Blinking LED stops blinking?

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Kookie
Posts: 23
Joined: Wed Mar 05, 2003 1:01 am
Location: Detroit, MI
Contact:

Blinking LED stops blinking?

Post by Kookie »

May I enlist your help with a simple problem I'm having? I'm using a red blinking LED in an automotive application. The problem is when the light is on for a few hours it stops blinking and goes solid on. I'm not sure why this is happening. I have to turn it off for a while for it to reset into blinking mode. Now this is a blinking LED, no external circuit for blinking. So, what's going on inside this LED that this might be occurring?<p>What I'm using this for is a simple theft deterrent light that is lit when I switch on a kill switch. Its a double pole switch that one pole disconnects the neutral switch. (The car can not be started if it is not in park or neutral. Mechanically it may be, but electrically it thinks its in gear and disables the starter circuit.) The other pole brings in the blinking LED and resistor in series. Very simple and low current. I'd prefer it to be blinking for the simple reason of higher visibility. (By the way, this is for a 1973 El Camino that I've totally rebuilt and this is one of the last little things I need to fix.) So anyway, I'd appreciate some suggestions as to why this blinking LED loses its blink-ability. Thanks.
bruinbear714
Posts: 26
Joined: Thu Apr 17, 2003 1:01 am
Contact:

Re: Blinking LED stops blinking?

Post by bruinbear714 »

It probably has something to do more with the LED than anything else...<p>Does this behavior repeat itself in different temperature environments? The led may have a temperature coefficient.
User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Re: Blinking LED stops blinking?

Post by Chris Smith »

Inside the LED is a cmos circuit that may have been spiked by higher than acceptable voltages at one time in its life. Most blinking LEDs that I have had fail happen because of over voltage. What dropping resistor are you using, as They don’t require any resistors as long as they are within their range or voltage.
Kookie
Posts: 23
Joined: Wed Mar 05, 2003 1:01 am
Location: Detroit, MI
Contact:

Re: Blinking LED stops blinking?

Post by Kookie »

Hmmm, your right its probably damaged from over voltage at one point or another. I remember setting it up for 12V operation, not remembering that the vehicle can operate at as high as 14V or so. (This LED is meant to be used (on) when the vehicle is off - not running (no lower than 12V, nominal about 13V), but the LED can still be switched on when the vehicle is running, and the alternator can put out high with no load. Remember, this is an old car.) I believe I used a 150ohm resister in series. Frankly, I don't remember what exactly I used. It was 6 months ago that I set this up and heat shrunk it all. It worked for a while then petered out with it's weird behavior. I abandoned the whole thing until now. <p>Maybe my best bet is to use a zener diode? (Didn't think of that until now.) That should protect it from even a situation like a bad voltage regulator I would think. (I've seen a regulator go bad where it started burning out bulbs all over the place with an over voltage situation.) I just don't want to use too much current. I want to use as little as possible. That blinking LED is rated at 55mA at 5 - 2.5V (3V typ.). I don't want to be in the position of not being able to start the damned thing after an extended period of non-use. (Been in that position just to power the damned clock.) I have to use some current to bias that zener then too. Not sure how much. Guess I'll have to go get one and find out. <p>Thanks for helping me think this simple thing through. I'll get another blinker and start over. Maybe with the zener. Think that's a good idea for such a wide supply voltage range? <p>Oh, one more question. A cmos circuit in the blinker? What's that typically look (set up) like? Just curious at this point.
chessman
Posts: 292
Joined: Tue Jan 14, 2003 1:01 am
Location: Issaquah, WA
Contact:

Re: Blinking LED stops blinking?

Post by chessman »

I use 680 for LEDs on automotive circuits....seems harmless enough.
russlk
Posts: 563
Joined: Thu Dec 06, 2001 1:01 am
Location: New Hampshire
Contact:

Re: Blinking LED stops blinking?

Post by russlk »

Your blinking LED does need over-voltage protection because transients, particularly from the starter, can exceed 60 volts.
Kookie
Posts: 23
Joined: Wed Mar 05, 2003 1:01 am
Location: Detroit, MI
Contact:

Re: Blinking LED stops blinking?

Post by Kookie »

680? What's that? Please explain for me. :confused:
chessman
Posts: 292
Joined: Tue Jan 14, 2003 1:01 am
Location: Issaquah, WA
Contact:

Re: Blinking LED stops blinking?

Post by chessman »

680 ohm resistor.<p>Good call Russ ;) I didn't know that.
User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Re: Blinking LED stops blinking?

Post by Chris Smith »

Two ways to protect the led are a voltage divider [two resistors] that split the voltage to the minimum running voltage [center tap] of the LED or a 1/4 watt rated 7812, 7808, 7806 voltage regulator. <p>Using a zener to Shunt to ground only increases the circuit current draw, and having the led work only in the off position will also help.
toejam
Posts: 253
Joined: Wed Dec 05, 2001 1:01 am
Location: n.c.
Contact:

Re: Blinking LED stops blinking?

