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Transistor as switch with low leakage current Answered

I don't really know much about transistors, alas, but I am looking to use a transistor as a switch in a device where it's important to me to that the batteries (3xAAA) last about a year. This means that I want to minimize leakage current. I would like to use about 20 mA at 2.7V to control 0.5-1 A at 3.3V (I have 4.5V at my disposal, so I can handle a 1.2V drop through the transistor). The device is a water leak alarm which I assume uses almost no current except when it's alarming.

Any suggestions on what to use? 

A lot of people use TIP120s as switches, but I've also seen comments that indicate that they have an unpleasantly large leakage current. And I don't need the high voltage/current/amplification capabilities of the TIP120.

Or should I just use a solid state or electromechanical relay?


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6 years ago

A TIP120 has no good amplification, it is a switching transistor.

But if you want to switch 1A directly you have no option for small size transistors.

An old smoke detector works for about 2 years on a single battery and it only needs minor mods to work as a water sensor.

There are Instructables that cover water sensors and quite a few schematics can be found using Google if you want to build your own.

Might help if you describe you setup a bit more or upload some images.


Reply 6 years ago

My setup is that I'm adding an ESP8266 wifi module to a Leak Frog leak detector. I want to keep the existing beeper but have it also email me when it detects the leak. There is a spot on the circuit board where I can tap in to get current when a leak is detected, but I can't draw enough current to power up the wifi module there.
The wifi module is programmed to connect to wifi and email me as soon as it powers up.


Reply 6 years ago

In that case any transistor rated above the power consumption of the WiFi module will do.

standard BC547 will handle up to 100mA, a low power WiFi module uses
far below 20mA to operate, so basically any transistor will do to switch
it on.

You module uses above 200mA at full power level but has a standby current of a few µA which means it could run directly and the transistor would only need to activate the transmission.

Be aware that the module only operates on 3.3V and will be fried at 5V.
If you want to power the module over the transistor for automatic transmission aim for a transistor capable of at least 300mA.

I usually start by checking my electronics scrap and reading the datasheets for what I find.

But a quick search on Fleabuy for a 500mA transistor works too ;)