ECT and voltage divider help?

[Bernd Felsche]

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.

>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.

>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.

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.

>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
flip-flop is just one way - using the high-level comparator output as
the SET and the low-level comparator output to RESET the flip-flop.

The reference levels of the comparator would be how you set the
on-off levels. Use the potentiometers in series to ensure that the
high reference level will always be higher than the low!

>Any ideas how I can do this?  I don't know a lot of the solid state
>devices.  Would I be using a transitor, op-amp, etc?  

>Will tapping into the ECT wire to read voltage somehow screw up the 
>reading to the ECM by acting as a sink or source?

>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.

The computer (micro-controller) would eliminate all the small 
relays and a heap of nested wiring. Flash-programmable computers are
very cheap - the micro-controller itself typically costs a few
dollars; the big bucks arise due to sophisticated output (mainly
driver transistors), housings and environmental requirements - heat
sinks, etc.

In your application; if you're not worried about how it looks, you
should be able to throw something together for about $50 - assuming
you have access to a suitable desktop computer to write and compile
the programs; and then download them to the micro-controller.

I know what the necessary circuits look like on my car when using
"conventional" setups that do the same sort of thing == UGLY.

>My question is, that he low-amp signal relay (needs to handle about 200
>mA max to trigger the fan relay coil) seems kinda big and bulky.  Is
>there a solid state device that can replace this?  Typically, a relay
>is used to drive a big load with a small signal, but I want to drive a
>small load with a small signal.  Just wondering if there was some kind
>of small IC that would do that instead of a big relay.

A high-power transistor. say a (MOS)FET - don't know how an IGBT would
handle being on all the time.

>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. :-)

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
	http://www.perth.dialix.com.au/~bernie - Private HP
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