# Reversing resistance

#### TedZ

Joined Nov 25, 2011
6
Using sample values say i have a sender that provides a range of 0 to 100 ohms and i have a gauge that requires 100 to 0 ohms. I can think of a lot of complicated ways to do it but i can't think of a simple way using a few discreet components. Circuit needs to have reasonable temperature stability. Anyone have any ideas.

Thank you.

Ted

#### Wendy

Joined Mar 24, 2008
23,396
Welcome to AAC!

The term you are looking for is negative resistance, and there are many, many ways to do this, some more complex than others.

#### MrChips

Joined Oct 2, 2009
30,517
Bill, I think the OP just wants a conversion where 0 ohms -> 100 ohms and 100 ohms -> 0.

A negative resistance circuit is something else. It is possible and does exist and is used in regenerative braking of motors, for example. I once had to modify the motor speed control of a cassette recorder for data recording and had to build a negative resistance circuit to give instant start and stop of the cassette motor.

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#### TedZ

Joined Nov 25, 2011
6

I am familiar with the term negative resistance but that is a marginally linear circuit that is not going to be very simple.

I was hoping for a simpler way. There may not be one.

Ted

Joined Dec 26, 2010
2,148
If the requirement is to obtain a reversed indication on a meter in response to a sensor variation, then a truly resistive output may not be required. It may be sufficient to have a DC amplifier deliver an appropriately scaled output voltage or current to the instrument.

For instance, the original sensor could be put into a bridge arrangement driven from the same supply as will be used to drive the meter, set up to give zero output at maximum resistance. The resulting output will increase as the resistance is wound down. The result will not be perfectly linear, but may be acceptable for some purposes if the sensor is a fairly small percentage of the resistance of its bridge arm. The net result of a current output proportional to the bridge error could then give a fair approximation to an inversely varying resistance.

#### SgtWookie

Joined Jul 17, 2007
22,230
Sounds to me like TedZ wants to adapt a fuel gauge from one motorcycle or automotive brand to another - isn't that about it, TedZ?

#### TedZ

Joined Nov 25, 2011
6
Thanks Adjuster, that's a thought i need to try it with the gauge.

Wookie i'm a boater besides doing a lot of electrical, electronic work. My old Lyman is 6 VDC and the senders are rare. Trimming 12 VDC sender is no problem but reversing the resistance is. However the thought of just providing a voltage that the gauge is looking for could be done in a lot of ways, Adjuster's probably the easiest. Doing it with a circuit allows me to but a little board under the dash with say a NIC (INIC or VNIC i need to think about it and see what really drives the meter) and that would be pretty linear and easy also.

Thanks. You set me in the right direction saying control the voltage, forget the resistance!!

Ted

#### JMW

Joined Nov 21, 2011
137
Lyman? You definitely need to send pix. There is no way to determine what is going on w/o pix. Learned to ski behind one in '57

#### TedZ

Joined Nov 25, 2011
6
OK Need to have 10 characters!

Ted

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#### THE_RB

Joined Feb 11, 2008
5,438
Most senders are resistive amd the gauges are analogue voltmeters.

It is probably as easy as inverting the position of the voltmeter in reference to 12v or ground, or reversing the sender position in reference to 12v or ground.

If that fails an opamp can be used to invert the analogue voltage signal.

Joined Dec 26, 2010
2,148
I had not realised this was a fuel gauge, although looking back at the original I really ought to have guessed. One thing that the OP meeds to take very seriously is that however the sensor is connected, it must never be driven with currents or voltages beyond those it was designed for.

For instance, one mistake with a variable resistor is to connect it so that a very high current will flow when the resistance is turned down. This may include things like connecting excessively large capacitors directly across the resistor in an attempt to get a stable reading. In the context of a fuel gauge, this could easily lead to a fire, or worse.

#### TedZ

Joined Nov 25, 2011
6
Thanks. Oil pressure and temperature senders. No fuel gauges in a 1956 Lyman!

The sender is purely resistive. The guages get six volts DC and depending on the resistance to ground, read progressively. At say 0 ohms it is full scale. At 100 ohms it is at zero. I need for example the exact opposite to occur, BUT... It has been suggested to control the voltage to the gauge period and use the senders varying resistance in a circuit of some sort. I haven't spent enough time on it but my latest thought is using a 555 driven by the voltage through the sender as a VCO and as DA converter to provide a DC output that can have any slope and range your heart desires. I can filter the output to make lousy DC but it is only a gauge!!

Ted

#### SgtWookie

Joined Jul 17, 2007
22,230
This might sound like a silly question, but does the engine have a generator?

I really don't know; I haven't fiddled with a powerboat of that vintage. What's the engine?

Many years ago, our family had a '55 Evinrude 7-1/2hp outboard on a boat my great-granddad built. I picked up a '56 Johnson 7-1/2hp and rebuilt it. I also had a 1927 Johnson 1-1/2 HP outboard that I built a boat for when I was a teenager. But, never had an inboard powerboat - we were more sailors.

With a 6v system, your battery should measure ~6.3-6.4v, and when the motor is running, the generator should output 6.9v to 7.1v. Is that about what you get?

Have to ask these kinds of questions to get background info.