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AJay

Solid state Voltage regulator

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I have been experiencing increasing problems with my temperature gauge and fuel gauge and have decided to do something about it. After some investigation i found that the gauges operate a a regulated 10volts, this is controled by a very archaic device called a voltage stabiliser........

The Smiths voltage stabiliser is a mechanical voltage regulator that reduces and maintains the instrument operating

voltage at 10v dc. Inside the stabiliser is a bimetallic strip, an insulated heating wire coil, and contact points.

1) The contact points are located inside the stabiliser housing. One side of the points is on the “B” terminal, the

other side is on one end of the bimetallic strip attached to the “I” terminal.

2) The bimetallic strip carries current between the contact points and the “I” terminal.

3) The heating wire coil is wound around the bimetallic strip. It is connected between the contact point on the

bimetallic strip and chassis ground.

“E” terminal (ground or Earth)

“I” terminal (to Instruments)

“B” terminal (from Battery) Contact Points (inside, not visible

When the ignition switch is turned on, the “B” terminal sees full battery voltage. Initially the stabiliser points are closed.

Current is carried across the contact points, through the bimetallic strip, and provided to the “I” terminal. Thus, when the

ignition is first turned on, full battery voltage appears on the “I” terminal of the stabiliser. Remember that one end of the

heating wire is also attached to the contact point on the bimetallic strip. Thus, when the ignition is switched on, current

also begins to flow through the heating wire to chassis ground. The resulting heat causes the bimetallic strip to change

shape and “open” the points. This breaks the current flowing to the “I” terminal and through the heating wire. The

voltage on the “I” terminal drops to zero (0) volts when the points open and the heating wire cools. Without heat the

bimetallic strip returns to its original shape closing the points. This restores current flow to the “I” terminal and the

heating wire. The cycle repeats several times a second.

The result of this on-off cycling produces a reduced “average” voltage on the “I” terminal. The voltage is not a steady

10v, but switching occurs with a frequency that approximates 10v. Because of this fast switching, it is virtually

impossible to measure the operating voltage of a stabiliser directly using a volt-ohm meter.

Now for the interesting bit.......

All the above can be replaced with a little device called a 10volt regulator

DSC01292.jpg

This little device can be fitted within the old Stabiliser for an origional look

DSC01288.jpg

The old unit can be dismantled by bending back the tabs on the side of the casing and removing the bottom section.

DSC01289-Copy.jpg

This shows the old bimetalic strip as discussed above.

This is bimetalic strip is removed and the regulator replaces it being soldered to the contacts.

DSC01294.jpg

DSC01295.jpg

Carefully place the circuit board back into the housing. Use pliers to gently fold the sheet metal back to secure it.

DO NOT allow the short wire to become pinched.

DSC01296.jpg

Test your work. Connect your vehicle’s ground to the case of the stabilizer. Connect battery voltage to the “B”

terminal. Connect a volt-ohm meter between the “I” terminal and chassis ground. If you do not observe 10v

you will need to open the unit up and check the circuit again.

Edited by AJay

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The finished item

DSC01297-Copy.jpg

This can then be reinstalled in the car with the case attatched to origional fixing as this provides the ground throught the case.

The Voltage stabiliser filter (a small black box installed between the regulator and the instruments) can now be removed, this was used to proved the instrumenst with something resembling a smooth dc supply.

The whole cost of this conversion was 49p

The voltage regulator came from CPC

http://cpc.farnell.com/stmicroelectronics/...equestid=531009

I now have rock solid water temp and fuel gauges :)

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Or you could buy a solid-state regulator that looks just like the original piece from JAE in California.

For less than a Dollar??

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Just followed the link to the regulator. This part will stop regulating when the input voltage drops below 12.5v. That means at 12v you are getting out 9.5v and at 11.5v you get 9v.

Worth noting when you are idling or just have accessories on.

These things are known as linear voltage regulators. If you want stable voltage even when the battery is discharging, you need a switch-mode regulator.

Dave Everett

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@Ajay, as mentioned in the post above, it can’t work at lower voltages. It can only work if you already have a stabilized 15V power supply in your car somewhere. Since most models probably will just have a 12V supply to operate from, a small voltage regulator circuit has to be used in that case. Most switch-mode supplies will require a small circuitry with it which won’t be that easy to fit it. 

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@Ajay, as mentioned in the post above, it can’t work at lower voltages. It can only work if you already have a stabilized 15V power supply in your car somewhere. Since most models probably will just have a 12V supply to operate from, a small voltage regulator circuit has to be used in that case. Most switch-mode supplies will require a small circuitry with it which won’t be that easy to fit it. 

original posts are 5 years old

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The LM2940T-10.0 only drops 0.5V, or 0.2V at lower currents.

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Thanks for your post.  I have a 1967 Series Land Rover with similar gauges and the voltage regulator recently shorted out (in the middle of a busy intersection, naturally).

 

I followed your instructions except I did not bother to incorporate the regulator housing since I am not concerned with aesthetics.  The regulator I used is a 1.0 amp 10 V (NTE1953) . The problem I have now is that the gas gauge reads "full" at all times.  Since I am not an electrician I am not sure why.  I tried reversing the polarity but then it reads "empty."  Thanks for any advise.  I would really like to keep a solid state regulator if I can figure out this issue rather than go back to the old fashioned one.

 

Sincerely,

 

Dave G.

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Hi Dave

The gauges are really quite voltage sensitive and a few 100mV will have them moving.

Yours are clearly over-reading with the regulator that you've made which suggests the voltage it too high.

Have you checked the output because it sounds like it could be short-circuit and giving you the full battery voltage?

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