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Getting appropriate 5VDC power from a 24VAC transformer


#1

I need to run my Panda II from a 24VAC transformer, and I’m wondering if something like http://www.murata-ps.com/releases/mps098a_oki-78sr.html would be appropriate. I’m not familiar at all with SMPS, me being an old-school linear regulator type of guy, but this thing seems to be the bee’s knees, where a linear regulator would dissipate WAY WAY too much power as heat.

Specifically, I’m a bit concerned about EMF and noise on the 5V line. Is this thing appropriate?


#2

24v AC? That device is DC input.


#3

I looked at the data sheet for the device you specified and it appears as though it would do the job. Of course you would have to rectify and filter the 24VAC transformer output since the device is a DC-DC converter.

As far as ripple is concerned properly filtering the output with the recommended capacitor(s) should work.


#4

You can use a suitable bridge to convert to DC, place a decent sized inputt filter cap, and any drop in 78xx switch mode replacement.

For < 2% ripple you could try

http://www.dimensionengineering.com/DE-SW050.htm

at $15, you many want to check for other products too.


#5

I agree with the heat thing, But I think that the 24vac will be the biggest worry, as rectified this will put you close to the max Vin for this device or an LM78XX. As to the EMF I don’t think that you will have any issues, as the power is low. As to noise, that is one for you to decide if your project is critical in this area, I think you will be OK though as the ripple looks easy to filter out.

Yours Simon M.


#6

Right, I was planning to rectify/filter the 24VAC input.

I think that after smoothing, diode losses, etc, I’ll probably be somewhere in the 30V-32V range, so while I agree it’s on the high end, I think I can just sneak by. The Datel (muRata) equivalent module even specifically allows 40V input as long as the output current is 350mA or less.

Yeah, this specific module is under $4 from Mouser. That’s quite a steal.


#7

Do you have any recommendations for a rectifier for this? Any chance you could please share the full component listing you used to get from 24v AC to the 5c DC? I’ve been looking to do the same thing for a while but my noobish electronics have limited me somewhat…

Much appreciated.


#8

A search on google should reveal several schematics for this. You can get a kit that uses a 7805 IC and replace the 7805 with the Murata device.

If you have not built a circuit from scratch, this should be your first project :slight_smile:

Assuming a 2A load you can use a rectifier like RS201-G. This devce supports 2A DC forward current and 50v peak reverse voltage. The reverse voltage should be ideally 3 times the RMS of the input AC voltage. This is about 50 volts in your case. So this device should be allright.

The DC output from the rectifier needs to be smoothened. The value of this capacitor will depened on the amount of ripple you can tolerate on the rectified voltage and can be calculated using the output current, input voltage and the frequency of the AC voltage.

To make it easy use something between 500 to 1000uf @ 35v electrolytic or equvivalent. Connect this between the rectified output and ground

Feed this voltage to the Murata device to get your switched 5v output.

You can place a 10uf capacitor @ 6v between the 5v switched output and ground to stablize the power supply and cover for any burst demand.

You can also place a 0.1uf ceramic capacitor between the 5v and ground to block residual AC.

Once again I have not used the Murata devices to say for sure but switched power supplies could be noisy and may introduce undesired noise into the rest of the circuit. You have to use best practices for power routing, ground planes, trace widths and component placement. This is critical when precision analog components are used on the board.

Power supply design can get complicated and there are tons of papers out there to read on this topic.


#9

Rectifying 24VAC will give you about ~28VDC. This means you will be dropping 23V across a linear regulator like the 7805. The current though a linear regulator is the same on both the input and output sides. The more current you draw the more heat/power the regulator must dissipate.

Power is: P = I * E, so you can figure how much power the regulator will have to dissipate, i.e.

P = 0.100A * 23V = 2.3W
P = 0.500A * 23V = 11.5W

The TO-220 version of the part can only dissipate about 2W without a heatsink and perhaps 20W with a nice big heatsink. If your circuit uses any appreciable amount of current than you will be wasting a lot of power by using your regulator as a heater :slight_smile:


#10

Jeff

The plan is to use a Murata 78SR module. This is a switch mode circuit in a module that has a TO-220 like package making it a drop in replacement for the 7805.


