ECT and voltage divider help?

[Daniel Houlton]

Bernd Felsche wrote:
>
>Daniel Houlton writes:
>
>>I'm working on a schmatic for a small controller for an electric 
>>radiator fan and I have some questions on reading the ECT sensor.
>
>>Doing some testing on my truck I found that the sensor is fed 5V from
>>the ECM (measured with the wire dis-connected from the ECT), but 
>>connected it only reads 3V when cold.  The voltage then drops to 
>>around .65 V at normal operating temp.
>
>Looks like nominal behaviour for any NTC device. You're probably
>seeing the result of a constant-current supply being used to
>determine the sensor's resistance - a constant current through a
>variable resistance will give a variable voltage.

That was my first thought as well, but the numbers weren't agreeing.
i.e. The voltage and resistance when cold gave a different current
than the same readings when warm.

After finding a page showing a resistor internal to the ECM and how
it's set up as a voltage divider though, I did the math again to
find the resistance of the unknown resistor in the ECM and I got 
the same answere (within a few percent anyways) with both the hot 
and cold readings.



>
>>It's not really important, but I was somewhat confused how hooking
>>the wire to the ECT sensor caused the voltage to drop from 5V to 3V
>>(or .7V or so when hot).  After some research on the net, I found that 
>>it's set up as a voltage divider.
>
>Test the sensor in isolation; switch off the engine, unplug the
>sensor and measure the resistance using a multi-meter. If the
>resistance falls with increasing coolant temperature, then it's
>probably an NTC resistor.

Yes, this is how I measured the resistance.  Unplugged the connector
and measured the resistance, then plugged it back in, turned the key
on and put a volt meter on it to get the voltage.  And yeah, the
resistance drops substantially as it warms up.


>
>>So, my question is, how do I switch something based on this voltage?
>>Basically, I want to monitor the voltage to the ECT sensor and when it
>>drops to around .4 or .5 V I want to trigger an output (that eventually
>>drives a relay).  Then, while the output is triggered and the voltage 
>
>First; you need to isolate your circuit from the other one using the
>same sensor. Using an op-amp is the easiest way of doing that - Op
>amps have input impedances in the megaohm range. Isolating the
>sensor gives you a buffered output signal - you might want to have a
>small gain set up on the op-amp for convenience - else keep it at
>unity.

Can you give me a hint on doing this?  I've been looking at a little
Radio Shack book on op-amps.  It give several circuits, but I don't 
know what they're used for.  i.e., Basic Inverting Amplifier, 
Transconductance Amplifier, Transimpedance Amplifier, Single-Supply
Amplifier, Summing Amplifier, etc.  They have brief descriptions, but
none indicate that they'd be used for sensing a voltage level without
changing that level.  Is this just a basic non-inverting amplifier?

Why is a small gain a convenience?  What's it do?


>
>You can then feed the signal into a "comparator"; an op-amp can be
>used for that as well. The comparator is basical a "switch" that has
>a low output when the sensed input is less than the reference
>voltage, and the output is high when the sense level is higher than
>reference. Depending on the op-amp, you can then drive the relay
>"directly" from the comparator output.

I saw the comparator circuit too.  It's just an op-amp without a
gain feedback right?

Op-amps need + and - voltage.  How would I get the - voltage?  I can
get 14.7V from the battery, but how do I get it to be negative as
well?

Also, how would I get the .6V reference that I want?  Just tap into
the 14.7V power and use a resistor or pot to drop it all the way to
.6V?  That sounds the easiest.



>
>>goes back up to around .7 V to turn the output back off.  And I want
>>to be able to fine-tune the on and off voltages with a couple pots.
>
>If you need hysteresis, then the circuit gets more complex. You'd need
>another comparator and then combine outputs to set and reset the
>corresponding driver output. There are dozens of ways of doing that; a

That looks like what my little book would call a Window Comparator.  Is
that right?  It looks like this would do that.  I'm not so sure I need
to though.

I'm thinking now that I can just trigger the fan relay when the ECT
sensor drops to about .6V (this is when the ECM first kicks up the 
rpm for the standard engine driven fan), then set up a 555 timer so 
that it runs for say 30 - 60 seconds after the voltage goes up over
.6V again.  (This would be a monostable or one-shot 555 configuration
right?).  That way it wouldn't cycle on and off so frequently.


>>Also, I'm including several inputs that can trigger the fan relay like
>>the A/C clutch, air compressor clutch, and a couple extras as well as
>>a manual on, manual off (which over-ride the other inputs) and an 
>>automatic setting.
>
>In that case, you may be better off with a small computer with an
>analogue to digital converter and several digital inputs. You then
>also have the ability to drive the radiator fan at a speed according
>to the load by using a pulse-width-modulated drive instead of a relay.

I've been playing with Atmel controllers for running additional 
injectors as well, but this projects seems simple enough to not really
need a micro.  (Besides, I'd actually like to finish it.)


>>Oh yeah, one more thing.  I've found sockets for different types of
>>relays in Jameco and a couple other catalogs, but they aren't very
>>high amperage and I'd rather use a common automotive 30A relay.  I
>>can't find board mounted sockets for these automotive relays though,
>>just pigtail sockets with wire leads.  Anybody know where I could find
>>a board mount socket for these?
>
>Turn the problem upside-down. See if you can fit your logic circuits
>into a relay case, find a relay plate and plug it in. A relay plate
>is just a moulded plastic section designed for you to plug in a
>relay - you clip a few together and wire the connections as needed
>using conventional automotive spade connectors.
>
>Auto-electricians will never guess you have a computer in one of
>those tiny relay cases. :-)

Actually, I did find one.  A board mount socket for automotive relays
in an old Newark catalog. 


thanks alot,

--Dan
houlster at inficad.com



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