# 555 timer reverse polarity circuit

Discussion in 'General Electronics Discussion' started by welshdragon, Jul 23, 2012.

1. ### welshdragon

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Jul 23, 2012
G'day All
I need to build a circuit that enables >30v +/- DC to reverse polarity every minute. I was told to use a 555 timer and a relay but i haven't a clue how to work out a circuit and what i need to get.
Any help would be much appreciated.
Chris

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3. ### CocaCola

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Apr 7, 2012
What is the intended purpose and what kind of current for that 30VDC? The circuit is simple enough, I'm just concerned about the load on the relay...

Have you Googled up 555 timer circuits? There is a WEALTH of online tutorials for them, and tons of good info... It all makes for a good read for this project...

4. ### welshdragon

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Jul 23, 2012
Thanks for the replies, the amperage will be <30ma.

5. ### CocaCola

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Apr 7, 2012
Then you can likely just use a transistor and simplify things over a bulky/noisy relay...

6. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Quieter and more elegant than a relay but a discrete H Bridge is going require more drive circuitry.

Edit: Oops I see he has a bipolarity supply. Won't need an H Bridge for that but it's going to hard to make the circuit simpler than a DPDT relay.

Last edited: Jul 24, 2012
7. ### CocaCola

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Apr 7, 2012
Yep, a transistor or (solid state relay) is certainly quieter and more elegant then a DPDT, but the DPDT is certainly the simple solution here...

8. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Please clarify this. does any part of this circuit currently exist?
Does the load have to be committed to ground?

I ask these questions because a typical method of reversing polarity is not by reversing the power supply voltage. This requires two power supplies. One Pos and one Neg with respect to circuit common or ground.

Typically we use a circuit called an H-Bridge. An H-Bridge swaps the polarity of the load while the supply remains a constant positive voltage with respect to circuit common or ground.

Most DPDT relay circuits that are used for polarity reversal are actually mimicking an H-Bridge.

9. ### welshdragon

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Jul 23, 2012
G'day
As of yet nothing has been built.
It doesn't need to be grounded.
The power will come from a dual 30v current limited bench power supply.
The + & - are connected to electrodes which are submersed in a solution.
My purpose for this circuit is to alternate the polarity on these electrodes (anode/cathode) approx every minute.
Why do i want this ?: to minimize oxidization on the electrodes.
regards
Chris

10. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Thanks for clarifying that. Since you have a dual (+-) supply a SPDT relay will suffice. Do you want a solid state alternative? If so, it will be more complex but should last forever.

11. ### welshdragon

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Jul 23, 2012
G'day
Ideally i would like to build a circuit that is as simple and robust as possible.

12. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
That would be a 555 driving a SPDT relay. I'll draw it up for ya.

13. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Give this a look see and tell me if you understand it. The +12V supply for the 555 is an inexpensive wall wart. I could have added a 12V regulator and powered it off the 30V leg of your power supply but wall warts are easy to use and cheap. Besides, all of us have multiples of them that are acquired over the years.

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14. ### welshdragon

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Jul 23, 2012
G'day,
Thank you for taking the time to help me with this design.
I do not quite follow how the 30v +- changes polarity. I understand the +- before the SPDT relay but cant follow where it goes after.

15. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
You stated that you have a dual (+ -) supply. The wiper of the relay just switches between your + and - post with respect to your power supply common.

16. ### welshdragon

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Jul 23, 2012
Thank you, after physically drawing the SPDT relay circuit on a sheet of paper i now understand how it works. I will now be able to put a shopping list together of all the components needed.

Please advise if any of the capacitors or resistors have to be of a certain type or wattage respectfully.

Many thanks

17. ### Electrobrains

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Jan 2, 2012
If you wouldn't have a +/- 30V supply, you could (if your application is isolated towards your supply), also use a single channel supply with a DPDT relay.

If you use a sensitive relay like for instance RT424012, you could also pick your 12V easily directly from your 30V supply.
You would then need an 18V, 1.3W Zener diode, like for instance 1N4746A.
It will heat up a bit, but it should be no problem. (tip: leave the wires a bit longer than usual, they will act as heat sinks).
See optional changes to CDRIVE's diagram.

The new parts you can see here:

http://www.newark.com/jsp/search/br...nsearch&Ntt=rt424012&Ntx=mode+matchallpartial

http://www.newark.com/vishay-semiconductor/1n4746a-tr/zener-diode-1-3w-18v-do-41/dp/33C2794

The components around the 555 timer are not critical. I suppose CDRIVE has dimensioned them well. For a bread board assembly, I would use normal 0.4 or 0.6W resistors. I would choose the big capacitor to be an electrolytic one with minimum 16V rating. In fact the higher voltage rating the better, because the leakage current would decrease. Better would be a tantalum cap, but it would cost much more. For precision circuits even a better cap would be some type of film capacitor, but as I understand, you don't need that. Be aware of polarities of diodes and capacitors.

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• ###### Polarity Changer.jpg
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18. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I've updated the schematic to indicate parts list. In the event that you don't want to power it from a 12V wallwart I've also added a 7812 voltage regulator.to the circuit. This will enable you to power it from the +30V leg of your supply. I would not recommend using Electrobrains Zener design. Zeners aren't intended (as Voltage Regulators) to be wired like that.

The timing cap (C2) is Electrolytic. You're not going to find a Tantalum cap, (as Electrobrains suggests), @ 400uF.

Your contacts current is low, so the relay isn't critical. You can get one at Radio Crap or the link in my notes. You don't need a relay contact rated > 1A AC. The higher the contact current rating the lower the coil resistance will be and the more current it will demand. You don't need it in this application.

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19. ### welshdragon

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Jul 23, 2012
G'day
I will be using a 12v wallwart for this project as i need to set a constant current on the 30v supply to achieve a constant 30ma across the electrodes which is adjusted down to 24v at the end of the process but still maintaining 30ma.
Would it be an easy alteration to add leds to the circuit to show the polarity change happening visually ? if so, please advise.

20. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
It would have been helpful to know this before I drew the schematic. Replace K1 with a DPDT relay and wire the LEDs like this.

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