two 24VAC Sources to one 5VDC

[bill12780]

Hello all,

This is my first post here. So just a bit of background. I am NOT a fully educated Engineer. (read--Associates in Electronics with 25 years of experience) However, I do play one at work. So somtimes I ask pretty stupid questions.

Anyway, I have a project that is really perplexing me. I have a control module (Another company) that outputs TWO(2) 24VAC Sources and share a common. One source actuates a valve open, the other, Closed.

I need to interface OUR sensor to controll these two sources via a ucontroller. These 24VAC sources are the only avaliable source of power to the circuit. So I want to rectify them and use them to make a 5V source. Problem is that they are never on at the same time and they switch back and forth depending on what the controller module is telling the valve to do.

I have worked out the details but It requires two relays and two isolation transformers, and two bridge rectifiers.

Anyone got any ideas how to do this other then what I have come up with?

Any advise here is greatly appreciated.

Best Regards,
Bill12780

 

[itsmoked]

The diodes smoked with extreme prejudice.. They wouldn't in my scheme above! Oh this was with two bridge rectifiers?!?

I don't see how two bridge rectifiers can possibly work.

You want to diagram your scheme Scotty? I want to see how you are going to sort out the two pseudo grounds.

You can't tie a bunch of separately derived DC returns together if they are powered from non-floated supplies.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

Had assumed (and we all know where that leads!) that the two supplies were isolated. However what was I thinking of? Auctioneering diodes? If we assume that the supplies are isolated the bridges will self-auctioneer anyway, with the highest output hogging the load. Will scan a sketch in at work if I get time.

With the added information in Bill's last post we needn't worry about isolation. One PS with a changeover on the input. Just needs a big reservoir capacitor to ensure that the relay changes over without causing a dropout in the supply.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Scotty; Reminds me of today's effort. 400 ton press control installation. I had a jury-rig to power my control system since the 480V wasn't yet run. I had hooked a chopped off power cord with a big DPST switch directly to the control system side of the 750VA control transformer. Pulled the fuse that allowed the 120A back thru the transformer so I wouldn't create 480V.

For weeks I'd been running the system this way.

I get it on site and mounted. Pick up the local extension cord plug it into my plug. Throw the switch in my hand. BAM! Bright aquamarine blue/green flash! What was that??!

I try to turn off the switch. It's welded closed. I'd let all the plasma out of the switch, with extreme prejudice, I might add. A breaker down the hall is tripped...

I had, of course, bonded the 120V Neutral to the safety grounds all over the box which was now mounted to a grounded 67,000lb steel machine. I look at the extension cord... The ground pin has been cut off. This allows it to be plugged into its outlet either way. Guess which way I got it.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bill12780]

Man I have to tell you...I could learn so much working with either one of you two. My boss is...lets say...well lets not say anything...

Yes the scheme was with two bridges...I thought the same thing...."To many Grounds!" But I thought I will try it cause basically I am a hack. And what followed was that tell tell smell of burning semiconductor.

I do have the whole thing working. I am gonna try and post a schematic latter today so you can all see what I was doing for sure. AND to just get any opinions anyone may have.

and your right about the cap. I just did the math for the filter cap and doubled it (2200uf) seems to be fine. I only see a small "glitch" on my scope during the switch and my meter never even moves. (I am just using a toggle switch to simulate the control box right now.)

AND! I checked the +5VDC by running a different microcontroller circuit and it worked fine.

I will get that schematic up today sometime.

Thanks again for all your help!

Bill12780