Running 1,5V Watch on 12V supply system?

Joerg Mayer

unread,

Oct 17, 2009, 6:13:31 AM10/17/09

to

Hello NG,

i'd like to run a 1,5V external watch without battery, but attached to the
12v power supply system of my scooter.

how do i reduce the voltage?

i thought of using a resistor in the input lead

eg the watch consumes 50mA at 1,5V:

12V - 1,5V = 10,5V
10,5V / 0,05A = 210 Ohm.

Will this ever work?

regards
Joerg

tomcas

unread,

Oct 17, 2009, 9:10:59 AM10/17/09

to

The problem is the 12v supply will vary from 13.7 volts while charging
to 9 volts while starting. A better solution is to add a Zener diode.

http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm

Jim Crowther

unread,

Oct 17, 2009, 4:38:02 PM10/17/09

to

A 1.5V zener? Doesn't exist. Better a 100k resistor feeding to deck
via two silicon diodes in series (1N413 or similar signal diodes would
be fine). Feed the watch from the junction between the resistor and the
'top' diode. That'd give 1.4V, quite sufficient for the watch, which
will draw only a few micro-amps (not 50ma!!!). I'd also add a 100pF
capacitor across the diodes just to catch any glitches.

--
Jim Crowther

tomcas

unread,

Oct 17, 2009, 8:09:19 PM10/17/09

to

Opps- you're right they don't go below 2.4 volts. He could use a LM317
with a 20 and 100 ohm resistor to set the output at 1.5 volts.

http://www.radioshack.com/product/index.jsp?productId=2062601
http://datasheetreference.com/lm317-datasheet.html

Jim Crowther

unread,

Oct 17, 2009, 10:18:03 PM10/17/09

to

In alt.scooter, on Sat, 17 Oct 2009 20:09:19, tomcas wrote:

>Jim Crowther wrote:
>> A 1.5V zener? Doesn't exist. Better a 100k resistor feeding to
>>deck via two silicon diodes in series (1N413 or similar signal diodes
>>would be fine). Feed the watch from the junction between the
>>resistor and the 'top' diode. That'd give 1.4V, quite sufficient for
>>the watch, which will draw only a few micro-amps (not 50ma!!!). I'd
>>also add a 100pF capacitor across the diodes just to catch any glitches.
>>
>Opps- you're right they don't go below 2.4 volts. He could use a LM317
>with a 20 and 100 ohm resistor to set the output at 1.5 volts.
>
>http://www.radioshack.com/product/index.jsp?productId=2062601
>http://datasheetreference.com/lm317-datasheet.html

KISS

--
Jim Crowther

Joerg Mayer

unread,

Oct 18, 2009, 4:40:03 AM10/18/09

to

Thanks a lot for your help guys. I think i will try it the way Jim
described.

Why can't i just put a resistor in front of the watch? would it get too
hot? or is it because of the voltage is not really constant at 12v?

all the best
joerg

Jim Crowther

unread,

Oct 18, 2009, 9:58:51 AM10/18/09

to

In alt.scooter, on Sun, 18 Oct 2009 10:40:03, Joerg Mayer wrote:

>Why can't i just put a resistor in front of the watch? would it get too
>hot? or is it because of the voltage is not really constant at 12v?

A watch draws very little current, and intermittently. So it's not a
constant load, and therefore a fixed resistor feeding it from a 12V
supply might actually present between say 11V and 1V - the former would
not be friendly to the watch. The supply can also be very spiky, even
directly across the battery. At the far end of the loom it can pick up
all sorts of crap from the HT wiring as well.

--
Jim Crowther

Joerg Mayer

unread,

Oct 20, 2009, 1:15:34 PM10/20/09

to

Am Sun, 18 Oct 2009 14:58:51 +0100 schrieb Jim Crowther:

> A watch draws very little current, and intermittently

Thanks Jim.