555 timer, PWM circuit failure

Discussion in 'General Electronics Chat' started by HiProfile, Aug 31, 2009.

  1. HiProfile

    Thread Starter Member

    Mar 15, 2009
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    I think I know the issue, but it the evidence just seems conflicting.

    I built a PWM circuit based on a 555 timer (link to circuit) and it worked well enough the few times I tried it, except it suddenly would not turn back on after turning it off. It's running 4 fuel injectors (4-5 amps) at a relatively low frequency.

    I originally designed it to run a single 5.4 watt solenoid, but later addapted it to run 4 injectors (~12-15 watts each). However I was thinking of the flyback diode in terms of voltage, not current - so I ran 4-5 amps using a single 1 amp diode (1n4003). Could that have fried the 555 chip?

    I've already checked the mosfet and flyback & steering diodes, all seem fine. The 555 chip is won't come out easily, so I can't readily test it. One last thing is the injector +12vdc wire was unsoldered when I initially inspected after failure, but resoldering it didn't fix anything. [It had to have happened as I picked it up, since it initially turned off as I turned the switch off]


    So all I can guess atm is a voltage spike killed something from the flyback diode not rated for enough current, or the possitive lead on the motor coming off induced some destruction...:(
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you use back emf diodes across the solenoids?
    Without them the back emf can distroy the circuit connected.
    The diode should be connected with the kathode to the powersupply
    and the anode to the swtiching transistor.

    Greetings,
    Bertus
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Something like this...

    [​IMG]

    I like that source schematic you linked to, the club meets several blocks from my house. It is lacking some info like the coil diode, however. The transistor type is unimportant, both need that diode.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,692
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    Hey Bill,

    I stumbled across this the other day. Congratulations. :D

    [​IMG]

    John
     
  5. Wendy

    Moderator

    Mar 24, 2008
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  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    I would not be surprised to see that in a bookstore.
     
  7. HiProfile

    Thread Starter Member

    Mar 15, 2009
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    bertus, those emf diodes are the flyback diodes I spoke of. I only had a 1 amp diode across 4-5 amps worth of solenoids, aka inductive load.


    I have 3 questions:

    1) The coil diode would go across the motor leads (far right in your pic), right?? I called them flyback diodes, as that's the name I came across somewhere.

    2) Is R2 in your pic required when I use a mosfet like the IRFZ46N? I recall reading something about that, but thought it was only for a regular transistor.

    3) On the circuit I linked to, you're basicly saying a regular transistor can be used in place of the mosfet, right? This would only be for low-freqency use (72Hz or less).
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    MosFETs are more efficient for high currents, but either can be used as long as the drive doesn't exceed specs of the individual device. Frequency isn't that important.

    I'm old school, so I tend to lean toward BJTs when I draw. MOSFETs tend to have better characteristics IMO.

    Yes, a resistor is needed. It doesn't need to be large (100Ω will do), it is to stop ringing as the input switch, which I understand is a common problem with driving MOSFET inputs.
     
  9. HiProfile

    Thread Starter Member

    Mar 15, 2009
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    Okay I think I understand - I probably fried the 555 chip. The chips I have are rated for 200mA output, and it probably fried it after a bit of usage. Too bad my eyes can't see infrared heat, that would solve SO many problems!

    I guess my last question would be about the output pulling too much current from the 555, how it affects the chip. That sounds like the reason my circuit doesn't work now, right?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    The problem is the inductance from the coil is high voltage, it is this high voltage that is frying electronics. The diode shown in the schematic I posted surpresses that.

    I just finsihed a circuit that uses the flyback principal. Doesn't relate to your circuit, but you might find it interesting.

    CMOS 555 Long Duration LED Flyback Flasher

    It is also used for buck boost conversion.

    But flyback EMF has to be controlled, or it can cause damage.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Fuel injectors usually run on fairly low voltage; say 3v to 3.5v. If you're running them on 12v, you are probably stressing them quite a bit.

    An IRFZ46 power MOSFET needs a Vgs (voltage on the gate with respect to the source terminal) of 10v to insure that it is fully turned on. A standard bjt (non-CMOS) 555 timer can source or sink up to 200mA when Vcc is 15v.

    Since pin 3 of the 555 uses a Darlington voltage follower, the output won't go much higher than Vcc-1.3v, even when lightly loaded. Therefore, in order to ensure that your MOSFET's gate gets up to 10v, you'll need to supply the 555 timer with at least 11.3v.

    Use a 47 Ohm resistor from the 555 output to the gate of the MOSFET.
    Also, it's a good idea to connect a 10k Ohm resistor from the MOSFET gate to source terminals. That way, in case the 555 or the 47 Ohm resistor fails, the 10k resistor will turn off the MOSFET. It will also help protect the MOSFET against ESD if the 555 is removed.
     
