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By windmill john
#93568
Whilst I know how a relay works and can test for bad earths etc., but beyond that…. So, please help but answer in children’s terms!
As the old quote goes: “It is better to be thought an idiot, than to open one’s mouth and confirm it”.
So, excuse my ignorance below.

As far as I can tell, if you have resistors in parallel, you have an increase in current. Is this why (I believe) there is a second resistor in parallel with the air temp sensor?

If I am right, please explain something. If you have a 10 k ohm thermistor and you add a 5 k ohm resistor in parallel to get about 3.3 k ohm of total resistance, why not just use a 3.3 k ohm thermistor?

Are they made? Does using parallel resistors do something different to the current?

Laugh if you feel the need, but it plays with my head.
By RocketRR
#93569
This explains it really well for you I hope.

In the parallel circuit, the net resistance decreases as more resistors are added, because there are more paths for the current to pass through.
This circuit of the booster plug uses this as a potential divider and thus as mentioned below as you have the same source voltage using Ohms law accurate current flow across the sensor can be calculated.
The two resistors have the same potential difference across them. The current through them will be different as they have different resistances.
The total current in the circuit is the sum of the currents through each branch.
You can use Ohms law V=I x R therefore I = V÷R to calculate the current thru each branch and across the total circuit using the known resistance values and also using 12v as your supply volts (potential). So in this case of the Booster plug its about trickery really, you can interpret Ohms law to also work out the voltage drop across each branch which for us again is important once I has been calculated. In a lot of complex circuits where high accuracy is required you can see this has a lot of uses to make sure components across the divider parallel circuit have the correct current applied.
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By windmill john
#93570
Thanks for the reply. Possibly about to shoot myself in the foot.

Why could you not use a lower resistance thermistor than a thermistor plus a resistor in parallel?

Or is that the trickery you mentioned?
By RocketRR
#93580
Yes absolutely correct the constant 12v supply we talked about earlier is thus shared across the two resistors fooling the ECU depending on the current draw.
Remember the thermister has an operational effect depending on the inlet air temperature even though offset by the parallel path... this is the whole point of the two paths. Remember as well a low resistance single thermister would be high current which would itself be a problem, lower resistance higher current greater heat dissipation. For example a 1k thermister on the 12v potential would be 0.12a whereas this would be reduced by the parallel path in the divider circuit. Mmm not sure if I made that any clearer :roll:
By Andy C
#93584
John.

If you want to know the equivalent resistance of a number of resistors in parallel just punch the numbers in here: https://www.digikey.co.uk/en/resources/ ... s-resistor

It's all explained there.

Re thermistors, they can be NTC or PTC Negative or Positive Temeperature Coefficient, NTC Resistance drops with rising temp, PTC vice verca, they come in a standard range of values and Temp coefficients, and I dont believe that 3.3k is a standard value, hence why you use a parallel resistor to decrease the effective resistance of the Thermistor.

Both NTC and PTC thermistors also have a property which is usually called the Beta value, this relates to how much the resistance changes as the temp changes - they are not all the same.

In terms of an ECU, the Power plug adds resistance, which effectivley adds resistance to the Temp Sensor which makes the ECU deliver a little more fuel in the "open loop" mode, the colder the temp the higher the thermistor resistance so a little more fuel flows, as the temp rises and the thermistor resistance drops the amount fo fuel wil be slightly reduced to compensate for the rise in temp.

Hope this helps.
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By windmill john
#93588
Thanks both. I’ll be blunt, you’re both going too deep for me. Best I can see and something RR mentioned, without the second resistor, the current flow is too high for the thermistor; shorter life?

Feel free to step away, but I am hoping one day, I see the light and suddenly it all makes sense. It not the maths I need help with, it’s the understanding.
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By Wheaters
#93590
I’ve always likened electricity to water flowing in pipes. High voltage across a wire is similar to high water pressure in a pipe. Put two resistances (or pipe restrictions) in series, one reduces the flow, the second reduces it even further. Put them side by side, in parallel, they both allow some flow at the same time so don’t restrict the current/flow to the same extent.
By Andy C
#93592
windmill john wrote:
Sat Oct 17, 2020 4:33 pm
Thanks both. I’ll be blunt, you’re both going too deep for me. Best I can see and something RR mentioned, without the second resistor, the current flow is too high for the thermistor; shorter life?

Feel free to step away, but I am hoping one day, I see the light and suddenly it all makes sense. It not the maths I need help with, it’s the understanding.
The second resistor that is connected in parallel with the thermistor serves to lower the resistance - it has nothing to do with current flow. The 47k resistor in parallel with the 10k thernistor gives a total resistance of 8.2k which one assumes is better suited to the ECU than 10K.

In terms of current slightly more current will flow through the 47k resistor than the 10k resistor simply because it is a lower resistance.

You really dont need to worry about the maths - there are plenty of current / voltage / resistance calculators on the web.

Does this help at all ?
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By windmill john
#93593
Andy C wrote:
Sat Oct 17, 2020 5:18 pm
windmill john wrote:
Sat Oct 17, 2020 4:33 pm
Thanks both. I’ll be blunt, you’re both going too deep for me. Best I can see and something RR mentioned, without the second resistor, the current flow is too high for the thermistor; shorter life?

Feel free to step away, but I am hoping one day, I see the light and suddenly it all makes sense. It not the maths I need help with, it’s the understanding.
The second resistor that is connected in parallel with the thermistor serves to lower the resistance - it has nothing to do with current flow. The 47k resistor in parallel with the 10k thernistor gives a total resistance of 8.2k which one assumes is better suited to the ECU than 10K. I can understand the thinking here.

In terms of current slightly more current will flow through the 47k resistor than the 10k resistor simply because it is a lower resistance. Just go through slightly more current when 47k is higher resistance than 10k? Surely less current would flow? Ohm’s law.

You really dont need to worry about the maths - there are plenty of current / voltage / resistance calculators on the web.

Does this help at all ?
By RocketRR
#93594
In short its electronics to fool your engine into getting more fuel in and it's done by getting bits to work by fiddling the numbers to fit.
Please let's talk engine oil and brake pads again lol

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