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555 RFI problem: reprise

J

Jordan

Jan 1, 1970
0
Some weeks ago I asked for help with a car blinker circuit, that was
prone to being affected by the (magneto) ignition system.

I think I've found the cause - the 555 IC itself.

I've only checked it on a bench so far, using a spark generator to
simulate the unsuppressed high tension ignition spark.
After breadboarding the circuit for the umpteenth time, I used a
different brand of 555 - a National Semiconductor. I'd previously used
KIA brand, thinking they'd all behave the same. Not the case - the NS
555 doesn't behave erratically with the spark nearby, whereas the KIA
555 always does.
No filtering was used either, just a cap on pin 5.

Lesson learnt!
Here's hoping it'll work OK on the vehicle now.

Many thanks to all who suggested fixes.

Jordan
 
Jordan said:
Some weeks ago I asked for help with a car blinker circuit, that was
prone to being affected by the (magneto) ignition system.

I think I've found the cause - the 555 IC itself.

"Cause" is a little strong here. It's a mix of the brand of 555, the
circuit you designed, the construction technique, and the environment.
Some of those are under your control, others aren't!
I've only checked it on a bench so far, using a spark generator to
simulate the unsuppressed high tension ignition spark.
After breadboarding the circuit for the umpteenth time, I used a
different brand of 555 - a National Semiconductor. I'd previously used
KIA brand, thinking they'd all behave the same. Not the case - the NS
555 doesn't behave erratically with the spark nearby, whereas the KIA
555 always does.
No filtering was used either, just a cap on pin 5.

Presumably the EMI is being conducted in through either ground, power,
or load lines.

Or possibly more than one of the above. (Although common-mode EMI for
such a simple circuit may not be a big deal.)

The cap on pin 5 is very important in any but the most gentle
environments. Bypassing and maybe regulation on the power will help too
(while not really keeping out any sort of truly harsh RFI).

Possibly RF chokes or ferrite beads (I'd lean towards the RF chokes at
first) would help too.

Certainly not all 555's are equal. Just as not all 7805's etc. In the
perfect world we'd engineer our designs so that it didn't matter which
brand the purchasing department bought this week :).

Tim.
 
J

Jamie

Jan 1, 1970
0
Jordan said:
Some weeks ago I asked for help with a car blinker circuit, that was
prone to being affected by the (magneto) ignition system.

I think I've found the cause - the 555 IC itself.

I've only checked it on a bench so far, using a spark generator to
simulate the unsuppressed high tension ignition spark.
After breadboarding the circuit for the umpteenth time, I used a
different brand of 555 - a National Semiconductor. I'd previously used
KIA brand, thinking they'd all behave the same. Not the case - the NS
555 doesn't behave erratically with the spark nearby, whereas the KIA
555 always does.
No filtering was used either, just a cap on pin 5.

Lesson learnt!
Here's hoping it'll work OK on the vehicle now.

Many thanks to all who suggested fixes.

Jordan
are you sure you wasn't working with a CMOS version?
 
J

Jordan

Jan 1, 1970
0
Presumably the EMI is being conducted in through either ground, power,
or load lines.

Tim.

No - it's RFI, or other "wireless" effect. It happens with no wire
connection, with separate power supply etc.

I had several suggestions, and tried most of them. Only changing the
brand of 555 worked.

J.
 
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