paublo999
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| posted on 21/8/09 at 10:18 AM |
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RPM/tacho feed voltages question
Hi All,
I was wondering if one of you kind electronics wizards could give me some advice please.
I am trying to feed a traction control box with an rpm signal. The rpm voltage feed is off one of the coil outputs and is pulsing between battery
voltage (12-14v) down to 2.5v. The traction control box is looking for a dip in voltage of 1.6v or lower?
I know that the voltages are not important bit of info here and that the tacho is using the frequency of the pulses to calculate rpm. So could I just
pop a resistor inline to say half the voltages or is that being too simplistic? The tc unit manual states that it will accept either 0-12v or 0-5v
signals.
Any education in my direction would be much appreciated as this is really not my area of expertise. I am speaking to the manufacturer but would like
some advice in advance of their proposal.
Thanks
Paul.
My videos http://www.youtube.com/user/ninjabogeyracing
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MikeRJ
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| posted on 21/8/09 at 10:38 AM |
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Firstly, is the TC unit ok with the very large voltage spikes it will see from the coil? These will be of the order of 200-300 volts, and quite short
in duration.
If so, then you can quite simply remove the 2.5v offset using either a zener diode (3v should be fine) or a stack of normal silicon diodes wired in
series. A silicon diode drops around 0.6v at lowish currents, so 5 of them should ensure you get down to zero volts.
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paublo999
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| posted on 21/8/09 at 10:48 AM |
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Hi Mike,
We piggy backed of the existing feed to my ETB digidash and it all appears ok. The TC manual also states that a coil feed is ok.
I'm off to google a zener diode now and get myself up to speed.
Will this drop both voltages by a percentage or both by "3v"?
Cheers for the reply.
Paul.
My videos http://www.youtube.com/user/ninjabogeyracing
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paublo999
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| posted on 21/8/09 at 11:34 AM |
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Been suggested to try a 10k resistor before the feed to the tc and then 10k resistor to ground which should divide the voltages.
will try this at the weekend.
Paul.
My videos http://www.youtube.com/user/ninjabogeyracing
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MikeRJ
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| posted on 21/8/09 at 11:58 AM |
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Diodes will give you a voltage offset, i.e. subtract a fixed voltage so you maintain most of the signal but still get down to zero.
A potential divider will reduce all voltages by a fixed fraction. This means you can never eliminate the offset voltage, and a small reduction in the
offset gives a much larger reduction in the high level voltage. However, dividing down by 0.5 will give an offset of 1.25, which should be low enough
to fix the problem.
The input impedance of the TC unit is important when using a simple potential divider; if it's a high impedance (i.e. 10 times or more higher
than the impedance of the divider - 5k) then everything should be fine. If it has a lower impedance input you may find this gives you a much lower
signal than you expect. If the manual doesn't mention this, then you'll have to suck it and see.
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paublo999
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| posted on 21/8/09 at 12:12 PM |
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Cheers Mike.
Really wish I did electronics at Uni now.. 
Thanks
Paul.
My videos http://www.youtube.com/user/ninjabogeyracing
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paublo999
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| posted on 23/8/09 at 03:49 PM |
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two 10k resistors only worked up to 3k rpm so looking for another solution now.
I have an friend who is now willing to help so should be fine.
Thanks all
Paul.
My videos http://www.youtube.com/user/ninjabogeyracing
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