Sciencemadness Discussion Board

Electrolysis

veganalchemist - 25-3-2017 at 05:52

Hi, I need to do some electrochemistry with magnesium electrodes.

I've got the electrodes but the current needs to be reversed every 15 seconds.

It's run at a constant current of 50 mA at about 2.7 V

Can't thinnk how to get the current to reverse without physically switching it by hand.

Could it not be switched every 60 seconds?

elementcollector1 - 25-3-2017 at 07:33

555 astable squarewave circuit? That should be pretty easy to put together.

Melgar - 25-3-2017 at 07:54

Learn how to use arduinos (this will be a really useful skill in the future, inevitably. community support is great), and set up an H-bridge circuit. You'll probably need at least two p-channel mosfets and two n-channel ones, as well as a way of measuring current. H-bridge circuits are usually used to reverse the polarity of a DC motor, so that it can change directions, but it should also work for what you're doing too.

If that's too complicated, use a DPDT relay for the H-bridge circuit, which you can figure out how to wire up just by searching for those two terms. I still recommend the arduino though, because of how configurable they are, and the fact that you can get the basic kind for around $6.

[Edited on 3/25/17 by Melgar]

unionised - 25-3-2017 at 08:26

Quote: Originally posted by veganalchemist  
Hi, I need to do some electrochemistry with magnesium electrodes.



Why?
There's likely to be an easier way.

woelen - 25-3-2017 at 10:59

You can do the reverse switching with 2 MOSFETs and an oscillator which has two outputs A and A', where A = ~A'. Such an oscillator is easy to make with a 555 circuit.

But my underlying question is, what do you need this for? What do you want to achieve?

Melgar - 25-3-2017 at 13:25

Quote: Originally posted by woelen  
You can do the reverse switching with 2 MOSFETs and an oscillator which has two outputs A and A', where A = ~A'. Such an oscillator is easy to make with a 555 circuit.

But my underlying question is, what do you need this for? What do you want to achieve?

The power supply necessary would be non-standard, to say the least, but it could be done from mains AC using two half-wave rectifiers where the AC voltage has been dropped down to like 5 volts with a transformer. You'd also need negative voltage regulators that could deal with the inevitable short-circuits that tend to happen in electrochemistry.

Considering that 90% of the cost of buying electronic components is usually shipping and handling, it'd probably be almost exactly the same cost to buy 5 power mosfets as it would be to buy one. The easiest way though, without resorting to a three-pin power supply, would probably be a DPDT relay with the current controlled by a MOSFET, no?

I always try to answer these questions even if the OP doesn't actually need what he says he does, because it frustrates me to no end when I need a solution to something, and I finally find a page where the OP asked EXACTLY the question I needed answered, only for the community to point out that some other solution (that wouldn't work for me) would be better for him.

Sulaiman - 26-3-2017 at 08:24

I think that a 2-pole changeover relay to switch polarity
e.g. 2p2w.jpg - 10kB

with the coil of the relay powered by something like this
http://www.ebay.co.uk/itm/Cycle-Adjustable-6-30V-Relay-Modul...
should work.

[Edited on 26-3-2017 by Sulaiman]

veganalchemist - 26-3-2017 at 09:31

Got it sorted. Tried it out and it works.

Using a 15 s on and off reley to power a 2-pole relay.

Using a Cebek cyclic timer, 0.3 s to 60 s.

Not sure why you have to use magnesium but it says won't work with C, Pt, Cu, Ni, Cu or Pb.


Cathodic coupling of aliphatic esters.

Tetrahedron Letters, Vol . 36, No. 27. Pp 4805, 1995




Paulo99 - 29-3-2017 at 07:21

What do you want to achieve?