V. reg grounded via diodes?

[DaveC]

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes are
grounded. A cap is also paralled to the diodes.

Can someone enlighten me as to the advantages of this design? All 5v power to
ICs, etc. is 5.02v (measured across the IC supply pin and ground pin), so I
presume that the entire power supply has been raised 0.64v above system
ground.

This is in an automotive alarm.

Is this diodes-to-ground a surge-protection method? Or...?

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[Jim Meyer]

DaveC <dave-usenet3016@mailblocks.com> wrote in message news:<0001HW.BB69A72000037E3C01EFD890@news.individual.net>...

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes are
grounded. A cap is also paralled to the diodes.

Can someone enlighten me as to the advantages of this design? All 5v power to
ICs, etc. is 5.02v (measured across the IC supply pin and ground pin), so I
presume that the entire power supply has been raised 0.64v above system
ground.

Thanks,

I don't think it's surge protection. Most likely, the designer
felt a little more comfortable with a 5 volt supply closer to the
upper edge of "normal" than the lower edge.

Jim

 

[Spehro Pefhany]

On Wed, 20 Aug 2003 22:30:56 -0700, the renowned DaveC
<dave-usenet3016@mailblocks.com> wrote:

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes are
grounded. A cap is also paralled to the diodes.

Can someone enlighten me as to the advantages of this design? All 5v power to
ICs, etc. is 5.02v (measured across the IC supply pin and ground pin), so I
presume that the entire power supply has been raised 0.64v above system
ground.

This is in an automotive alarm.

Is this diodes-to-ground a surge-protection method? Or...?

The designer may have needed a negative supply wrt IC ground for some
reason. Can you post the entire schematic in a.b.p.s.e. ?


____
| |
-------------|7805|--------------- 5V
|____|
|
+--+---+-------------- 0V
| | |+
- V ---
^ - ---
| | |
+--+---+--------------- -0.6V
|
===
GND

One guess is that some switches are returned to chassis ground and the
designer wanted to allow for the possiblity that chassis ground might
not be quite the same voltage in different places in the vehicle. With
TTL level inputs there's not that many mV to play with. Offhand I
don't see the purpose of the second diode but maybe it's desirable if
the product is mis-wired.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Russell Powell]

This sounds like an ESD protection scheme to me....
I'd guess the regulater would already have ESD protection
built-in.
"DaveC" <dave-usenet3016@mailblocks.com> wrote in message
news:0001HW.BB69A72000037E3C01EFD890@news.individual.net...

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes are
grounded. A cap is also paralled to the diodes.

Can someone enlighten me as to the advantages of this design? All 5v power
to
ICs, etc. is 5.02v (measured across the IC supply pin and ground pin), so
I
presume that the entire power supply has been raised 0.64v above system
ground.

This is in an automotive alarm.

Is this diodes-to-ground a surge-protection method? Or...?

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[DaveC]

On Thu, 21 Aug 2003 6:15:36 -0700, Spehro Pefhany wrote
(in message <m3h9kvgr1d744svl2rhqmnsbtqgc9j6gdr@4ax.com&gt:

The designer may have needed a negative supply wrt IC ground for some
reason. Can you post the entire schematic in a.b.p.s.e. ?

What you've drawn below is all I have. I have no schematic, only the PCB I'm
trying to troubleshoot. I sketch down parts of the circuit as I follow some
traces. I noticed this v.reg configuration when I saw voltages far above 5v
(with reference to system ground) at the ICs.

____
| |
-------------|7805|--------------- 5V
|____|
|
+--+---+-------------- 0V
| | |+
- V ---
^ - ---
| | |
+--+---+--------------- -0.6V
|
===
GND

One guess is that some switches are returned to chassis ground and the
designer wanted to allow for the possiblity that chassis ground might
not be quite the same voltage in different places in the vehicle. With
TTL level inputs there's not that many mV to play with. Offhand I
don't see the purpose of the second diode but maybe it's desirable if
the product is mis-wired.

This does make sense, I guess. There are 10A relays on the pcb that are
switched with 12v. These are referenced to system ground. Also, sensor
switches (door, hood, trunk) switch 12v, so maybe the diodes provide a bit of
isolation and protection from "undershoot"?

And yes, this type of auto accessory probably gets the power reverse-wired
regularly during installation.

The reason for the diodes is a bit beyond my meager design skills. Just
trying to understand the thoughts of the designer...

