How to use transistor to add resistor in series w/ circuit?

[bigjoncoop]

I know I'm over thinking this...

Is it possible to add a resistor in series with a wire/circuit using a transistor?

Im currently using a small trimpot to add a variable resistance in series with a circuit / wire, but instead of having to manually adjust it carefully periodically from adding between 80ohm - 100ohm I just want to use an Arduino to control a few transistors to either add a 80ohm, 90ohm, or a 100ohm resistor in series.

But I can't for the life of me figure out how use the transistor to put the resistor in series! I've already built the circuit but I wasn't paying attention and I'm adding the resistor in parallel which is obviously not what I'm trying to do....

Thx

[tom66]

Transistors need a bias signal, for a FET this is a voltage across the gate-source region and for a BJT then this is current through the base-emitter.

This means that if the source/emitter voltage varies, it can be difficult (though not impossible) to use a transistor to do this.

The simplest workaround is to use something like a relay or optoisolated FET.  A more complex workaround is something like an analog switch such as a CD4053 or 74HC4066.  Be aware that these switches still have limitations as their on-resistance will vary across signal levels, so for any analog system you would need to model this to ensure the change in on resistance is not significant enough to cause you issues.

[magic]

What sort of voltages exist at each end of the variable resistor and what current flows through it?

What power supplies are available?

[BrianHG]

Digital potentiometers (Digipot) exist.
SPI input, outputs using different ranged devices can go from 5k, 10k, 25k, 50k, 100k.
+/- 18v analog signals, or, 0-36v.

https://www.microchip.com/en-us/product/MCP41HV51

Other vendors have their own versions, like TI, Analog Devices...
Smaller resistance ranges exist, like 1k.

[BrianHG]

If your feed through signal is only positive to a positive load, then you can do this with a few 3 mosfets for the switches, and then 3 more for the level shifters with a few 1 meg resistors if you can tolerate a 1 megaohm load on your output.

The other option is 3 opto-mosfet switches which can have fairly low turn-on impedances below 1 ohm.

[bigjoncoop]

Let me add some more details:
(Not everything but enough to help you understand what I'm trying to achieve 😜)

The circuit I am adding resistance to is a car 2-wire temperature sensor (ECT Sensor). It's a 5v circuit with 2 wires. (5v & Gnd obviously)

As the temperature changes the sensors resistance changes. The higher the temperature the lower the resistance.

Currently I just cut one of the wires and have a modified potentiometer (that goes from 80 - 100 ohms) connected in series on the cut wire. That way I can ADD between 80 - 100 ohms to the circuit in order to trick the cars computer in thinking the temperature is just a couple / few degrees cooler then it actually is...

The purpose of this post is I'm trying to figure out a simple solution to use a Arduino in order to vary the added resistance (80 - 100 ohms) instead of manually having to adjusting it.

The end goal is to have the Arduino monitor that the temperature sensor and automatically add the needed resistance to keep the car thinking it's at a certain temperature.

(originally I was trying to accomplish this by just using some transistors and resistors. I have boatloads of other components, but as we all know you can never have everything that you need and I'm just trying to avoid having to buy even more components to add to my collection if I can make due with what I already have...

I do have a bunch of 5v relays I can use to accomplish this if all else fails.

***Someone over on the Allaboutcircuits forum, just recommend this circuit. Your thoughts?

[ Specified attachment is not available ]

[magic]

Maybe this?

Code: [Select]

original thermistor --- 80Ω --- [10Ω || switch] --- [10Ω || switch]
where || means parallel connection of a resistor and a switch. Closing a switch effectively shorts the resistor. If the ECU always applies a higher voltage to one end of the resistor chain and a lower voltage to the other, the switches can simply be N-ch MOSFETs, with gates driven between 0V and 12V.

(Note that the "12V" in a car is not exactly 12V, it must be heavily filtered and protected from transients.)

[EPAIII]

Wouldn't FETs be better than transistors?