Spikes

Discussion in 'General Electronics Chat' started by richbrune, Dec 6, 2005.

  1. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
    106
    0
    I have a circuit with an LED in it that occasionally blows out.For a long time I didn't have any idea what might be causing this, but recently I found that the power supply that this circuit is designed to "piggy back" off of, has a rather large AC powered coil (inductor) that is shut on and off via a mechanical switch. I noticed that the "spike" caused by this was barely visible on the ocilliscope, and was very short in duration, but high in voltage. Since the circuit that is controlling the LEDs in it has a 12volt DC regulator, I tried a 14 Volt MOV across the positive and negative terminals of the Voltage Regulator, but the spikes were still visible, and appeared to be still well over 20 volts negative and positive. I tried to put a diode across the load (LED)with the black band toward positive, but after that I noticed the spikes were still visible, but they were only positive going, and the LED went out. I can see the LED go very bright for an instant as the coil is turned off so I'm wondering if that's what's causing my LEDs to go out. The 24 volt coil that I think might be causing the problem cannot be removed, so I'm wondering if there's a "local"way to protect my LED?.
    Thanks again for all the past help,
    Rich
     
  2. Gorgon

    Senior Member

    Aug 14, 2005
    113
    0
    Hi Rich,
    You don't tell the input voltage to the 12v regulator, but I assume it is 24v since you've got the '24v coil'. Dependig on the input voltage I would have put a Zener diode of some wattage(1-3W) over the input to the regulator. The voltage should be 2-5v more than the normal supply voltage, or the highest available supply voltage if it is not properly regulated.

    The 12v regulator can't be very sophisticated or it has been zapped too many times.

    The coil in question should have some form of bleeder circuit to get rid of the inducted voltage/ energy, maybe this has failed?

    TOK ;)
     
  3. dougp01

    Member

    Dec 6, 2005
    27
    1
    If you are powering your relay coil from the DC supply, simply put a reverse biased diode directly across the relay coil. It will be parallel to the coil connections.

    When the coil is energized, the diode will be off. When you remove power from the coil, (remember it is an inductor as well) the EMF in the coil collapses as a reverse potential and conducts through the diode. A 1N4004 should be good enough.

    -Doug
     
  4. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
    106
    0
    Hi Gorgon! How's the weather up there?

    The coil that's generating the spikes is a 24V AC coil for 120V motor contactors, and there's no bleeder circuit (possibly because it's all AC?) I'm sharing this transformer (24VAC) for my supply power, then a bridge rectifier and a 7812 linear voltage regulator. 470 uf Caps both before and after the 7812, across neg and pos. Larger caps didn't help.

    The spikes make it through the bridge, the 7812 Voltage regulator, and then through a 555 timer (the LED is blinking rapidly).

    When these spikes make it through to the LED, they are both NEGATIVE AND POSITVE polarity, so I'm assuming that the zener diodes would only help me for one polarity, right?

    I don't know how they make it through, but they're powerful. Would some kind of ferrite core possibly help?

    Rich

    sunny, clear, and 70 degrees F here today.
     
  5. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Right on the ouput of the regulator add a large cap to ground to help filter out the spike there, then maybe an inductor between the led and its source voltage to choke any pulse that the cap didn't remove.
     
  6. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
    106
    0
    Hi Brandon!

    I'll try a few inductors in front of the LED, but I've already tried putting caps as large as I can fit the space I have, and the didn't have an effect.

    Thanks again,
    Rich
     
  7. chulomex

    New Member

    Dec 1, 2005
    4
    0
    Can you draw up a quick schematic of the circuit?
     
  8. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
    106
    0
    Here's a sketch
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Something to consider, damping a fast spike with a large capacitor is actually not that efficient. Large capacitor is usually an elco, and this is made from wounded sheet of foil which has quite high internal inductance and capacitance. It doesn't react that fast but it is a large charge reservoir. The larger the elco is the worse its 'reaction time' is. So it might 'miss' the spike entirely, depending on how fast the spike is (a hint of the approx. spike width would be useful here). Instead of going 'big' try going 'small'.

    A few small and fast capacitors and medium elcos in parallel would perform better. Ceramics for fast response and elcos for high energy storage. Something like 0.1uF ceramics will do.
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The normal way of suppressing spikes on an AC relay or contactor coil is a 'snubber network'.

    On 110 or 240V this is typically a 0.1uF capacitor in series with a 100 Ohm resistor.

    They can be connected across the relay coil or across the contacts switching it (or both).

