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Registered Member #4537
Joined: Sun Apr 22 2012, 06:59AM
Location:
Posts: 9
Hi,
I have a serious problem which occurs randomly.
I have a low voltage 5v section with a LM555 set to "one shot" which means when i press a switch, the 555 output goes high for X miliseconds. the 555 output goes to an optocoupler that ignites a trigger coil (ignition coil) and the coil should ignite a Xenon lamp.
the problem is, when i trigger the 555, on the scope i can see that the output rises immediately to about 4-5v, but after 50 uSec, it has a big spike downwards, even lower than 0v, and it recovers again very quickly to the high level. the thing is, that i randomly have the flash about 100ms after triggering the 555. most of the times the flash happens immediately, but about 1 time every 20 triggers, the flash fires in some 100ms delay.
What i believe is happening, that the primary High voltage pulse is creating bad stuff on the 555, causing the spike downwards, which is causing maybe randomly the optocoupler not to fully turn on enough time, and only after the 555 output recover the spike downwards, it rises again and turns on the optocoupler fully again which triggers the lamp in delay.
both the Xenon lamp and the 555 share the same ground. they have to. the 555 is powered via L7805 from a 12v battery which is also charging the high voltage capacitors.
on lower voltages of the xenon flash and trigger it happens much less often.
any idea how to reduce the spike or make the 555 immune to the spike ? I already have plenty of 10uF ceramic bypass capacitors all around. tried even on the 555 output, no help...
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Without the schematic I can only guess what is happening. Verify the ground connections. The current pulse that passes through the lamp shall return directly to the capacitor. If the 555 ground is in the way you surely have a problem. Ground everything in a tree to the same point, and use bypass capacitors across the power pins of the 555 and between pin 5 and ground.
Registered Member #4537
Joined: Sun Apr 22 2012, 06:59AM
Location:
Posts: 9
HI, I do have a capacitor on pin 5, a bypass capacitor of 10uF ceramic on the supply of the 555, and on the input and output of L7805 - really everywhere
the 555 ground is coming from the opposite side of the circuit, from the L7805 although electrically connected with the other grounds (of the lamp and trigger coil)
I thought of the diode between the primary coil of the trigger transformer - and i made a test: i kept the trigger coil connected, and just disconnected the output HV wire from the lamp. it stopped doing these spikes. which means to me that the spikes are caused by the HV output of the coil - not the primary.
but then i did another test - i disconnected the + of the lamp from the big capacitor (to check how the HV pulse is returning to the circuit) - it didn't help. then i disconnected the "-" minus of the lamp from the capacitor (and common ground) leaving only the HV trigger output connected to the lamp, a few CM away from the circuit - and then again the spike stays.!!! with no way of return to the circuit. it seems to do the spikes when the HV output trigger is connected to a large plane (the lamp) regardless of a return path to the circuit... MAYBE when it is connected to a large plane is acts as a load to the trigger coil and thus increasing the spike on the primary after all ? it might be that
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Verify if you have high-impedance nodes in the circuit around the 555, that may behave as antennas receiving interference from the HV pulse. Try to place a grounded metal plate over the control circuit. And verify how you are connecting the oscilloscope, as the probes may act as antennas too.
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High voltage pulse affecting 5v logic
