11 replies to this topic

[Bryan T]

I need to tap power for a N+ project from an existing 24vdc bus. What is the best way to do this? Is there an existing break-out board that would do the job, or would it be best to make my own?

[eduncan911]

Couldn't you use a voltage divider with two resistors to lower the value? Just taking a stab, but it sounds like you want to take a 24vdc circuit and wire in parallel another circuit, but you only want 9vdc in that circuit. Google "Voltage Divider", or even "Voltage Divider Calculator" for some online help. The formula is: Vout = Vin * R1(R1 + R2) Here's a good hit, showing where you'd "tap" to get your 9vdc (shown as 6vdc here): http://www.thecarpcs...vider&Itemid=88

[wabash9000]

I use a LM7809 wired the same as the LM7805 or LM7812 is on this pdf http://musicfromoute...ly_assembly.pdf

[carb]

Bryan,

It depends on the amount of power you will need.

If you need only enough for the Netduino card you could use several methods including the ones listed, but if you need more I would sugest that you consider something like Switching Regulator.

The resistors and the linear regulators waste power, the more the voltage difference the more wasted power (in the form of heat). For instance if the regulator or resistor is dropping 18 vdc (leaving 6 vdc for the Netdunio) and you are using 100ma the lost power would be P=IxE or 18 x .1= 1.8 watts. If you use a voltage divider (resistors) and you change the load then the voltage would change also.

With the switching power supply you will have a lot less loss, some can also raise voltage and some are adjustable.

The one that I linked is just one of many, look around and read the reviews and data sheets. In the end pick what will work the best for you.

Chuck

[Bryan T]

Thanks for the responses. I have considered all of these. As mentioned, the voltage divider wastes power, and voltage varies with load. I was hoping to find a "ready-made" solution, perhaps the one Chuck mentioned. I really expected that there was an existing break-out board that I have not yet found, as I thought this was more common than it may actually be. I am powering the Netduino Plus, with an RS232 shield, using both serial and ethernet communication. The only other power requirement that I may add is a Seeed Studio relay shield. For simplicity's sake, I really considered the voltage divider route, but I would like to avoid the aforementioned issues. I could be persuaded, however...

[carb]

Thanks for the responses. I have considered all of these. As mentioned, the voltage divider wastes power, and voltage varies with load. I was hoping to find a "ready-made" solution, perhaps the one Chuck mentioned. I really expected that there was an existing break-out board that I have not yet found, as I thought this was more common than it may actually be. I am powering the Netduino Plus, with an RS232 shield, using both serial and ethernet communication. The only other power requirement that I may add is a Seeed Studio relay shield.

For simplicity's sake, I really considered the voltage divider route, but I would like to avoid the aforementioned issues. I could be persuaded, however...

Bryan which RS232 shield are you using? I have some relays that needs 12 vdc. I use a wall wart with a split pigtail and feed 12vdc to both the Netduino Plus and the relays.

[Bryan T]

Bryan which RS232 shield are you using? I have some relays that needs 12 vdc. I use a wall wart with a split pigtail and feed 12vdc to both the Netduino Plus and the relays.

I'm using the CuteDigi RS232 shield.

My application requires that I mount the N+ project on a conveyor that has an available 24vdc supply bus running the full length of the conveyor, but AC wall power is not readily available. I need to be able to mount the device anywhere along the conveyor where it is needed, and tap into the available bus. In this case, a 24vdc-to-(9-12)vdc conversion is the only sensible solution.

[wabash9000]

With the LM7809 that I mentioned, I built that entire circuit shown in the pdf I linked but added a +9 and -9 volt feed to it. Through my testing the step down in voltage has not resulted in any noticeable heat. I have a small heat sink attached to each one so that could help. The only time any of them got hot is when I accidentally shorted out the 5v regulator for 10 seconds. It got hot but not worryingly hot. I have hooked up different loads to it and measured the voltage and amps and it will output 1.5 amps all day long with a very stable voltage. Off your 24 volt rail it should be able to supply 2A(I'm limited by the 1.5 amps the transformer puts out). The regulator, heat sink, 2 caps, a diode, and a small board to solder it all to should be about $10-15, and should be easy to make. edit: also if you ended up needing 12volts for the relays, you can make a second one with the LM7812 to get a 12 volt source where you need it.

[zzzzzz]

Bryan, you will probably have to make your own - I would not use a 7809 as using a 24V supply there will be a lot of waste energy as heat in the reg chip(7809), try using something like a LM2575-ADJ - and design it for 9V, there are design tools such as "switchers made simple" to assist with the design Cheers

[Bryan T]

I'm going to show my ignorance here, but how does an LM317T compare to those mentioned above? I happen to have one of those I've been playing with.

[carb]

I'm going to show my ignorance here, but how does an LM317T compare to those mentioned above? I happen to have one of those I've been playing with.

It has been a long time since I have worked with the LM317 series IC. It has a different Pin out than the LM78XX / LM79XX series of regulators. The LM317 has adjust, output & input. It requires additional components to operate (potentiometer or resistors, capacitors etc.). Where as the LM7805 has input, ground & output. The LM7805's output is 5vdc, LM7812 = 12vdc etc. Depending on the load and input voltage you may need a heat shink.

The LM78XX / 78LXX series come in several load capacities and package styles, T-220, TO-92 (like a small transistor) or surface mounts. With proper heat disipation the LM78XX series is good for 1.0 amp, the 78LXX series about 100 milli-amp.

Chuck

[nakchak]

I'm currently using a LM2596 based power-supply circuit, can handle up to 45V DC input and as its switch mode doesn't get that hot and has upto 3A output current. Which makes it a pretty decent hobby PSU in general.

You can get ready made adjustable modules on ebay for a few $ or a fixed 5v version which you can power the ND directly with on the 5v pin, in my circuit i have 100uF output filter cap and i have a pretty decent DC output i.e. the noise levels are tiny at most in the 1 to 2 mV range.

Nak.