Pullup resistors

[nedge2k]

Would someone mind explaining the whole pullup resistor thing? What's their purpose? How do you know if you need one or which one to use? TIA!

[Efi Guy]

If you use a resistive type sensor such as a oil pressure or oil temp into an input that looks for voltage then you need a pull up resistor and a 5 volt supply to give it the voltage dependant on the resistance of the sensor.

[stevieturbo]

On topic, I was asked about 4 wire coils and a P8Pro.

What role does the pullup resistor have here, and where does it get wired in ? And what ohms would be needed ?

I think it goes between either 12v/5v and the ecu coil trigger ?

[MarcoV6T]

I guess you're talking about amplified coils.
The P8, opposed to the S- series, does not have internal pull-up resistors on the coil outputs, meaning that the outputs are floating when not active(off). As most amplified coils need a positive signal to fire them, you need a pull-up on the DTA coil output, and reverse the action.

Thus when not firing the coils, the outputs need to be grounded(on), when firing a coil the dedicated output turns off, and the pull-up resistor fires the coil.

A good starting point for a pull-up resistor would be 1K, or less. the smaller the resistor, the faster the switching time, but also the more heat it will produce. You'll also notice that when the DTA is configured for amplified coils, the case heats-up when the engine is not running.

Most coils do handle 12V on the trigger, but if it can't, then wire the pull-up one side on the +5V output(or use a resistor divider from +12V), and the other to the output.

[nedge2k]

If you use a resistive type sensor such as a oil pressure or oil temp into an input that looks for voltage then you need a pull up resistor and a 5 volt supply to give it the voltage dependant on the resistance of the sensor.

Just to drag this back up....

So I have a 2-pin oil temp sensor which is 2800Ω @ 10ºC and 80Ω @ 120ºC

How do I determine which rating of resistor I need? Also, how do I then translate the temp/resistance scale I have into mV to use in DTA? Is it V = IR? If so, does the pull-up resistor determine the value of I?

TIA

Edit: Found some info here: http://www.autosportlabs.org/viewtopic. ... 93120bd144

Mentions a 5k resistor, others mention a 1k or even a 2.2k - does it actually matter? Similarly, some say bridge the two pins with the resistor, some suggest you just have it on Vin, which is it?

Amazed DTA manual/wiring sheet doesn't mention much on this.

[stevieturbo]

Wiring diagram, bottom corner of page 6 mentions it.

http://www.dtafast.co.uk/Downloads/Wiri ... iagram.pdf

The actual resistance isnt important, but obviously it will have an effect on the reading. Unfortunately no real easy way of scaling it though without testing which for temperature sensors can be awkward.

[nedge2k]

Had a play with the Vout=(Rsens/(Rsens+Rpullup))*Vin formula in excel and basically it seems that the higher the pullup resistor, the smaller the voltage swing.

For instance, with a 5k resistor across the above sensor, the swing should be 1820mV to 78mV. With a 1k resistor, the swing should be 3705mV to 366mV. Playing further, I found that for this sensor, the optimal resistor with the biggest swing was a 480 with a 4282-707mV swing. Any smaller and it starts to go down again. The lower resistor will cause more current draw but I assume the higher voltage resolution will make the reading more reliable/accurate?

Regarding the actual scale, if you have a sensor with no scale info, just contact the tech support dept. of non-OE manufacturer and ask them for a copy of theirs. Plenty of sites out there that will cross-ref OE part numbers to aftermarket - and vice versa.

[MarcoV6T]

On which pin will it be connected, Ana1-3, or oil temp input?

The dedicated oil temp sensor input on the S80, does not need(has it internally) a pull-up resistor.

Thus, which input will you be using for that sensor?

[nedge2k]

The dedicated oil temp sensor input on the S80, does not need(has it internally) a pull-up resistor.

Unless I'm going mad, can't see that mentioned in the manual or wiring diagrams? Makes sense though, seeing as there's a few pre-defined sensor scales - made me wonder why they hadn't specified a resistor to use anywhere.

So I take it that is true for all the temp. sensor inputs? Is it a 1k internal pullup like the coils (so i can figure out the scale)?

[lumley32]

DTA uses a 1K to 5v on any pin thats for temperature.