DC voltage control

Discussion in 'The Projects Forum' started by rectorp02, May 21, 2012.

  1. rectorp02

    Thread Starter New Member

    May 21, 2012
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    First, thanks in advance for your help.

    Here's a quick rundown of what I am doing. I use electrolysis to restore cast iron. Common practice has been to use a battery charger to supply the needed DC. I have converted a server power supply for this purpose. It works great but I would like to be able to control the voltage output in an efficient and inexpensive manner. I'm thinking a pot but I am unsure about which one would be correct but I am open to suggestions. With my limited knowledge I am assuming I need something like this but it is not inexpensive.

    Your thoughts and suggestions would be greatly appreciated.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    It is not efficient to control the voltage at the output of the power supply.

    You either have to control it internally to the power supply, which means getting access to the circuit diagram, or you have to build your own DC control circuitry, which is almost being back to square one.

    What are your requirements in volts and amps?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    I think what you actually need to control is current.

    Anyway, you might be able to put light bulbs in series with your system and thereby reduce the total current going to the electrolysis bath. It would take some experimentation to choose the right bulb and number of bulbs. Not elegant, but it could work.
     
  4. rectorp02

    Thread Starter New Member

    May 21, 2012
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    thanks for the input. I was looking for a simple and easy fix and I guess there just isn't one. Thanks again.
     
  5. #12

    Expert

    Nov 30, 2010
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    What isn't simple about wiring up some light bulbs? What's easier?
    It certainly isn't easy to try to use a 2 watt potentiometer to control an electrolysis bath.
    How about a Variac controlling a cheap, unregulated, battery charger? You wouldn't even have to change the plugs on the power cords.
     
  6. rectorp02

    Thread Starter New Member

    May 21, 2012
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    Wiring up some light bulbs is indeed a simple process. I have done quite a bit of research involving the electrolysis process and believe there are some basic concepts. First it is the amperage that does the work. Electrolysis begins to take place at around 1.5 volts. The ideal voltage is around 9 volts. Having said that I understand that the variables are numerous and each situation is different. I am attempting to set up a system that will be able to do anywhere from one piece to a dozen or so. Since my understand is that amperage is more critical than voltage I was thinking that by controlling the voltage, and thereby the amperage, I would be able to do some experimenting and have some measure of control when the number and size of the pieces change.

    I am here because of my limited knowledge in this area. Please don't take anything I say as sarcasm and if it seems as such it is only my lack of knowledge and I do appreciate you guys sharing your knowledge. First i would think that wiring light bulbs would be a bit cumbersome from an experimental standpoint. I assumed that using a pot would be a simple thing to accomplish (just 3 connections). The power supply I am using right now has a 500W output and I was assuming the pot would have to be rated 500W too, is that correct? I am attempting to get away from battery chargers, which is what I used previously. First one of the things I am trying to accomplish is coming up with an inexpensive alternative affordable to casual cast iron collectors. A good many of these would be used in a 5 gallon bucket or a rubber maid container or something similar with things such as rebar, coffee cans, used fencing, and the like as the sacrificial metal. Second, most chargers are half wave rectified and I was under the impression that the full wave rectification provided by power supplies would be more efficient.

    This is strictly a hobby type set up and inexpensive is the key word. These are not something I intend to sell but share what I have learned so that others can build their own. Right now I have taken a $12.50 power supply (versus $90 plus for a battery charger) and used it to clean some skillets by just jumping the supply on and hooking up a pos and neg lead to the container and it works. I am building an enclosure with a volt meter, ammeter, and quick disconnects (Anderson connectors) to make it easy to use and swap.
     
  7. #12

    Expert

    Nov 30, 2010
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    A 500 watt rheostat (potentiometer) is massively expensive ($250 and up). That is why fast switching regulators were invented. Fast switching keeps the heat problem under control by having the power transistors completely on or completely off in a time rate of modulation so the power is controlled by time on/time off, many times per second. They can be designed to control current instead of voltage but either way is pretty complicated compared to potentiometers or light bulbs.

    The problem with "controlling voltage, thereby amperage" is that the resistance of metals in a bath varies by batch, concentration of solute, and by the amount of time they have been in the bath. As the metal gets cleaner, the resistance goes down. However, with careful watching, you can get a fair degree of control by adjusting the voltage by hand. Have you been doing that or just "plug it in and count off the time"?