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[JeffM]

...The designer may have needed a negative supply...
Spehro Pefhany

Nope. Ground is still ground.

diode ... desirable if the product is mis-wired
Spehro Pefhany

Wouldn't help at that point in the circuit.

...ESD protection scheme...
Russell Powell

Not even close. Thanks for playing.

the designer felt a little more comfortable
with a 5 volt supply closer to the
upper edge of "normal" than the lower edge.
Jim Meyer

Jim has it right
--and the 2nd (reverse-biased) diode is overkill.

 

[Bushy]

Jim has it right
--and the 2nd (reverse-biased) diode is overkill.

No it's not, as I learnt during my apprenticeship, all those years ago, it's
called a "Brother-in-law Diode". A diode that serves no useful purpose in
the circuit but is included because the circuit designer's brother-in-law
owns the diode factory!

Hope this helps,
Peter

 

[K Wind]

"Spehro Pefhany" <speff@interlog.com> wrote in message
news:m3h9kvgr1d744svl2rhqmnsbtqgc9j6gdr@4ax.com...

On Wed, 20 Aug 2003 22:30:56 -0700, the renowned DaveC
dave-usenet3016@mailblocks.com> wrote:

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes
are
grounded. A cap is also paralled to the diodes.

Can someone enlighten me as to the advantages of this design? All 5v
power to
ICs, etc. is 5.02v (measured across the IC supply pin and ground pin), so
I
presume that the entire power supply has been raised 0.64v above system
ground.

This is in an automotive alarm.

Is this diodes-to-ground a surge-protection method? Or...?

The designer may have needed a negative supply wrt IC ground for some
reason. Can you post the entire schematic in a.b.p.s.e. ?


____
| |
-------------|7805|--------------- 5V
|____|
|
+--+---+-------------- 0V
| | |+
- V ---
^ - ---
| | |
+--+---+--------------- -0.6V
|
===
GND

One guess is that some switches are returned to chassis ground and the
designer wanted to allow for the possiblity that chassis ground might
not be quite the same voltage in different places in the vehicle. With
TTL level inputs there's not that many mV to play with. Offhand I
don't see the purpose of the second diode but maybe it's desirable if
the product is mis-wired.

The forward biased diode as shown will lift the regulator by whatever its
forward voltage is with respect to ground. If 1N4005s are used I would
expect the output to be around 5.6V. If not, I would take a look at the
diodes and cap to see why. 5.2 volts tells me that something is wrong.

Ken

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com

 

[K Wind]

"DaveC" <dave-usenet3016@mailblocks.com> wrote in message
news:0001HW.BB69A72000037E3C01EFD890@news.individual.net...

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes are
grounded. A cap is also paralled to the diodes.

Can someone enlighten me as to the advantages of this design? All 5v power
to
ICs, etc. is 5.02v (measured across the IC supply pin and ground pin), so
I
presume that the entire power supply has been raised 0.64v above system
ground.

This is in an automotive alarm.

Is this diodes-to-ground a surge-protection method? Or...?

5.6V supplies are not uncommon. I would check continuity between the ground
pins of the ICs and the 12V ground or measure the voltage between the output
of the regulator and a known 12V ground. If if you have continuity or still
measure 5.02V, something is wrong with the diodes or cap. It seems to me
that if the ground of the circuit flows through the diodes to 12V ground, it
will make the output of the regulator dirty. The current flowing out of the
ground terminal of the regulator may be constant, but the rest of the
circuit isn't. If the current changes, the voltage across the diodes will
change, thus changing the output voltage of the regulator.

Ken

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[Fred Bloggs]

Fred Bloggs wrote:

The 5v regulator (3-pin type) has its ground leg attached to
parallel-backward-connected 1N4005 diodes. The other end of the diodes
are grounded. A cap is also paralled to the diodes.
Can someone enlighten me as to the advantages of this design? All 5v
power to ICs, etc. is 5.02v (measured across the IC supply pin and
ground pin), so I presume that the entire power supply has been raised
0.64v above system ground.

This is in an automotive alarm.

Is this diodes-to-ground a surge-protection method? Or...?


This is a common method of GND loop isolation . The circuit common is
the GND pin of the regulator- and that is why you measure 5.02V between
VCC and GND at the IC. The capacitor bypass is for regulator
stabilization and *input* bypass- which is chassis referenced. The
double diodes clamp the ckt common at +/-0.7V wrt chassis- there must be
other bypass elements not shown. Diagram below:

Please view in a fixed-width font such as Courier.


