Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

--
*Why is it called tourist season if we can't shoot at them?

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:

Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?
A number of possible reasons. They may have wanted 4.3V for a reason.
Most likely, though, in case the supply got connected to 12V either due
to a wiring fault or because someone assumed all live connections should
be 12V.

--

Roger Hayter

On 09/05/2020 14:39, Roger Hayter wrote:
Dave Plowman (News) wrote:

Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?
A number of possible reasons. They may have wanted 4.3V for a reason.
Most likely, though, in case the supply got connected to 12V either due
to a wiring fault or because someone assumed all live connections should
be 12V.


They put it to give some union ****** a job.

--

On 09/05/2020 14:39:50, Roger Hayter wrote:
Dave Plowman (News) wrote:

Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?
A number of possible reasons. They may have wanted 4.3V for a reason.
Most likely, though, in case the supply got connected to 12V either due
to a wiring fault or because someone assumed all live connections should
be 12V.

I assumed the latter, where either accidentally, or intentionally the
pin is touched or strapped to 12V.

I've worked on 24V equipment and the criterion was similar. Where the
unit must not be damaged by applying 24V to any pin.

On 09/05/2020 13:49, Dave Plowman (News) wrote:
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

can't be that old if it has a TPS.....

In article ,
Jim GM4DHJ ... wrote:
On 09/05/2020 13:49, Dave Plowman (News) wrote:
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

can't be that old if it has a TPS.....

TPS is as old as EFI.

--
*Frankly, scallop, I don't give a clam

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Roger Hayter wrote:
Any guesses as to the purpose of this diode?
A number of possible reasons. They may have wanted 4.3V for a reason.

Doubt it, as it merely feeds what are effectively pots.

Most likely, though, in case the supply got connected to 12V either due
to a wiring fault or because someone assumed all live connections should
be 12V.

That does seem the likely explanation. Although there is also a zener to
take care of possible over voltage.

--
*My wife and I had words. But I didn't get to use mine.

Dave Plowman London SW
To e-mail, change noise into sound.

On 09/05/2020 16:09, Dave Plowman (News) wrote:

That does seem the likely explanation. Although there is also a zener to
take care of possible over voltage.


The zener will only protect from over voltage if the current is limited
to less than the power rating of the device. If the protection diode
wasn't in place and 12V from a battery, capable of supplying 10s or 100s
of Amps were connected t0 the 5V output the zener would instantly
destruct, possibly giving a short circuit.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On Sat, 09 May 2020 15:15:32 +0100, "Dave Plowman (News)"
wrote:

In article ,
Jim GM4DHJ ... wrote:
On 09/05/2020 13:49, Dave Plowman (News) wrote:
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

can't be that old if it has a TPS.....

TPS is as old as EFI.

How big is the smoothing cap after the diode? If large, maybe the
diode is to let the 5V source be switched off instantly without being
held up by the capacitor until it empties of charge.
--
Dave W

On 09/05/2020 15:15, Dave Plowman (News) wrote:
In article ,
Jim GM4DHJ ... wrote:
On 09/05/2020 13:49, Dave Plowman (News) wrote:
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

can't be that old if it has a TPS.....

TPS is as old as EFI.

not old then

Is there anything on it that could feed back a voltage of the opposite
polarity to the ecu?
Brian

--
----- --
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Dave Plowman (News)" wrote in message
...
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

--
*Why is it called tourist season if we can't shoot at them?

Dave Plowman London SW
To e-mail, change noise into sound.

Brian Gaff (Sofa 2) wrote:

Is there anything on it that could feed back a voltage of the opposite
polarity to the ecu?
Brian

The diode would not help with that problem.

--

Roger Hayter

On 09/05/2020 13:49, Dave Plowman (News) wrote:
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?


The electrical environment outside the ECU is harsh - my money is on
stopping ECU operation being affected by imported "crap" arriving via
the power supply lines.

On Saturday, 9 May 2020 13:50:10 UTC+1, Dave Plowman (News) wrote:
Been fixing the Lucas ECU on an old Jaguar. Similar to the type used on my
Rover.

The fault was on the 5v regulated supply. Being a 50 year old design, all
done with discrete components. Now fixed.

But looking at the design, one thing I'd like explained. The 5 volt rail,
as well as powering internal things, is exported to 'power' the throttle
position sensor, coolant temperature sensor and air flow meter.

After the regulator, and just to this external 5v feed, there is
additional smoothing etc. And between two of the smoothing caps, a diode.
Which reduces the 5v to the nominal 4.3 +/-0.2v of the external rail.

Any guesses as to the purpose of this diode?

without seeing the circuit, hard to know. I've used diodes before as nonlinear resistors in cap - nonlinear R - cap filters, but that's not a common way to use em.


NT