Adjustable Regulator with On/off
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Hello,
I'm needing to supply regulated power to a device ( Digital Camera 8.4v at
about 500ma) and be able to turn it on and off to conserve battery life when
not in use. Initially I was thinking of a transistor from a stamp pin that
would then trigger a relay to send power to a standard 3 lead voltage
regulator. However I've been looking at some 5 lead regulators that have an
On/Off lead that could possibly be driven from a stamp. This would be a much
more elegant solution.
Has anyone controlled a regulator's On/Off pin before? I assume sending 5v to
the pin will turn on, and sending nothing (with pull down resistor) will turn
it off. Is this right? Should there be a resistor between the stamp and the
regulator's On/Off pin? Also, I'm unsure if the regulator it's self draws
current when it is off. I need long battery life so this is very important.
The regulator I'm looking at is at:
http://www.national.com/pf/LM/LM2941C.html
I figure the adjustable version is best so that I can adjust the voltage if I
need to change cameras.
Thanks,
Stewart
I'm needing to supply regulated power to a device ( Digital Camera 8.4v at
about 500ma) and be able to turn it on and off to conserve battery life when
not in use. Initially I was thinking of a transistor from a stamp pin that
would then trigger a relay to send power to a standard 3 lead voltage
regulator. However I've been looking at some 5 lead regulators that have an
On/Off lead that could possibly be driven from a stamp. This would be a much
more elegant solution.
Has anyone controlled a regulator's On/Off pin before? I assume sending 5v to
the pin will turn on, and sending nothing (with pull down resistor) will turn
it off. Is this right? Should there be a resistor between the stamp and the
regulator's On/Off pin? Also, I'm unsure if the regulator it's self draws
current when it is off. I need long battery life so this is very important.
The regulator I'm looking at is at:
http://www.national.com/pf/LM/LM2941C.html
I figure the adjustable version is best so that I can adjust the voltage if I
need to change cameras.
Thanks,
Stewart
Comments
>
>The regulator I'm looking at is at:
>http://www.national.com/pf/LM/LM2941C.html
>...
> I assume sending 5v to
>the pin will turn on, and sending nothing (with pull down resistor) will turn
>it off. Is this right? Should there be a resistor between the stamp and the
>regulator's On/Off pin? Also, I'm unsure if the regulator it's self draws
>current when it is off. I need long battery life so this is very important.
Read the data sheet carefully. The LM2941 is ON when the pin is
pulled low (grounded), and OFF when its voltage is above about 1.3
volts. It takes an active low output from the Stamp to turn the '2941
ON.
Yes, you could put a resistor of ~220 ohms for good measure between
the Stamp pin an the ON-OFF control, but it is not necessary. Also,
no need for a pullup or pulldown resistor. However, you do have to
take care if you are going to use the SLEEP command on the Stamp,
because when the Stamp wakes up all of the outputs briefly turn into
inputs, and if your power supply is ON, it will turn off briefly
during that interval. To avoid this, put a capacitor (~1uf) between
the ON/OFF terminal and ground. The current out of that terminal
when grounded is about 50 microamps.
Yes, the regulator will draw current when it is OFF. I don't know
about the LM2941 specifically, but it is not a micropower device, and
might draw a milliamp or so when OFF. (That is a guess, subject to
test. The data sheet says nothing about it, so you have to wonder.
It is designed for automotive apps, where 1 ma is not too much).
There are micropower regulators like the LT1121 that draw as little
as 20 microamps when off, but I don't know of any with that low an
OFF current that can also supply 1 amp when ON. The LT1121 is good
for only 150 ma.
If you want truly zero-power off state, you will have to put a mosfet
switch or relay in fron of the regulator.
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
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