Hi ALL,

I am planning to build the BOX for my LabVIEW project. I will need to power up my flow turbine which needs 25V 50mA power supply.
I am quite new so, I will need your advise what is the best solution to step down from 240V 50Hz to desire DC voltage (what transformer should I use).

Many Thanks
Greg

 

Your "Flow Turbine" (FT) needs 25 volts at 50 mA. That's a pretty small FT. Here's the thing: If you supply the proper voltage and have 50 mA or greater the FT will operate properly. Suppose you have 25 V and 5000 mA (5 A). The FT is still going to draw only 50 mA.

Off hand I'd suggest you search transformer suppliers and see who has a transformer rated for what you need. I'd recommend a transformer that gives you the proper voltage and double the amperage. That way the transformer doesn't run hot.

Also, you need to understand there's a difference between AC and DC. NO, not the difference you already know, the difference that AC is rated in RMS whereas DC is rated in voltage. What's the difference? RMS is the Root Mean Square of the AC sine wave. To calculate the DC value you need to multiply the AC RMS by 1.414 to get the filtered final DC voltage. And It would be recommended that you filter the output. Further, you need to rectify the AC into DC. Just how you plan on doing that ALSO makes a difference. If you use a single diode then you're chopping half of the sine wave out. You still get the full DC voltage but you lose half the power. Using a single diode you have to calculate a voltage drop through the diode of (typically but not specifically) 0.7 volts. Using a full wave rectifier you convert the negative sine wave into a positive voltage, restoring full power. But now you're dropping (typically) 1.4 volts.

There are other considerations as well. You can go with a 48 volt secondary with center tap. Using just two diodes, one on one leg and the other diode on the other leg, using the center tap as your common, you can get full power and drop only 0.7 volts because at any given moment current is flowing through only one diode. I went with 24 volt center tap because they're common. 24 volts RMS rectified and reduced through the diode is:

24 x 1.414 - 0.7 = 33.24 volts. That may make your motor run hot because it's not designed to run on that much voltage. So depending on how you filter your DC output can make a difference as well. Without the filter you're going to see about 23.3 volts, which is under voltage for your FT. Using a center tapped 18 volt AC transformer will put you at about 24.75 volts. I'm sure your FT won't be bothered by that.

It starts with engineering. Knowing the specs and the requirements as well as the losses will give you the final numbers. Off hand I think a good filtered DC from an 18 volt transformer will probably do nicely. After all, your FT is only drawing 50 mA. The next thing you'll want to ask is how to design your filter.

 

Thank you Tonyr1084

 

Do you have a link to your flow turbine? If you can run on 24 VDC you can just use a simple 24 VDC Wall Wart power supply with 24 VDC being very common. Most are configured for World Wide Input Voltage 100-240VAC 50/60Hz and your current requirements are really not much. Here is an example of what I am suggesting. Generic ge New 24V 1A AC/DC Power Adapter Power Supply, Black. Since you are working with a project box you can also use a connector like these. Most frequently called wall warts use a standard 5.5mm connector. Will you have any other power needs?

Since I do not know your location I just used a simple Amazon link but power supplies like this are very common globally. The link is merely to give you a thought. There are also called AC to DC adapters as well as a few other names.

Ron

 

Thank you Ron for your advice. I have attached specification sheet (model V2).

Greg

 

It looks like the power supply can be anywhere between 10V and 28V DC so an easy to get 12V supply would work. Or an old laptop 19V supply.

 

As you will need something to convert the quadrature output to a form that can be displayed or sent to a computer for logging it would seem sensible to use the same basic power supply for that unit. As this unit will probably be just a microcontroller it could be powered from the 12 volts via an LM7805 or similar regulator. For such a small current I don't think it would be worth using a switch mode regulator to save the small amount of power that would be waisted.

Les.

 

Thank you Ron for your advice. I have attached specification sheet (model V2).

Greg

You're welcome. My read, as was mentioned is any supply between 10 and 24 VDC will work fine as a supply voltage. Additionally as was mentioned these units afford a quadrature output (two outputs with a 90 degree offset). The quadrature output is useful if you need to detect the direction of flow for any reason but if that is not necessary then either output can be used to simply indicate flow.

Do you have a display picked out yet? I ask as many displays afford excitation outputs for the sensors they can be used with. Also, you may find a display which can share the same DC power supply.

Ron

 

Thanks Guys.
I have found old laptop power supply 19V which I think should be ok.

I was originally looking at 25V as I was advised to by NI Technician to this voltage level to power it up.

Have a nice weekend all.

 

Given that most flow turbine flowmeters are fairly expensive devices I suggest using a regulated supply with an adequate margin of current capability. Digikey is one reputable supplier that has an easy to use on line catalog. Also, they are reliable.The smallest regulated supply that I would recommend is 100mA, or perhaps 150 mA. They are usually available for both 120 and 240 volts input, depending on how you connect the AC supply.