Hello Folk, My first time here. I need help. I have a transformer that takes in 220VAC and puts out 12 or 24 VAC. I need to talk that output and change it to a DC that keeps changing its polarity every few minutes. The output needs to stay between 500mA and 650mA. I need to be able to feed at least three parallel loads with that range of current. Have looked at switches and relays but nothing makes sense to me. The circuits I come up with keeps becoming complicated. Can any one help plz??? I am not an electronics person so please keep that in mind when you suggest solutions
Thanks

Hi Folks,
I had posted a question earlier. Your responses made me realize that I need to do a better job explaining what I am doing and look for help with. Attached picture does it better.

Summery:
I have 220VAC~240VAC coming out of the wall. Need to change it to clean 9VDC~21VDC. Then I need a way to flip polarity every minute of so. Only one polarity is needed at a time. Load is dumb. It just sits and burns. Current range required at load is 500mA~650mA. The picture explains more.

Following questions are on the picture too:

There could be more loads (three total perhaps). Is there a way to keep current to each load the same 500mA~600mA range regardless of how many loads are there? (say max three) I can put a turn knob to set flow of current depending on how many loads there are.

What would it take to create all this for more loads? Which component would have to be altered?

Could this work better if I choose a transformer with different output capability (higher or lower power or output)?

Is there a way to reduce the frequency of the AC current from 50~60 Hertz to maybe 1~5 Hertz. If that can be done I can go directly from transformer to selection knob. (then there is no need to convert to DC or to switch polarity) The current at load needs to stay the same though.

 

You would need a DC power supply that runs with your AC input and produces both polarities of DC outputs, and then a way to switch between them.

 

There are many ways to do this. Does you 12/24v transformer have a center tap or perhaps two 12v windings that can be placed in series? If so then you can use a "center tapped full wave bridge rectifier" to produce a plus and minus voltage. A simple SPDT relay can then switch between the two. Look in vol 3 of the eBooks listed on the top the page and read about diodes and rectifiers.

How many minutes do you need exactly? A simple LM555 timer IC should be able to operate the relay if the time isn't excessive.

How pure must this DC voltage be? What is the load that you are driving?

 

Yup. And if your transformer doesn't have a center tap, or a dual secondary, you could either get a new transformer, or, create a single-polarity DC supply and then also use a switchmode power supply (SMPS) that produces both positive and negative DC voltages from a positive DC voltage.

What is your approximate maximum budget for this project? And at what DC voltages do you need the currents you specified?

 

You can make a DC supply with the transformer. Then use an H-bridge or a relay to reverse the polarity to the load.

http://www.google.co.uk/imgres?imgu...a=X&ei=vh4NTYuECMyLhQecy423Dg&ved=0CDkQ9QEwBQ

Figure 2 here:
http://www.trainelectronics.com/Reverse_loop_dual_coil_relay/index.htm

 

500mA-650mA power, is that PER load, or all 3 loads in parallel.

e.g. Power supply of 1.5A or 500mA?

An H-Bridge solution would be the most straightforward if the loads aren't somehow permanently grounded.

What are the loads, and what is the frequency of polarity reversal?

 

You can make a DC supply with the transformer. Then use an H-bridge or a relay to reverse the polarity to the load.

http://www.google.co.uk/imgres?imgu...a=X&ei=vh4NTYuECMyLhQecy423Dg&ved=0CDkQ9QEwBQ

Figure 2 here:
http://www.trainelectronics.com/Reverse_loop_dual_coil_relay/index.htm

Oh, duh! I should go to bed earlier, apparently.

That's right. If your load doesn't need both polarities at the same time (and is floating, i.e. isn't connected to anything referencing your AC Mains ground or neutral as "ground", or, if your DC supply is floating and the load doesn't care), then you can just use a single-polarity DC supply and swap the connections to the two supply outputs to reverse the polarity seen by the load.

 

Hello,

Your new post is merged in the firts post of the earlier started thread.
(the merging went not as I wanted, sorry).
You do not need to start a new thread.
Now you can continue here.

Bertus

 

There are many ways to do this. Does you 12/24v transformer have a center tap or perhaps two 12v windings that can be placed in series? If so then you can use a "center tapped full wave bridge rectifier" to produce a plus and minus voltage. A simple SPDT relay can then switch between the two. Look in vol 3 of the eBooks listed on the top the page and read about diodes and rectifiers.

How many minutes do you need exactly? A simple LM555 timer IC should be able to operate the relay if the time isn't excessive.

How pure must this DC voltage be? What is the load that you are driving?

Thanks for your response. I have reworded the question. There is more detail too. Would you mind having a look please. Thanks