MC3334 question

[dn]

> Michael Fawke wrote:

> Pardon my ignorance, but I am a computer programmer and my knowlege of
> analogue is _very_ limited....

Ya, I feel sorry for you electrically challenged individuals... 
(just kidding! 8-)

> On the MC3334, when used to drive an MJ10012 (?) with current sense, the
> circuit diagram I have shows a .027 Ohm resistor bewteen ground and the 
> MJ10012, with the sense taken from the MJ10012 side. In Tim Drury's article
> on Distributorless ignition, he gives a circuit for the ignitor which does
> not have this resistor. I am assuming the resistance is necessary for the
> current sense to work, - so what can be used to create this resistance, or
> can it really be discarded (as per Tim Drury's design)?

After a little data book browsing,  I see MC3334 has an overcurrent limit
threshold of minimum 120 mV, typical 160 mV, maximum 190 mV.  This means 
that at the current level you want to limit at, the voltage across the
current sense resistor has to rise to around 160 mV to trip the current 
limiter circuit.

In the data book, they show a circuit with a 0.075 ohm resistor, and a further
adjustable voltage divider into pin 8 (the current limit input).  This circuit
is setup to limit at 5.5 A.  If you multiply 5.5 by .075 ohm, you get 412 mV 
across the resistor Rs.  This is divided further by a resistive divider 
circuit, comprised of RD1 and RD2.  They give a formula in the data sheet for 
calculating RD1 and RD2 for the desired current limit.  Tim Drury may not 
have used the current limit, but I wouldn't recommend it unless you use a 
coil which requires a separate ballast resistor.  The different value for Rs
in your circuit is 0.027 ohms, which would give a current limit of about 6 A.
Your circuit possibly doesn't use the voltage divider shown in my data sheet, 
which is added to make the current limit adjustable.  

As far as finding a resistor for Rs, low value power resistors are kind of 
scarce.  You could consider paralleling several smaller resistors to get the 
required value.  Let's see, this resistor would dissipate about 2.5 W @
6 A (I^2*R).  You could parallel 4 X .1 ohm, 1W resistors (or 10 X .27 ohm
.5W) to give .025 ohm @ 4W (.027 ohm @ 5W), more than sufficient for the task.

> Also, how do you pick a coil to work with this design? Most coils I have
> seen (single output) are sold with no info on resistance. Can the M3334 (when
> used with current sense) use _any_ coil? I am currently using a coil with 
> a ballast resistor and would prefer not to fork out another $50 Australian
> for another coil :)

Any coil will work, the circuit works by limiting the maximum current into
the coil (which keeps it from heating up at low RPM's), which also eliminates
the need for the ballast resistor.  The ballast resistor ALSO limits current 
at HIGH RPM's, though, which decreases ignition energy and kindof defeats the 
purpose... If you decide NOT to use the current limit in the MC3334, then you 
WOULD need the ballast resistor.  To disable the current limit, tie pin 8 
to ground.

regards,
dn







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 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
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