Connecting two temperature gauges to a single sender

[Puggy]

Hello all,

Hopefully a simple question. I have two engine water temp gauges, one in the pilot house and one on the cockpit. Currently they have a switch to change between them. I want to wire them together so they read concurrently. If i wire them together in parallel, they both work but read 50% of the correct temperature. Thinking about this, i assume the temp sender has a changing resistance according to the engine temperature, and the gauges are basically ohm meters, and connecting two of them has altered the resistance in the circuit in some way. But my physics is struggling from this point on.

I am sure there is an electrical expert who could explain the principles and suggest a way of making it work? Can I add a resistor to the circuit? Or should i wire in series instead?

Thanks for your help

Piers

 

[nimbusgb]

Best solution would be a current mirror made from a simple operational amplifier or like this.

use gauge 1 as the reference and put gauge 2 in line with Q2 - Vout

 

[VicS]

Above is one solution but without details of components and how to connect it will be difficult to do.

Worth trying them in series I'd think. Even if the readings are not absolutely spot on they may be near enough.

 

[scottie]

Buy a dual station sender if it is available VDO do them for example

 

[nimbusgb]

Above is one solution but without details of components it will be difficult to achieve.

Worth trying them in series I'd think. Even if the readings are not absolutely spot on they may be near enough.

In series the resistance of the two meters will be doubled thus halving the current through the circuit. ie series or parallel you are going to read 1/2 the real deal.

get a digital multimeter put it in series with one of the gauges measure the current. My guess is that any general purpose NPN and PNP pair will work. Only a couple of hundred mA going to be needed at most.

 

[VicS]

In series the resistance of the two meters will be doubled thus halving the current through the circuit. ie series or parallel you are going to read 1/2 the real deal.

Depends surely on the reistance of the sender compared with the resistance of the two meters. If the resistance of the meters is low compared with the resistance of the sender you might get a usable result.

Correct though if the resistance of the meters is high.

Thought it worth a try. All the wiring is there in one place ( at the switch) so very easy to do.

"Any general purpose pnp or npn pair will do" . means nothing to most people.

 

[Converging]

please could someone put that in knob head speak please?

 

[VicS]

please could someone put that in knob head speak please?

Which bit dont you understand?

 

[VicMallows]

I want to wire them together so they read concurrently.

I'm going to go off on a tangent: Seems odd that you are so concerned with monitoring the temperature? Do you really spend time looking at the temperature gauges? (I'm guessing this is a MOBO). Surely you have an alarm which sounds if the temperature becomes unacceptable and can then look at the appropriate gauge?

Vic (causing trouble)

 

[Ubergeekian]

Hopefully a simple question. I have two engine water temp gauges, one in the pilot house and one on the cockpit. Currently they have a switch to change between them. I want to wire them together so they read concurrently. If i wire them together in parallel, they both work but read 50% of the correct temperature. Thinking about this, i assume the temp sender has a changing resistance according to the engine temperature, and the gauges are basically ohm meters, and connecting two of them has altered the resistance in the circuit in some way. But my physics is struggling from this point on.

No such thing as an ohm meter[1]. All mechanical meters are ammeters[2] and all electronic meters are voltmeters.

Old-fashioned automotive style temperature senders are thermistors (temperature-sensitive resistors) and the gauges are ammeters. That sound like the setup you have: when you put two ammeters in parallel each one takes and reads half the current.

So ... put them in series. Easy. The additional resistance may reduce the readings a wee bit, but probably not enough to matter. Check with an ohmeter[3] - if the resistance of a gauge is less than 10% of the sender resistance at operating temperature you'll be fine.

[1] Well, you can get a meter to measure resistance, but it does it by measuring the current flow for a known applied voltage, so it's really an ammeter[4]
[2] Except for some very rare electrostatic voltmeters. I have one. It's three feet high and weighs 30kg. I use it as a towel rack
[3] See [1]
[4] Which may be implemented as a shunt and an electronic voltmeter.

 

[VicS]

So ... put them in series. Easy. The additional resistance may reduce the readings a wee bit, but probably not enough to matter. Check with an ohmeter[3] - if the resistance of a gauge is less than 10% of the sender resistance at operating temperature you'll be fine.

That's exactly what I had in mind when I suggested trying them in series

 

[VicS]

I'm going to go off on a tangent: Seems odd that you are so concerned with monitoring the temperature? Do you really spend time looking at the temperature gauges? (I'm guessing this is a MOBO). Surely you have an alarm which sounds if the temperature becomes unacceptable and can then look at the appropriate gauge?

Vic (causing trouble)

No, a Nauticat 43 ketch

 

[Puggy]

Thanks all for suggestions. I will try in series and see what the result is. As VicS says, not a MoBo but Nauticat 43 ketch. Yes i do have an alarm etc, but i just don't like the clunky switch arrangement to switch the instruments upstairs, and wondered if i could improve it. I also reckoned i can't be the only person to have pondered this question..

I see the ASAP supply Faria instruments and offer a sender unit for twin temp gauges so might consider putting that on my christmas present list...

P

 

[VicS]

I see the ASAP supply Faria instruments and offer a sender unit for twin temp gauges so might consider putting that on my christmas present list...

Be sure its a suitable sender for the instruments you have.

If it works in series it will be the simplest solution, with no extra wiring ...

esp if you don't know your PNP from your NPN, or your current mirror from Alice's looking glass.

 

[Puggy]

Be sure its a suitable sender for the instruments you have.

If it works in series it will be the simplest solution, with no extra wiring ...

esp if you don't know your PNP from your NPN, or your current mirror from Alice's looking glass.

Yes. Good advice. I will try the series first. I certainly dont know my PNP from my NPN (and Wikipedia wasnt that helpful!)

 

[VicS]

Yes. Good advice. I will try the series first. I certainly dont know my PNP from my NPN (and Wikipedia wasnt that helpful!)

The ones in nimbusgb's diagrams are NPN.
PNP have the arrow pointing the other way.
That's pretty near the limit of my knowledge

But by tomorrow there will be a dissertation on here from WilliamH explaining the difference I'll bet!

 

[omega2]

When the siren goes off it's too late, the engine is"cooked" Buy a dual station sender ASAP do them.