Step down transformer or ?

[Chris Perry]

I need to energize a small, low power board (12vDC / 35 mA) from a panel providing 24vDC, 500 mA max. Both are outdoors in a weather tight enclosure with not a lot of extra space. What should I look for in a step down transformer for this application or is there another practical solution. All comments and suggestions are appreciated. Thank you.

 

[Harald Kapp]

Welcome to electronicspoint.

As your source is 24V DC, a transformer is not suitable. It can transform only AC.
At 35mA you could use a simple linear regulator based on an LM317, see e.g. here.
Or, more effective in terms of power losses avoided, a step-down converter module like e.g. this one.

 

[Gryd3]

Or, more effective in terms of power losses avoided, a step-down converter module like e.g. this one.

I would certainly go this route, It's a little more than the linear regulator, but the cheaper alternative will simply waste the additional power as heat...
The step-down converter is much much more efficient. (Mind you, the panel you have is large enough to power it with either solution, but if the idea crosses your mind to put a battery in there to run while it's cloudy or night-time, the step-down converter will help stretch out more life)

 

[Chris Perry]

Thanks for your advice. I will go with a module for simplicity - 2 wires in, 2 wires out and adjust to 12v with my meter. (I would not be able to make a LM317 ciruit without detailed instructions.) I see that the converters are widely available. Last questions: Do I need to be concerned about heat in the enclosed cabinet with other circuitry and is it better to have a higher rated module, say 20 amp, or go with a 2 or 3 amp unit based on the 35mA demand. Also, the main board (gate opener) is powered by a transformer but also has 24v battery backup. The 12v board is powered by a plug in transformer. The 12v board is a "supervised" relay that overrides the entire system during power outages which is why I want to power it from the operator board. Thanks again.

 

[Gryd3]

Thanks for your advice. I will go with a module for simplicity - 2 wires in, 2 wires out and adjust to 12v with my meter. (I would not be able to make a LM317 ciruit without detailed instructions.) I see that the converters are widely available. Last questions: Do I need to be concerned about heat in the enclosed cabinet with other circuitry and is it better to have a higher rated module, say 20 amp, or go with a 2 or 3 amp unit based on the 35mA demand. Also, the main board (gate opener) is powered by a transformer but also has 24v battery backup. The 12v board is powered by a plug in transformer. The 12v board is a "supervised" relay that overrides the entire system during power outages which is why I want to power it from the operator board. Thanks again.

You may not need to worry about heat in this particular instance. The small board draw is roughly 0.5Watts which is pretty low. It might get warm, but I highly doubt it will get hot. This is more of a concern the the cheaper linear regulators mentioned above.
As far as over rating components go. Yeah, it's better to over-rate the supply, but providing 20A to a board that is rated to pull 0.035A is grossly overkill. You would be more than generous with anything that provides more than 1/8th of an Amp.
In any case, it may be worth looking at the board a little closer, if there are any components that may suddenly draw more power, you should over-compensate a little more.
As far as your gate setup is concerned, I'm a little curious why the board responsible for control during a power outage runs off a plug-in transformer...

 

[Chris Perry]

You may not need to worry about heat in this particular instance. The small board draw is roughly 0.5Watts which is pretty low. It might get warm, but I highly doubt it will get hot. This is more of a concern the the cheaper linear regulators mentioned above.
As far as over rating components go. Yeah, it's better to over-rate the supply, but providing 20A to a board that is rated to pull 0.035A is grossly overkill. You would be more than generous with anything that provides more than 1/8th of an Amp.
In any case, it may be worth looking at the board a little closer, if there are any components that may suddenly draw more power, you should over-compensate a little more.
As far as your gate setup is concerned, I'm a little curious why the board responsible for control during a power outage runs off a plug-in transformer...

The 24v LA500 gate operator (MAIN) is powered by a built-in transformer and has battery backup. The 12v AC or DC vehicle exit sensor (SENSOR) is an accessory which causes the gate to open when a vehicle exits the property. On activation, a N.O. contact on the sensor closes briefly and operates a relay on the main to open the gate. Normally, the main auto closes the gate after a preset interval. During an outage, the main runs on the battery but the sensor holds the contact closed which keeps the gate open until power is restored. This is because the sensor is "supervised" similar to an alarm system and goes to fail safe mode without power. Obviously, this overrides the advantage of having battery backup.
The documentation for the sensor states without explanation that the board can be modified to be "unsupervised". I suppose this may be as simple as removing a capacitor however the manufacturer is non-responsive and I believe out of business. Thanks.