Post by toejam »

If you want it work all the time use a lm317 as a current regulator and set it for the current the led is optomised for. That only takes a resistor or two.
User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Re: Blinking LED stops blinking?

Post by Chris Smith »

Blinking LEDs are not current sensitive, they are voltage sensitive. You need to clamp the voltage, not the current. Any thing over the preset voltage wipes out the Cmos sensitive circuitry. Doesnt take much.
Kookie
Posts: 23
Joined: Wed Mar 05, 2003 1:01 am
Location: Detroit, MI
Contact:

Re: Blinking LED stops blinking?

Post by Kookie »

Hey, I really appreciate the discussion here. I think I'll try a 7805 with a diode in series and a couple of caps for good measure. That should get me around 4.3V for the blinky LED. I didn't realize the blinking LED's were so voltage sensitive. Makes sense now as to what happened. Thanks alot guys.
rshayes
Posts: 1286
Joined: Tue Mar 04, 2003 1:01 am
Contact:

Re: Blinking LED stops blinking?

Post by rshayes »

The 12 volt power system in an automobile is not particularly well controlled. When cranking the engine on a cold day, it may drop to the 7 or 8 volt range. It may go up to about 14 volts when charging the battery. These are steady state limits.<p>Depending on the wiring connections, it may be possible to get voltage spikes of several hundred volts when haevy loads are abruptly disconnected. The 7805 type of regulator is only rated to withstand input voltages of about 30 or 40 volts. It can be protected by adding a resistor in series with the power supply lead and a shunt zener diode or other surge suppressor. The regulator needs at least 8 volts input to deliver 5 volts out. The resistor should be chosen to drop a little less than 4 volts when the load is drawing maximum current. The zener diode should be chosen such that it is not conducting with the normal supply voltage, but will conduct heavily before the voltage rises high enough to destroy the 7805. Remember that zener diodes have a fairly wide tolerance, usually about 10%, and that they have a strong positive temperature coefficient for voltages above the 5 or 10 volt range. A 27 or 33 volt part is about right for a 12 volt system assuming a rating of 40 volts for the 7805.<p>The 7805 needs a bypass capacitor close to the regulator for stability (within a couple of inches). This would be in parallel with the zener diode being used as a transient suppressor. This should be about 10 microfarads or so, and rated at least 50 volts. I usually parallel this with a .1 microfarad ceramic capacitor to take care of high frequency bypassing since the impedance of most electrolytic capacitor begins to rise in the 100 kilohertz region. Similar parts can be used on the output of the regulator to provide a low output impedance at high frequencies.
Kookie
Posts: 23
Joined: Wed Mar 05, 2003 1:01 am
Location: Detroit, MI
Contact:

Re: Blinking LED stops blinking?

Post by Kookie »

Geez-o-pete, this is getting complicated fast. Ok, I concede. <p>stephen: I think I know what your saying, I think. I am a little confused about the "resistor in series with the PS lead" though. Also, how do you calculate the resistor for the zener since its there basically for transient voltages. Wouldn't it have to be a really high wattage? Not sure...<p>Here is a picture of what I think you mean so far:<p>http://mywebpages.comcast.net/Kookie/pscir.JPG<p>Are you saying that R1 (in the picture) should drop about 4V at 80mA output of the 7805? Is that to protect it from drawing too much current at the output? Also, R2's value should just be enough to bias the zener at 27V right? That is there to protect it from transient voltages, so does the wattage of that resistor have to fairly high?
Kookie
Posts: 23
Joined: Wed Mar 05, 2003 1:01 am
Location: Detroit, MI
Contact:

Re: Blinking LED stops blinking?

Post by Kookie »

Hey, I found this... I believe this is similar to what you talked about stephen... is this the explanation:
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>
Brent Brown says:
I use a regular 7805 in a 12 or 24V automotive environment. My design has a 1N4007 series diode, 10ohm 1W series resistor, 33V 1.3W zener to ground, and 1000uF 35V cap to ground on the input side of the regulator. Adding the resistor here is the key to getting the most from a simple design:-
The resistor limits the peak currents into your zener (during spikes) helping it do its job and ensuring that it doesn't snuff it on the first spike.
The resistor and capacitor make a low pass filter that helps reduce noise into (and out of) your circuit.

Zeners are a little slow perhaps? - then the low pass filter helps here too.
I suggest using a 16V zener for a 12V only application. I also noted that most 7805's have a 35V absolute maximum input voltage rating, but better than some other regs that have been suggested. <p>My circuit draws about 40-50mA which makes the regulator get warm on a 24V supply so I have a small heatsink on it. <p>The 1N4007 diode in series with the input prevents damage from reverse supply connection. It also stops the 1000uF cap from discharging if the supply voltage falls (eg. engine cranking), so if your circuit doesn't draw too much you have a short term UPS!
<hr></blockquote>
So, is this what was meant?<p>revised picture: http://mywebpages.comcast.net/Kookie/pscir2.JPG Should I throw in your 10uF cap too?
Post Reply

Who is online

Users browsing this forum: Ahrefs [Bot] and 28 guests