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electronics help needed

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Gadabout:
Hi, I am in need of someone more electronic savvy than myself(that means everyone :-) )to help me with a small electronic job, its just telling me what resistors I need to get a voltage drop at certain points, am going to try and attach three diagrams to hopefully explain. I need to know the value of the resistors shown in the diagrams to achieve the voltage outputs in each example please
thanks
Mark

RussellT:
Hi Gadabout

Interesting problem, presumably you're trying to recreate a cruise control switch.  Unfortunately I don't think it's quite as simple as you think as it depends what's in the cruise control ECU CCS connection.  As your diagram shows you get the highest voltage when all three resistors are in series - so the highest resistance.  If the effective internal resistance of the ECU connection is for example 10,000 ohms then having 10,000 ohms in the switch would halve the voltage.  I think to do the calculation you need to measure the voltage at the CCS connection with nothing connected to it and probably with the cruise control engaged.  You also need to connect a resistor in the circuit and see what the voltage changes to.  From that it should be possible to calculate the resistors you want.

On the other hand it's probably easier to connect a variable resistor in place and reduce the resistance until you get the voltages you want and take the resistor out and measure the value.  I would have thought you'd be safe enough starting with a 10k variable resistor - which should give a maximum current of less than 2mA at battery voltage.  That shouldn't be enough to damage anything.  Make sure you start with the resistance at maximum.

Maybe others more expert than me will have different views or more to add.

Russell

mc:
You should be able to get a reasonable resistance measurement, by simply measuring between CCS and GND with the module shut down (that old a vehicle, it'll probably shut down pretty quickly, but would still be wise to give it a couple minutes with the ignition off before measuring).
Try measuring the resistance with a multimeter connected both ways, in case there are any diodes that may upset resistance readings.


Once you know the internal resistance of the module, some series resistance calculations should give you the resistances needed.

philf:
Hi Gadabout,

I agree with Russell and think there's not enough information given to work the values out.

I would try to apply 7.2v to the CCS connection and then measure what current is flowing. This would tell you the input impedance of the Cruise Control ECU which has to be part of any calculation. (Voltage/Current=Resistance - Ohm's Law)

I came up with 3 formulae for the three scenarios:

I used the mean voltages and A, B & C are the three resistors working from top to bottom. D is the input impedance of the Cruise Control ECU.

(B+C)/(B+C+D)=(12-3.45)/12

C/(C+D)=(12-1.6)/12

(A+B+C)/(A+B+C+D)= (12-5.65)/12

Once you know D, C can be evaluated, then B and finally A.

Phil.

[Edited]

RussellT:
I thought about trying to measure the internal resistance of the cruise control ECU, but I thought that it's likely to have transistor switches so it might be difficult to get the correct value.

Russell

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