Voltage regulators fail high

[TonyMS]

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

Cheers

Tony MS

 

[Hammy]

On Thu, 12 Feb 2009 16:13:39 +0000, TonyMS <news@montgomery-smith.org>
wrote:

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

Cheers

Tony MS

You can use a SCR crowbar which blows the fuse,or something like this.

http://i44.tinypic.com/103iypt.png

This will a least save you on fuses if its just due to wiring errors.

The comparator senses the output voltage and compares it against the
reference if it's greater the comparator pulses the base of Q1 which
with Q2 makes a latch this switches off the P-FET at the 7805
regulator input shutting it down.

I didnt include any caps other then C1 for some noise imunity, I also
didnt calculate resistor values but you should get the idea.

Theres allso dedicated monitoring IC'S.

You may instead want to figure out why they keep failing do you have
them properly heatsinked?

 

[Hammy]

On Thu, 12 Feb 2009 11:48:35 -0500, Hammy <spamme@hotmail.com> wrote:

You can use a SCR crowbar which blows the fuse,or something like this.

http://i44.tinypic.com/103iypt.png

This will a least save you on fuses if its just due to wiring errors.

The comparator senses the output voltage and compares it against the
reference if it's greater the comparator pulses the base of Q1 which
with Q2 makes a latch this switches off the P-FET at the 7805
regulator input shutting it down.

I didnt include any caps other then C1 for some noise imunity, I also
didnt calculate resistor values but you should get the idea.

Theres allso dedicated monitoring IC'S.

You may instead want to figure out why they keep failing do you have
them properly heatsinked?

Two things I forgot; put a small signal diode at the comparators
output cathode towards the transistors just 1n4148 will do . The other
thingis place a small capacitor across R6.If you don't have that cap
there the transistors will latch as soon as you apply power to the
circuit.Start out with 10pf increase as needed.

Usually most devices operating off 5V can take an absolute maximum of
7V so set your protection to activate at around 5.8 or 6V. But check
the datasheets on what the devices are that the 7805 is supplying and
Check the absolute max voltage they can withstand. Then set your
Protection accordingly.

You could also place an led in the transistor circuit so you know that
it has been tripped.

 

[Eeyore]

TonyMS wrote:

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually it's
125C on 7805s.

Graham

 

[Hammy]

On Thu, 12 Feb 2009 22:44:01 +0000, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually it's
125C on 7805s.

Graham

I've abused the 7800 series of regulators. They have thermal
protection I know it works from experience. The only possible
explanation for having ten volts at the output is hooking it up
backwards, that's about all I can think of.

 

[Jamie]

TonyMS wrote:

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

Cheers

Tony MS
Yes, you put a fusible link in there with a TVS diode as the

cross bar to blow it..


http://webpages.charter.net/jamie_5"

 

[Jamie]

Hammy wrote:

On Thu, 12 Feb 2009 22:44:01 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:



First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually it's
125C on 7805s.

Graham


I've abused the 7800 series of regulators. They have thermal
protection I know it works from experience. The only possible
explanation for having ten volts at the output is hooking it up
backwards, that's about all I can think of.
or not having the common pin connected.

http://webpages.charter.net/jamie_5"

 

[Hammy]

On Thu, 12 Feb 2009 19:08:42 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

or not having the common pin connected.


http://webpages.charter.net/jamie_5"

Yep never thought of that.That would do it.

I've accidently left them into a dead short for a few minutes and they
survivied even the T0-92 L series. Your eyes start to get buggy after
a couple of hours looking at a breadboard

 

[Michael A. Terrell]

TonyMS wrote:

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

Are you using thee proper capacitors across the input and output pins
to ground?


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The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Randy Day]

In article <E7udncqHNKRwXgnUnZ2dnUVZ_hSWnZ2d@earthlink.com>,
mike.terrell@earthlink.net says...

TonyMS wrote:

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?


Are you using thee proper capacitors across the input and output pins
to ground?

The SOT-89 version (78L05) looks similar, but
has the input and output pins in reverse order
to its larger cousins...

 

[Phil Allison]

"TonyMS"

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

** A 5.1 volt zener ( 5 watt rating ) across the supply rail will save any
parts that could be damaged.

Add a fuse of fusible resistor in series with the supply to the reg IC too.

