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Re: car blinker circuit?

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The horn will be putting all sorts of voltage spikes onto your supply
voltage. This is because most horns are a coil (electromagnet) and pair of
contacts. Each time the horn diaphram is attracted to the
electromagnet, it causes the contacts to open, the diaphram then
returns to it's original position allowing the contacts to touch
again. The magnetic field in the coil collapsing as the current is
shut off causes a high voltave spike, it is this series of spikes that is
causing the timer circuit to reset at odd intervals instead of the cycle
rate you have selected by the component values you used.

You would need to filter these spikes out of the supply to the timer chip
to stop this from happpening..... try putting an inductor in the + supply
line, with a cap to neg on the 555 side of the coil. It could also be
worthwhile putting a 17 volt zener accross the cap to help shunt any
remaining spikes to 0v.



+ ---- ciol ------|-------- 555 power input
                  |
                  |
                 ---   filter
                 ---   cap
                  |   (17 volt zener in
                  |   parralell if desired)
                 0v

The inductor in the supply line will try to keep the current into the
filter cap constant, as inductors resist change in the current through
them. The filter cap will smooth out the remaining voltage spike left on
the supply.



Pip


Re: car blinker circuit?
Thanks Pippa

I'll try that.
I've not worked with inductors. I see various types - RF chokes, ferrite
coil packs, mini coil packs. Which type should I use?
1 to 1000 uH of the RF chokes available - choice not critical?

I really need to fix this, as I've found that just running the engine
causes the same problem!

Jordan

Pippa Reeves wrote:
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Re: car blinker circuit?

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Hi Jordan,

something like CAT. NO. LF1272 from Jaycar should do the job nicely. It is
a 100uH rated at 3 amps DC.... I don't know how yoou have the output of
this flasher circuit wired up, so I assumed that you would probably be
running the full load current through the choke.... that's why I suggest
one that is pretty beefy physically and low DC resistance. If the load
current of the lamps is being switched by a FET or bipolar transistor,
relay etc, you may possibly be able to seperate it's current draw from the
supply that is running the 555 and use a physically smaller choke just
filtering the 555 power, but not the lamp supply.


As for the cap, I would throw something like a 220 uF electrolytic, with a
100 nF polyester cap in parralell..... You may ask why the little baby
size cap accross the electrolytic? It is there because the construction of
an electrolytic makes it not very good at filtering very high frequencies,
whereas the little polyseter one, is designed for RF stuff, so has really
good high frequency response. Because the thing you are trying to filter
out is high voltage pulses, these have a very fast rise time, so to a cap
they tend to look like a very high frequency pulse.


None of these component values are really critical.... after all you are
just trying to 'stomp' on high voltage pulses that are superimposed onto
the supply voltage.

have fun,

Pip


Re: car blinker circuit?
www.telstar-electronics.com


Re: car blinker circuit?
The Telstar unit won't work on 6 Volt systems.

I've given up trying to get the 555 circuit to be stable, having tried
most of the suggestions for filtering. It seems that the RFI from the
magneto is just to strong.
So, I'm using a standard 6V blinker can and load resistors in parallel
with the LEDs - not the favoured solution but it works OK for now.

Heaps of thanks to all for trying to help.

Jordan

Professor wrote:
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Re: car blinker circuit?
The Telstar unit will operate the flashing easily down at 6V... but the
audio feedback may or may not function.

Professor
www.telstar-electronics.com


Re: car blinker circuit?
According to their FAQ:

"Question: Will FlashAlert work on 6V systems?
Answer: No... FlashAlert operates from 8 to 16VDC."

If you say it only affects the audio feature, how do you come to know this?

I'm not sure if Atmel makes the chip for it, but it's the only maker
I've been able to find that makes IC's specifically for electronic
blinker cans, and they also don't support 6V.

Even if it could be used, I might still have the same EMI problem, as
the ignition magneto is unshielded and unsuppressed - I don't want to go
that far.

Thanks
Jordan

Professor wrote:
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Re: car blinker circuit?
How do I come to know this. I'm the designer...

Professor
www.telstar-electronics.com


Re: car blinker circuit?
It's nice to be able to talk to the designer.
Do you have any comments regarding Atmel's recommended supply voltage
range (9V to 15V for U643B)?
I'm assuming you're using their IC's.

Jordan

Professor wrote:
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Re: car blinker circuit?
Actually, the FlashAlert uses the 556 timer chip...

Professor
www.telstar-electronics.com


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