#11

I used a KBU8K rectifier (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=KBU8KFS-ND) because I love overkill. For my filtering cap I used a 4700uF 63V, again, because overkill is awesome.

My order is currently in-transit from Digi-Key, so I’ll throw it together and let you know how it went when I’m done. Now, if I could only get my hands on a dirt-cheap o’scope…


#12

[quote]For my filtering cap I used a 4700uF 63V, again, because overkill is awesome.
[/quote]

This cap would have been so much better: http://shop.conrad-uk.com/components/passive-components/capacitors/electrolytic-capacitors/high-capacity-electrolytic-capacitors/477079.html

:slight_smile:


#13

LOL


#14

[quote]
This cap would have been so much better: http://shop.conrad-uk.com/components/passive-components/capacitors/electrolytic-capacitors/high-capacity-electrolytic-capacitors/477079.html[/quote]

Lol. Yeah, seriously.

In the end, I had to use a 1000uF for physical space reasons :frowning:


#15

I finally got around to bread boarding this circuit (thank you for the help!), and it seems to work fine with a small low rated adapter (24v dc .3 amp) but when I use it with a larger 24v ac adaptor rated at 1 amp the fuse blows straight away. The adapter I’m using is below - is it not suitable?

http://jaycar.com.au/productView.asp?ID=MP3032&keywords=24v+ac&form=KEYWORD

I just need to drive a solenoid, so 1amp seems excessive anyway, but can’t find any lower rated ac adapters with Aust. power plug.

Guess question is, can this adapter be used somehow? Or can anyone recommend a source to get a 24v ac adapter with lower rating?


#16

Hey Ross - happy aussie christmas !

You say 24v DC once there; DC and AC transformers will definetely need different circuits. That wasn’t what you did was it?


#17

Hi, and merry Christmas to you!

The circuit has a bridge rectifier in in to convert the ad to dc, but I didn’t have an ac adapter, so tested with a dc adaptor before I bought ac adaptor. I put in a 1amp quick blow fuse, and it blew straight away (twice).

I bread boarded the circuit, and have soldered it onto a fez cobra prototype extension board.

Admittedly I’m pretty amateur with electronics, I really don’t understand why it would work with a lower rated adaptor. Could it be because it’s unregulated ac adaptor? Or any other ideas?


#18

the rectifier is for turning AC into DC, and there’s no point; its already DC. I am far from an expert so don’t actually know how it would react but I certainly don’t see it as a valid test. Certainly the fact that the fuse blows when you connect AC might imply that there’s a wiring issue?

Do you have a multimeter handy, can you check the outputs along the way (on the output side of the rectifier ) with no load connected to it? Also did the fuse blow when you had a load connected or just when you applied the input power?


#19

Also, did you ever tell us why you needed to run 24v AC? I’m interested to know why you don’t just run your AC and DC circuits separately instead of trying to rectify & transform?


#20

I want to run off 24v AC as Fez is controlling 24v AC water valves. I don’t want to have to have two power supplies running. Perhaps I wasnt clear with the AC / DC tests. I used a 24v DC adapter as a test power souce while building the circuit. It should have no bearing with the recifier except a sight voltage drop across it. Once I had it working with the 24v DC adaptor, I went and bought the AC adaptor (I didnt want to waste the $30 on an adaptor if I couldnt get the circuit working).

I’ve tested the outputs along the way with the 24v AC adaptor, and the correct 5v is being fed to the Fez connectors, so no issue there. Just after the rectifier, it is reading approx 24v DC, so it is rectifying the input as expected.

I’ve checked and double checked the wiring / soldering on the circuit, and all is ok (no visible shorts etc) so i’m a bit lost as to what is causing the issue. It also has the same issue when running on the bread board - which has also been checked and double checked and matches the circuit provided earlier in the thread - which leaves me at the assumption that either the adaptor is not suitable, or I need to add something else to the circuit - but have no idea what…