Anecdote:

Once had to repair a suit case size box of electronics that accompanied an
Italian made electronic piano accordion.

It was chock full of 7400 series ICs, all running from the same 5 volt
regulator - a TO3 pack type.

The reg IC was not properly fitted to its heatsink nor was there any thermal
grease used - so it eventually failed HIGH -at about 11 volts IIRC.

Took nearly all the digital ICs with it - over 120 of them, thankfully
they were all in sockets.

When I completed all the repairs, I added an SCR crow bar to the 5 volt
rail.




...... Phil

 

[TonyMS]

TonyMS wrote:

I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there any
way I can protect my boards against such failure?

Cheers

Tony MS

Many thanks for all these helpful replies, which I'll study at greater
leisure.

One other suggestion that occured to me last night was simply using a
high wattage resistor to drop the 12V down to say 7V so that a failure
can't lift so high

Cheers

Tony MS

 

[IanM]

TonyMS wrote:

TonyMS wrote:
I've been using 5V regulators - L7805ACV and others. I've had some
failures, no doubt due to careless wiring on my breadboard.

The problem is that, when they fail, they fail high. Ie, instead of
regulating my 12V supply down to 5V, they leave it at around 10V, and
damage other components on the board.

First question, is this normal behaviour? Second question, is there
any way I can protect my boards against such failure?

Cheers

Tony MS

Many thanks for all these helpful replies, which I'll study at greater
leisure.

One other suggestion that occured to me last night was simply using a
high wattage resistor to drop the 12V down to say 7V so that a failure
can't lift so high

Cheers

Tony MS
78xx regulators need a minimum of 2V 'headroom' so they might as well

not be there if only fed 7V. If you feed therefore feed them a minimum
of 8V, and they fail short, well 8V can smoke the logic nearly as well
as 12V.

The one thing that hasn't been mentioned is a reverse protection diode,
connected cathode to the 7805 input, anode to the output. If you have
anything that dumps current into the 5V rail, e.g. multiple supplies,
large reservoir caps downstream of the regulator, or badly designed
motor drive circuits, reverse current through the regulator can blow it.

Personally, I'd make up a PSU delivering the voltages I commonly need
for breadboarding and stop trying to run regulators on the breadboard.
The only exception to that is if I'm deriving a low current clean supply
for one chip using a 78L05 or similar.

I've been using the same hacked Sinclair Spectrum +2 PSU as a small
bench supply for the last 15 years with no trouble. IIRC it uses a LM723
regulator which let me hack in a 1.5A current limit circuit and I modded
it with a pair of 78M12 and 79M12 regulators for the +/- 12V supplies.
The other mod it has is a 4 pole toggle switch on the output so the
project on the bench is isolated when its off and I can solder stuff
without blowing out CMOS inputs. Its not the supply for every job, but
it's pretty handy for breadboard work.

If you *must* continue with the main regulator on the breadboard, make
up a little module on veroboard with your heatsinked 7805, a fuse or
polyfuse on the input, the decoupling caps, a reverse protection diode
and a SCR crowbar circuit on a suitable header to plug into your
breadboard. Use separate pins for 0V in and out even though its the
same rail so you don't loose regulation if the ground lifts and you'll
not over voltage any more logic.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:

 

[Chris Jones]

Hammy wrote:

On Thu, 12 Feb 2009 22:44:01 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:


First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually it's
125C on 7805s.

Graham

I've abused the 7800 series of regulators. They have thermal
protection I know it works from experience.

I have heard of people shorting the output to ground and using them as
thermostatically controlled heaters - they heat until the thermal
protection temperature and then stop heating above that temperature. I
think the reliability is probably not good when used like that, because the
manufacturer may have set the thermal cut-out at a temperature that will in
the long term damage the die, the die attach or the moulding compound, and
also the electromigration lifetime due to current density in the metal
tracks would be reduced at high temperature.

Still, if you need to perform that heating function without needing high
reliability, then I can't think of a cheaper way to do it. It would be
nice if someone would make a part that was designed and specified for that
application, so that it could be used with more confidence.