+------------------------------------------+------+------
| \ |
| / e
| 5V REG \ |/
| +-------+ +----|
| | | ckt | |\
+ o--+------IN OUT---+----+------+-------+ | c
Vbatt | | | | | | | |
/|\ +--COM--+ | +vcc+ +vcc+ | | | | big I
| | + | | | | | |\ | | |
| | 5V === | | | |---| > --+ | | |
| | - | +gnd+ +gnd+ |/ <---+ | \|/
| | | | | |
unreg +-------+----+------+-------+ little |
| I |
| +---||---+ )||
| | | relay )||
| +---|<|--+ coils )||
| | | )||
+---|>|--+ |
- | |
| <--- It -----> |
-------+-----------------------------------------------+-------
| heavy duty noise and transients
--- from other junk on chassis does
/// not enter/corrupt circuit
chassis

Also- the diodes allow the 5V logic to drive chassis referenced 5V
relays with the normally reverse biased diode acting as a common clamp.
Additionally, the logic can put 5.7V across the coils guaranteeing solid
operation at elevated temperatures where coil current requirements
increase. Isolation from chassis is still preserved.
Please view in a fixed-width font such as Courier.

+-------------------------------------------------+------
| |
| |
| 5V REG |
| +-------+ |
| | | ckt o
+ o--+------IN OUT---+----+------+ /
Vbatt | | | vcc | +-----> o
/|\ +--COM--+ | '---' | | | big I
| | + | | | |\ | |
| | 5V === | |--| > -----+ | | |
| | - | '---' |/ | | | \|/
| | gnd | | | |
unreg +-------+----+------+ | | |
| | |
| +---||---+ 5V )|| 12V )||
| | | relay )|| relay )||
| +---|<|--+ coil )|| coils )||
| | | )|| )||
+---|>|--+ | |
- | | |
| | |
-------+-----------------------------------+-----------+-------
|
--- <--- It ----->
///
chassis heavy duty noise and transients
from other junk on chassis does
not enter/corrupt circuit

 

[DaveC]

On Fri, 22 Aug 2003 6:37:08 -0700, N. Thornton wrote
(in message <a7076635.0308220537.497e4d3@posting.google.com&gt:

Its very simple, it ups the regulator output voltage by 1 diode drop.

Yes, I can see this.

The purpose of this design is...?
--
Dave C.
dave-usenet3016@mailblocks.com

 

[DaveC]

On Fri, 22 Aug 2003 4:48:13 -0700, K Wind wrote
(in message <1qn1b.122691$ib2.26603161@twister.neo.rr.com&gt:

5.6V supplies are not uncommon. I would check continuity between the ground
pins of the ICs and the 12V ground or measure the voltage between the output
of the regulator and a known 12V ground. If if you have continuity or still
measure 5.02V, something is wrong with the diodes or cap. It seems to me
that if the ground of the circuit flows through the diodes to 12V ground, it
will make the output of the regulator dirty. The current flowing out of the
ground terminal of the regulator may be constant, but the rest of the
circuit isn't. If the current changes, the voltage across the diodes will
change, thus changing the output voltage of the regulator.

Voltages measure thus:
5.02 from negative terminal of v.reg to 5v rail all across the board
5.6 from system ground to 5v rail all across the board
12.8 (approx) from system ground to 12v rail all across the board

Voltages are clean, as viewed on a 'scope, all across the board.

This circuit, as designed, has worked for about 15 years. Never had a
problem. Now some component in the circuit has decided to lose its mind.
Could be one of the two custom ICs (40-pin microcontroller; 8-pin
somethingorother), or a linear or non-linear component.

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[DaveC]

On Fri, 22 Aug 2003 6:41:12 -0700, Fred Bloggs wrote
(in message <3F461D73.1000603@nospam.com&gt:

Also- the diodes allow the 5V logic to drive chassis referenced 5V
relays with the normally reverse biased diode acting as a common clamp.
Additionally, the logic can put 5.7V across the coils guaranteeing solid
operation at elevated temperatures where coil current requirements
increase. Isolation from chassis is still preserved.

I don't know how this effects your explanation, but the relays are 12v
operated. The microcontroller drives hi-voltage (12v) drivers which drive the
relay coils with 12v.

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com