    I believe that even cheap battery chargers are full wave rectified. It's cheaper to rectify more than to use a bigger transformer.

    We really need a clue about the amperage.
     
  8. rectorp02

    Thread Starter New Member

    May 21, 2012
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    I have been using a battery charger and just checking the piece I am cleaning from time to time (SOP for all collectors I know). The amperage varies depending on the size of the piece and the cleanliness of the sides of the tank I am using (cut off LP gas tank) and varies between 5 to 15 amps. I have noted that at the higher amperage's the pieces clean quicker and more thoroughly. The solution is simply water and washing soda (sodium hydroxide) at a mix of 1 teaspoon per gallon. Since my research I am wondering if I should be checking the PH level, another subject.

    I take it from your response that I was correct assuming a 500W pot would be needed? Just curious for my own education, you're right about the expense and that defeats what I am trying to accomplish here. I suppose that an array of light bulbs of some sort could be hooked to some sort of switch scheme so that the number of lamps could be added or removed from the circuit without having to rewire would be a possibility. How feasible (expensive) would resistors be? Would they have to be rated 500W too? If my memory serves me correctly if resistors are paralleled the wattage is additive?
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    It doesn't matter how you hook up the resistors. The bottom line is the net resistance has to be equal to the target you are seeking and any power that is not consumed by the electrolysis tank has to be wasted and dissipated by the resistors. There is no easy way out.

    Just using some ballpark figures, if your tank uses 10A@40V that represents a load resistance of 4 ohms.

    Thus if you wish to reduce the current by half to 5A, you will need a 4-ohm 100W resistor.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Lightbulbs are the cheapest power resistors you can buy, and include visual feedback. ;)
     
  11. mcasale

    Member

    Jul 18, 2011
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    It seems that a bench top DC supply is what you need. It needs to have voltage AND current settings.

    These can be quite expensive, but there's a lot of cheap, decent quality stuff available now on the Internet. You can get used supplies on numerous web sites, including e-Bay. Search around on Google to see what's out there and if anything fits your budget.
     
  12. rectorp02

    Thread Starter New Member

    May 21, 2012
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    Thanks for the input. In electrolysis is is the amperage that does the work so I was actually looking for a means to increase amperage. Noramlly the process runs around 15 amps, this is determined solely by the set up. I understand that in a circuit where everything else remains constant that decreasing the voltage increases the amperage.

    To be clear on what I am trying to accomplish with this. I was hoping to find a way for average cast iron collectors to be able to accomplish electrolysis inexpensively. Surprisingly, to me anyway, a good many don't own battery chargers and don't wish to lay out $90+ just to clean a piece or two a year. I have determined through actual use that a $20 computer power supply will do the job nicely and some basic skills is all that is required. The addition of a volt and amp meter add about $15 and complicate the project just slightly but if you have the skills required to modify a power supply you should be able to handle this as well. So now for about a third of the cost of a battery charger you are in business. My understanding is that the electrolysis process works best at somewhere around 9 volts but to be honest there is precious little detailed info available cleaning cast iron with electrolysis. By detailed I mean what PH level, voltage, and amperage is most efficient. So my need is to be able to increase as well as decrease current and through experimentation attempt to learn what works best. To accomplish this I need to be able to increase as well as decrease the current flowing in the circuit without having to rewire or modify the circuit to accomplish this.

    I gather that the answer to my original question is that the easy way is not cheap and the cheap was is not easy.
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Something I have seen used which is cheap and simple was a series bank of power resistors (like 5W or 10W resistors) and an alligator clip. You just clip the lead onto different places in the bank to change the resistance.
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    This is a misunderstanding. To increase the current you have to increase the voltage.

    The current required will increase with the surface area of the work and decrease with increasing separation of the electrodes.

    I have no idea what current and voltage is required in your situation.

    I am going by the information you have provided. If in a typical setup you require 10V@20A then there are two ways to increase the current.

    1. You can use a variable voltage supply and increase the output voltage.

    2. You can start off with a fixed higher voltage, 15V or 20V and insert a series of fixed high power resistors, say 1-ohm 100W each. You will need about 10 of these.
     
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