    For 24V AC, you could try a standard one or change the component values proportionally, e.g. the capacitor anything up to 1uF polyester and resistor down to 10 Ohms. The standard values will probably be OK, though.

    This should drastically reduce the voltage spike across the coil, prevent the control contacts arcing & hopefully eliminate any leakage to the rest of the circuit.

    The basic cause of your problem is that when the control contact opens while there is current flowing in the relay coil, the inductance of the coil tends to try and maintain the current flow, causing the voltage across the coil rise almost without limit. This then arcs back across the control contacts, feeding a high voltage pulse into the rest of the circuitry.
    It also causes damage to the control contact & produces radio interference..
     
  11. richbrune

    Thread Starter Senior Member

    Oct 28, 2005
    106
    0
    Thanks to N935 and R Jenkins!
    Yesterday I put a .1uF cap directly across the LED (load) , and I still could see voltage on the scope, but the visible "spike" of LED luminescence was very attenuated. I think it might be adequate, but I'm going to also put snubbers around all the other major inductors.

    Thanks again!
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Rich,

    It seems to me your 555 is being recycled alot [causing that led to flash]. When that happens you can connect a 100 uF and a .01 uF between pin 8 and ground at the 555 to counter the effects of counter emf from a collapsing field of a relay de-energizing. This was the problem experienced on another thread [building the parachute deployment circuit for that rocket club ... whose flight to 50,000+ was a success last month]. With your circuit, a changing load caused the erratic supply voltage on the 555. The 7812 needs a minimum of 14 volts at the input.

    I was looking over your schematic and was wondering why the extra resistors [100 and 130] and the 1n4003 were in the power supply. They only reduce the available voltage at the input to the 7812 if the load approaches 100 mA. If you went for the maximum 500 mA load of the 7812, your input to the 7812 would be well under the minimum level. You might want to consider a new design if you draw more than a few milliamps.

    On your timer circuit, I was wondering about the extra resistors [surrounding the LED]. It seems you could have adjusted that variable for the 1.5 volts or so at the wiper to energize the LED.

    What were the source of those spikes? Were you sharing the 24 volt line with a heating or air conditioning control circuit?
     
  13. dougp01

    Member

    Dec 6, 2005
    27
    1
    There has been quite a bit of discussion on snubbing the spike. I looked at your drawing and it was not apparent to me where the contactor coil is wired in and how the spike is getting in.

    LEDs cannot tolerate a voltage reversal, no matter how show the duration!

    You can put an anti-parallel diode across the LED. This would be something like a 1N4001 thru 1N4003 that is connected exactly in parallel with the LED cathode to anode and anode to cathode. It insures the reverse current does not attempt to flow through the LED.

    One other thing, there is the possibility that you can blow the LM7812 Regulator. It is not recommended that you put an extremely large value cap after the regulator, since all your ripple rejection should be before the regulator. Instead the output cap should be about 1/100 the size it is now. Also you need a diode connected from the output of the 7812 back to the input. Connect diode anode to the output (pin 3) and diode cathode to the input (pin 1). You can use that 1N4003 to do the job.

    -Doug
     
  14. Gorgon

    Senior Member

    Aug 14, 2005
    113
    0
    Hi Rich,
    At the time of asking the weather was a little below 0 C, slight snow i the air. In the mean time the temperature dropped about 10 deg C(to -10 C) and some more snow, some 10-15 cm. Not much but pretending to be a real winter. This evening it is close to 0 again. I suppose it is raining tomorrow.

    Regarding the zener diode, it will protect you from both positive and negative spikes. It works like an ordinary diode that way. From your schematics I can't see wher your ground is connected, If you connect the ground at the end of the construction you will direct all noise and spikes trough you logic. The single ground connection (if any) should be at the power input, just after the bridge rectifier. I also wonder about the 2 resistors and the diode before the regulator. What is their purpose? The big capacitor should be connected close to the input terminal of the regulator, with nothing inbetween. You could also use a smaller capacitor in parallel with the 470uF, like 0.1 uF. You should also do the same on the output, add a 0.1 uF over the big one wich is far too big, a 10 uF is more like it. You should also use the same 0.1uF direct over the 555 power pins.

    How do you ground your probes when measuring the spikes with the scope? For me it looks like a typical ground strap pickup of noise.

    One thing you could try to verify that you really get negative spikes, is to connect a second LED reversed direct over the first. They will then protect each other against reverse voltage and you can see if the reverse pulse is real.

    TOK ;)
     
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