Re: Voltage Regulator - surprise

On Jan 16, 6:51 am, Fred Bloggs <nos...@xxxxxxxxxx> wrote:
BobP122 wrote:
On Jan 15, 9:50 am, default <defa...@xxxxxxxxxxxxx> wrote:

On Mon, 14 Jan 2008 15:13:48 -0500, John Popelish <jpopel...@xxxxxxxx>
wrote:

That it continues to output, is not a problem here (it drives a fan

250mA), but the regulator "cooks", fails completely and becomes
unusable.
Two questions:
1. Why does it output to failure - below 15V?
2. As it does becomes unusable (i.e. cooks/fails), what can I do to

Three terminal regulators would cook if the battery is connected to
the output and the input falls below the battery voltage - it is in
the application notes - they recommend an isolation diode on the
output or diode from the output to feed voltage to the input (which is
reversed biased during normal operation and keeps the regulator happy
when the input voltage is missing).

Having a battery on the output and no input is just like connecting
the regulator backwards in the circuit. A large cap on the output can
kill a regulator when power is removed.

The diode makes a cutout circuit unnecessary unless you are concerned
about the small power (quiescent current) the regulator wastes when
sitting idle
--

Hi again,
To eshjr
I can see where a relay configured to 'cutout' the battery would avoid
problems. Thanks.

To default
The battery is connected to the VR only. There aren't any direct
connections from the battery to the load.
Nor would I know how to. But now due to a curious nature - I'll be
looking into that! So thanks a lot - I think.

A big Thank you to all JP, eshjr, & default- much appreciated.
The fan is rated 6 to 15 volts. The rechargeable battery power supply
was 18Volts. My 'too simple' remedy was to use a 15V Regulator. Once
again I learned how true the adage "a liitle learning is a dangerous
thing".
After your explanation, I replaced the Voltage regulator with a 33ohm
5W resistor in series with the load (DC Fan 60ohms). I tried a 5.1Volt
5Watt Zener (in series with the Fan),but without any calculations it
just felt warmer to the touch. I'm hoping that this is a safer
solution. . .?
I'm still curious to know why the Voltage Regulator 'cooks' when the
input voltage drops to (around) 7Volts and the regulator's voltage to
way below regulation - (around) 5Volts. Shouldn't there be much less
heat generated at this stage - or at least not enough to heat the VR
to failure?

The thermal overtemperature cutout for these regulators compares a
reference voltage derived from an internal 6.3V zener diode to the
cut-in Vbe of an internal temperature sensing transistor positioned to
shunt the base drive away from the output transistors at a die
temperature of 175oC. At your low input voltage of 7V or so, the zener
bias current is too low which makes the zener reference voltage low, and
this in effect shifts the resulting thermal cut-out threshold to a
higher die temperature causing the 'VR' to fail. The output transistor
SOA protection circuit will also be inoperative, leaving the current
limiting protection as the only safety feature in place. You also risk
damaging your battery by running it down to one third of its full charge
cell voltage. You need to include a battery voltage monitor which
latches the whole battery load off when it drops below a threshold of
say 12V under load. Because this is a battery powered system, you would
be better off using CMOS logic and a MOSFET series pass element for the
monitoring and control.

I did have (recommended) caps at the input and the output. I'm
guessing that at the Fan's dropout Voltage (5Volts), a 'switching' on-
off oscillation occurred, and the momentary 'on current' was much
higher than 250mAmps, enough to burn the VR.

Not plausible...- Hide quoted text -

- Show quoted text -

BobP
Wow! and Thanks! Your explanation may be just a 'little too clever for
me', but there was a flash of Iight here and there, enough for me to
at least "believe" if not "undestand". By the way just in passing, who
did create the universe? It wasn't you or - was it? Please say it
wasn't.
Now I have a real value for 'deep cycling' batteries. That alone is
great information.
Another Question: The circuit for all this came from an old mercury
'heat' type thermostat.
The contacts are open to a set 'low' temperature (e.g. 70oF), at which
point the contacts close.
The contacts stay closed as long as the temp is below 70oF and stay
open as long as the temp is above 70oF.
Problem was, I needed the Thermostat to operate as 'cooling' or air
conditioning type. As such - it would have to turn on a small fan when
the contacts were 'open' and turn off the fan when the contacts were
'closed' - the exact opposite to its normal usage.
A kind of Solution: To do this, I used a PNP (Darlington) transistor
configured as a switch.
One Thermostat contact is tied to B+.
The Other Thermostat contact is tied to the base of the Transistor and
Ground (B-) through a resistor (10k).
With the contacts 'open', the base of the transistor base is grounded
through the resistor. i.e. Transistor on.
With the contacts 'closed', the base of the transistor base is
positive - i.e. Transistor off.
The circuit works - but is somewhat inelegant.. For a start - it draws
power even with the transistor off. i.e thru th 10k resistor - a mA or
two.
Its kept awake to find a better configuration (using the same 'heat'
type thermostat). Any suggestions?
BobP
P.S. I could have and maybe should have just 'recyled' the thermostat
- but then again, look at all I've learned. (There was a previous
"lesson" on discarded lithium-ion batteries. Enough just to say I'm
using
Nickel-Cadmium now.)
.

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