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'Any ways or devices to work as a relay'
2005\10\28@172101 by olin piclist

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John Waters wrote:
> I need to turn ON/OFF a small d.c. motor regularly, the d.c. voltage
> as applied on the motor is 1.5 V, while the current throwing through
> is less than 80mA. As the whole thing will be driven by batteries, I
> want all the power to go to the motor instead of the mechanism to
> turn it ON/OFF. Hence I would avoid using a relay since the current
> drawn by the relay coil will be quite big. Is there any
> power-budgetary ways to turn ON/OFF my motor?

The cheapest is a small NPN transistor and a base resistor.  This will drop
about 200mV and waste an extra mA or so.  For slightly more money a N
channel FET with "logic level" gate threshold voltages will be slightly more
efficient.


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2005\10\28@174143 by Russell McMahon

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>> I need to turn ON/OFF a small d.c. motor regularly, the d.c.
>> voltage
>> as applied on the motor is 1.5 V, while the current throwing
>> through
>> is less than 80mA.

> The cheapest is a small NPN transistor and a base resistor.  This
> will drop
> about 200mV and waste an extra mA or so.  For slightly more money a
> N
> channel FET with "logic level" gate threshold voltages will be
> slightly more
> efficient.


As Olin says. But if energy really counts then a FET solution is
liable to be significantly superior. Gate energy is essentially zero.
If a bipolar transistor drops 200 mV then that's 0.2/1.5 =~ 13% of the
energy lost in the transistor. A FET with an on resistance (Rdson) of
0.1 ohm will drop V=IR = 0.08 x 0.1 = 8 millivolts or 0.5% of energy
used, and FETS with much lower Rdson are available.

As Olin says, a logic FET will be needed if only 1.5v is available for
gate drive. FETs that operate well at Vg = 1.5V are relatively
uncommon but are available.

BUT - what drives this "relay" - if it's a switch then most switches
will handle 80 mA with about zero drop with ease. If it's a control
circuit then you need to ensure that it is efficient and draws little
current when the motor is off - especially so if the motor is on for a
small part of the time.

You say 1.5 V - is this from a standard "torch battery". If so the
1.5V will fall to around 1 V towards the end of the battery life.

More information will help people give you a better answer.



       RM

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