>> 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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