Voltage regulator to replace AA batteries??

[lago]

I am sure that there is someone here with more electrical knowledge than what I have and it won't take much.

I want to use a AC/DC transformer to replace the 2 AA batteries in the remote sensors for a weather station.

In checking the voltage of the transformers I have in my 'might need this sometime' box, all have a higher output than rated on the transformer.

I have 3 different 12v transformers and the output ranges from 15v to 19v. I also have a switchable transformer that selects 1.5v to 12v. All settings have voltage output at almost double the setting.

This may be a dumb question but what am I missing here? Do I need a regulator of some kind to reduce the voltage in order to replace the AA batteries.

Any suggestions?

Thanks
Lago

[pelligrini]

How reliable is the device you are testing with?

[lago]

It's one I got at BORG a few years ago, around $50 or so. It gives correct voltage when I check AA batteries and also AC circuits.

Lago

[LCHIEN]

The first thing is to realize that battery-operated devices usually operate over a wide voltage range as the battery falls off, so tight regulation is not needed. A 2 cell electronic device pulling low current is going to be able to operate over at least 3.3V down to about 2.4 or 2.6 volts, sometimes as low as 2.0 volts.

Transformers are theoretically flat output voltage but due to their internal resistance have a variable output voltage that's highest when unloaded and lowest at full load. The voltage drop is due to IR. Usually they are specified at full load which means that if you are testing them unloaded then they will easily be 10 or 15% or even higher. What they will be when you run your remote weather sensor will be a very low load so what you are measuring is close to what you will get.

Quite frankly power supply regulator design can be quite complex with your choice of linear and switching regulators plus many varieties of switching regulator, power dissipation efficiency drop out, line and load regulation etc but for your weather station with low current and wide input range you really don't need anything complex.

Find the lowest voltage, low current rating transformer and measure the output voltage. Make sure its a DC wart and not an AC wart.

Find some very common 1N4001, 1N4002, 1N4003 (anything up to 1N4006 will work) rectifier diodes and put them in series with the load, anode to the plus supply and cathode (band) to the plus load terminal. Add as many in series as it takes to drop your voltage to 2.4 to 3.3 volts range at the load. As a starting point each diode will drop about .6 to .7 volts. If your transformer measures 7.7 volts then you will want to drop that to about 3.3 volts. 7.7-3.3 is 4.4, divide by .65 and you get around 7 diodes. Try this and you should get about 3.15 volts. Add or subtract a diode to adjust it if necessary.

Good luck.

[cabinetman]

I am sure that there is someone here with more electrical knowledge than what I have and it won't take much.

I want to use a AC/DC transformer to replace the 2 AA batteries in the remote sensors for a weather station.

In checking the voltage of the transformers I have in my 'might need this sometime' box, all have a higher output than rated on the transformer.

I have 3 different 12v transformers and the output ranges from 15v to 19v. I also have a switchable transformer that selects 1.5v to 12v. All settings have voltage output at almost double the setting.

This may be a dumb question but what am I missing here? Do I need a regulator of some kind to reduce the voltage in order to replace the AA batteries.

Any suggestions?

Thanks
Lago

It seems that if you had a transformer that can put out 3 volts (2 AA batteries @ 1.5 V each), it should work...yes?

.