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Ray White
Joined: 02 Dec 2014
Posts: 7701
Location: Derby
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 Posted: Fri Sep 11, 2026 1:31 pm Post subject: Isolated power supply. |
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Please can someone (probably called Dave!) explain how an 'isolated 12 volt power supply' gizmo works when fitted to a positive earth car to get round electronic components being polarity sensitive.?
I have one fitted with my fancy hydrostatic petrol gauge and although it works O.K. I don't understand how.
It would be good to know how it functions as I am considering replacing my noisy brake servo pump with a quiet vane type that has polarity sensitive electronics built in and I am assuming I would need such a box of tricks to get round the issue of the car being positive earth?
If I could do that it would be a big improvement on the present arrangement.
Of course I may be completely wrong on many, if not all counts... but ignorance is bliss apparently. !
Thank you. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4418
Location: South Cheshire
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| Posted: Fri Sep 11, 2026 5:41 pm Post subject: Re: Isolated power supply. |
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| Ray White wrote: | Please can someone (probably called Dave!) explain how an 'isolated 12 volt power supply' gizmo works when fitted to a positive earth car to get round electronic components being polarity sensitive.?
I have one fitted with my fancy hydrostatic petrol gauge and although it works O.K. I don't understand how.
It would be good to know how it functions as I am considering replacing my noisy brake servo pump with a quiet vane type that has polarity sensitive electronics built in and I am assuming I would need such a box of tricks to get round the issue of the car being positive earth?
If I could do that it would be a big improvement on the present arrangement.
Of course I may be completely wrong on many, if not all counts... but ignorance is bliss apparently. !
Thank you. |
If the whole thing is insulated or isolated (specifically the outer case) then it can be bolted to anything and all (both in this case) the electrical connections have to be made with wires, rather then than relying on a case to chassis earth.So the device is essentially polarity independent. |
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Ray White
Joined: 02 Dec 2014
Posts: 7701
Location: Derby
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| Posted: Fri Sep 11, 2026 6:26 pm Post subject: |
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| Would I wire it up with what would have been positive earth to the battery going to the earth point on the black box? |
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Ray White
Joined: 02 Dec 2014
Posts: 7701
Location: Derby
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ukdave2002
Joined: 23 Nov 2007
Posts: 4418
Location: South Cheshire
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| Posted: Thu Sep 17, 2026 1:01 pm Post subject: |
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Hi Ray
What is the actual problem (specifically device) that you need to address? |
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Ray White
Joined: 02 Dec 2014
Posts: 7701
Location: Derby
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| Posted: Thu Sep 17, 2026 3:48 pm Post subject: |
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| ukdave2002 wrote: | Hi Ray
What is the actual problem (specifically device) that you need to address? |
The issue is to do with my brake servo vacuum pump. it will be remembered that I was unable to used the manifold depression due to the supercharger. When I did have it connected the engine would stumble and cut out when ever the brake pedal was depressed.
My solution was to install an electric vacuum pump in the void above the rear axle. The pump I have fitted is a simple "piston" type and although the system works well enough, it is exceptionally noisy when the ignition is switched on. Yes, the racket goes away after about 20 seconds but it is - to say the least - unpleasant.!
I have tried routing the exhaust to the underside of the car with a small silencer ... but it is still too loud.!
The obvious solution, it seems, would be to change the piston pump for a 'vane' type as they are very much quieter.
The downside of the more modern vane type vacuum pump is that they rely on electronics which are polarity sensitive and would be incompatible with my positive earth car.
To get around this problem, I thought perhaps I could use a 110W. 12volt insulated converter.; similar to the one that came with my hydrostatic petrol gauge?
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ukdave2002
Joined: 23 Nov 2007
Posts: 4418
Location: South Cheshire
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| Posted: Thu Sep 17, 2026 4:37 pm Post subject: |
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Can you not simply electrically isolate the pump from its mechanical attachment to the car, e.g. using nylon screws and an insulating gasket between the pump and the car chassis/earth , then you are free to wire the pump up with the -ve connection (switch in this case) to the pump case and the +ve connection to the pump terminal connection?
Even using an isolator you may still have to insulate the pump electrically from the chassis... |
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alastairq
Joined: 14 Oct 2016
Posts: 2261
Location: East Yorkshire
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| Posted: Thu Sep 17, 2026 5:27 pm Post subject: |
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Don't forget, rubber hoses, either...
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces. |
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Ray White
Joined: 02 Dec 2014
Posts: 7701
Location: Derby
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| Posted: Thu Sep 17, 2026 5:34 pm Post subject: |
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Thank you for those replies.
I am now better informed.  |
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