  12. HiProfile

    Thread Starter Member

    Mar 15, 2009
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    I think you're right about the injectors, to an extent. Peak & hold injectors have a 2-3 ohm coil, and use an additional in-line 6-8 ohm resistor. Saturated injectors have a 10-14 ohm coil, and get "saturated" with the full system voltage. I've already got that covered, so they won't burn out. My test units are useless extras anyway. It's also powered by a car battery, so it's always above 12 volts.

    I'll be making another circuit, except with slots for the timer, mosfet, and big cap (to adjust freqency). I'll also be adding those 2 resistors; however, what does the 10k ohm resistor in the schematic do? I think it goes from the gate & discharge pins to +12v. Is that more for the timer's benifit?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    In the schematic on the page you linked to, pin 7 of the 555 timer is an open-collector output that is only capable of sinking current. The 10k resistor is used to charge the gate of the MOSFET to turn it on. This results in a very slow turn-on time for power MOSFETs with a large gate charge, such as the IRFZ46, and subsequent heating of said MOSFET due to operation in the linear region (partially conducting).

    You would be better off to use something like Bill Marsden's 555 PWM circuit, only with a much larger pot than 10k.
     
  14. HiProfile

    Thread Starter Member

    Mar 15, 2009
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    The mosfet never seemed to get warm during use, however I never used it for more than 10 minutes.


    Which of Bill's 555 PWM circuits are you refering to? Most appear to not offer the full range of duty cycle I want, and the one that does seems to be missing R1, which seems important if I want to change the frequency (70Hz or less, on left):

    [​IMG]

    Otherwise I could just hook the mosfet's gate to pin 3 with 47 ohm resistor, right? It should provide ~10.3 volts when high, enough for the mosfet. BTW what's the deal with the triangle and rectangle shapes, do they signify the cmos version or something?
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, the schematic on the left. Replace R2 with a 100k pot, which will give you about 70Hz, and a PWM adjustment range from nearly 0% to nearly 100%. Remove the LEDs and resistors being powered from pin 3. You'll also need a 220uF and a 0.1uF cap across the 555's power pins to take care of transient spikes which are caused when the 555 timer's output changes states. And then yes, connect the MOSFETs' gate to pin 3 using the 47 Ohm resistor, and a 10k resistor from your MOSFET gate to its' source terminal.

    Most other places, you'll see a 555 drawn as a rectangle. Bill seems to like drawing them as a Schmitt trigger buffer/inverter with two inputs.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Add a 10Ω resistor between pin 3 and the two diodes. I'll be changing my drawings pretty soon to reflect that. Amazing what a fresh 9V battery does for an experiment.

    Gee Wookie, I just like drawing them as I see them. :p
     
  17. HiProfile

    Thread Starter Member

    Mar 15, 2009
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    Thanks for all the help!! After this I'm definately going to research the 555 timer the best I can, so I don't have to bother you guys with such "simple" questions.


    Right now it's 99% done, but I just want to double-check I have the right parts in the right spots you guys told me. Here's what I have so far:
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    HiProfile: yep, that ought to do it.
     
  19. elRey

    Active Member

    Feb 23, 2009
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    Last edited: Sep 11, 2009
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Please don't "hot-link" to other websites (unless you own it), because most webmasters consider that practice as stealing their bandwidth.

    The big difference is that on the other schematic, the charge path for the MOSFET gate is through a 10k Ohm resistor. A MOSFETs' gate is similar to a capacitor; it takes current flow to charge or discharge it. An IRFZ46 power MOSFET has a relatively large gate charge requirement (around 80nC without looking at a datasheet).

    Having a 10k resistor in the gate charge path means that the MOSFET will spend considerably more time operating in the "linear region" than it would with a lower resistance path.

    When a MOSFETs' Vgs is 0v (the voltage on the gate relative to the source terminal), the device is considered fully turned off (near-infinite resistance). When the Vgs is 10v, the device is considered fully turned on (Rds(on) is very low).

    Somewhere in the vicinity of 2v-5v is the "threshold region", where the MOSFET begins to conduct current between the drain and source. From that point until the gate reaches 10v, the MOSFETs' resistance is much higher than when in its' ON state, and it will dissipate power as heat.

    Also, pin 7 is not as capable as sinking current as pin 3 is. This will cause the MOSFET gate to discharge more slowly than if it were discharged by pin 3.

    The idea is to get the MOSFET out of the linear region as quickly as possible, without over-stressing any components.
     
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