestamp: May 12, 2015, 08:39:21 am
content: Replace the 3W load resistor with a 3W incandescent bulb. The low cold resistance should absorb the test pulse.

author: Goran
author_rank: Contributor
timestamp: May 12, 2015, 09:09:43 am
content: Hi all, thanks for the responses, much appreciated.Quote from: GodOfVolts on May 12, 2015, 01:27:52 amWell, time delay relay could be an option, but that specific relay is much more over the budget. If there is some relay that is less expensive, which contains following specification:- has one input (A) and two outputs (B and C)- B output is activated on a positive edge signal (A). If the signal lasts for some time (ex 1s), output is switched from B to C- can deal with a minimum of 1A current.I already have a 10Ohm (25W) resistor, which I could wire to output B, and LED bulb and 47Ohms resistor on output C.Quote from: GoneTomorrow on May 12, 2015, 04:37:43 amMonitoring thing cannot be removed, sadly. I am not very good with capacitors, so can you please explain me what would recommended setup be doing in theory, what values to they need to be, and what would be the connection?Quote from: Delta on May 12, 2015, 07:57:32 amWhat kind of relay we are talking about. Please bare in mind that if the pulse does not get through to LED, we still need to somehow persuade the control unit that there is a 15W load present, otherwise the error will pop up.Quote from: rolycat on May 12, 2015, 08:39:21 amI could try that out just to see if it works. Still, that would not be good for a final solution, since this 3W halogen bulb would light anytime when a LED lights too, and that would be awkward (denies the (esthetic) reason for replacing the halogen bulb with LED in the first place).

author: rolycat
author_rank: Super Contributor
timestamp: May 12, 2015, 09:20:53 am
content: Quote from: Goran on May 12, 2015, 09:09:43 amPresumably the existing resistor is behind the light fitting - could the bulb not be concealed in the same way?

author: cryptyk
author_rank: Contributor
timestamp: May 14, 2015, 11:34:43 pm
content: I'm the new guy here, and basically know nothing, but I thought this is what inductors are for...It would absorb your load, converting it to a magnetic field, for some time until it passes it through the coil.