Chris

 

[whit3rd]

On Feb 13, 2:41 am, TonyMS <n...@montgomery-smith.org> wrote:

TonyMS wrote:
I've been using 5V regulators - L7805ACV and others. I've had some
failures, ...

The problem is that, when they fail, they fail high.

There is one scenario that can kill 78xx regulators: if the
output voltage is higher than the input, damage can result.
That doesn't SOUND likely, but remember that power-loff
events always kill the input voltage, and the output
only drains slowly if there's an output filter capacitor.

The recommendation is to add a diode, cathode to (input)
and anode to (output) when using large filters on the output.

 

[Jamie]

Chris Jones wrote:

Hammy wrote:


On Thu, 12 Feb 2009 22:44:01 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:



First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually it's
125C on 7805s.

Graham

I've abused the 7800 series of regulators. They have thermal
protection I know it works from experience.


I have heard of people shorting the output to ground and using them as
thermostatically controlled heaters - they heat until the thermal
protection temperature and then stop heating above that temperature. I
think the reliability is probably not good when used like that, because the
manufacturer may have set the thermal cut-out at a temperature that will in
the long term damage the die, the die attach or the moulding compound, and
also the electromigration lifetime due to current density in the metal
tracks would be reduced at high temperature.
That is totally insane!

http://webpages.charter.net/jamie_5"

 

[TonyMS]

whit3rd wrote:

On Feb 13, 2:41 am, TonyMS <n...@montgomery-smith.org> wrote:
TonyMS wrote:
I've been using 5V regulators - L7805ACV and others. I've had some
failures, ...
The problem is that, when they fail, they fail high.

There is one scenario that can kill 78xx regulators: if the
output voltage is higher than the input, damage can result.
That doesn't SOUND likely, but remember that power-loff
events always kill the input voltage, and the output
only drains slowly if there's an output filter capacitor.

The recommendation is to add a diode, cathode to (input)
and anode to (output) when using large filters on the output.

Yes, I think this must have been my problem. I had a too large cap on
the 5V.

Still doesn't explain why it failed high though

Thanks

Tony MS

 

[Phil Allison]

"TonyMS"

The recommendation is to add a diode, cathode to (input)
and anode to (output) when using large filters on the output.

Yes, I think this must have been my problem. I had a too large cap on the
5V.

Still doesn't explain why it failed high though

** Course it does !!

You shorted the input voltage to ground.

BJTs fail short C to E.

Dick wad.




..... Phil

 

[Chris Jones]

Jamie wrote:

Chris Jones wrote:

Hammy wrote:


On Thu, 12 Feb 2009 22:44:01 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:



First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually
it's 125C on 7805s.

Graham

I've abused the 7800 series of regulators. They have thermal
protection I know it works from experience.


I have heard of people shorting the output to ground and using them as
thermostatically controlled heaters - they heat until the thermal
protection temperature and then stop heating above that temperature. I
think the reliability is probably not good when used like that, because
the manufacturer may have set the thermal cut-out at a temperature that
will in the long term damage the die, the die attach or the moulding
compound, and also the electromigration lifetime due to current density
in the metal tracks would be reduced at high temperature.
That is totally insane!

Here are some pictures of the heating plate than one guy mounted to his
diesel injection pump, so that vegetable oil would be warm enough to lower
the viscosity closer to that of diesel.
http://www.trappertricks.de/eheizung/
I count 10 regulator chips. He mentions that as well as all of the
tantalums, he needed an additional large capacitor otherwise he says it
oscillates and the tantalums blow up. I suspect they probably would sooner
or later anyway.

Chris

 

[Eeyore]

Hammy wrote:

On Thu, 12 Feb 2009 22:44:01 +0000, Eeyore
rabbitsfriendsandrelations@hotmail.com> wrote:

First question. What temperature are you running them at ? Do you
understand thermal calculations ? What is the max die temp ? Usually it's
125C on 7805s.

I've abused the 7800 series of regulators. They have thermal
protection I know it works from experience.

And I know from experience that if you abuse the thermal protection, they can
explode.

Sounds to me like you're abusing the part through ignorance. How about
answering my question too ?

Graham