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macmarch05/10/2010 10:00:22
147 forum posts
1 photos
I have a need to use a rheostat to control the current from a power supply giving 12 to 15 volts at up to 10 amps for a small anodising plant. Its working well by letting the tank take what it wants but if the current is too much over then the dyeing process becomes a bit of a lottery. 
 Looking around the net I find some at prices I can't afford. (£400)
I want one of about 5 ohms and 75 to 100 Watt rating. (I could have to drop up to 6 amps at 15v = 90W)
Has anyone got one they don't need (subject to an amicable negotiation on price) or can point me in the right direction?
 chhers
 
Ray
Andrew Johnston05/10/2010 10:13:54
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7061 forum posts
719 photos
Hi Ray,
 
Personally I wouldn't use a rheostat to drop the voltage, although it does have the advantage of being simple. Both RS and Farnell seem to have rheostats at rather less than £400. RS also do a wirewound 'adjustable' resistor, which might be suitable if you do not need to keep adjusting the resistance. Might be worth a look on their websites to see if they have anything suitable.
 
Regards,
 
Andrew
DMB05/10/2010 10:19:40
1585 forum posts
1 photos
Hi Ray,
FWIW, I remember anodising done in a school science lesson. I clearly remember the excellent teacher demonstrating the effect of relatively high and low currents. A high current quickly deposited more plating of a very soft sludgy nature, easily rubbed off.
A low current took a long time to deposit a much thinner plating which was more or less permanent. This plating was then coloured with a dye. This was 40-50 years ago, so cannot help with what the dye was or actual currents/voltages. Hope the foregoing may be of  a little help.
John.
Ian S C05/10/2010 10:35:38
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7468 forum posts
230 photos
The element for a bar heater, the type with a ceramic rod with the nichrome wire wound on the outside makes a good adjustable resistor, and its cheap, proberbly get one at a garage/ car boot sale. Ian S C
macmarch05/10/2010 11:01:52
147 forum posts
1 photos
Not quite the answers I was expecting but nevertheless problem solved.
Ian, using an old fire element, great idea but some of the fumes about when anodising are flammable. not overly so, but still a bit iffy.
Andrew. Just had a look on RS site. I hadn't seen the adjustable type only the 1 turn type.
At 12v and 100watt rating the element will stay black and I can mount it on the outside of the power supply so I can adjust occasionally and fit a roomy cover over it. nice and cheap to.
 
cheers all.
 
Ray
john swift 105/10/2010 11:41:50
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318 forum posts
183 photos
Hi Ray
 
what kind of power supply do you have , a a simple transformer and rectifier
or a 13v regulated supply you would use with a CB for instance ?
 
 how often do you need to adjust the current and how fine a control do you need
 
a simple solution could some thing like maplin's 3-15v  25a power supply  RP10L
 
but at £99-99  I expect  several high power aluminium clad resistors from RS or Rapid
 
and a simple diy switch ( to switch  one or more resistors in series or parallel combination ) 
 
will be the  less expensive solution your looking for
 
another diy sollution is to build an electronic regulator to control the output from your existing
power supply
 
the simplest using a low power potentiometer to control one or more power transistors to
emulate a high power potentiometer
 
 
diagrams to follow
 
 
   John
john swift 105/10/2010 14:01:26
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318 forum posts
183 photos
Hi Ray

just some ideas to replace rheostat
more details required to calculate missing component values
does this help ?
 
 
                   John
john swift 105/10/2010 14:32:17
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318 forum posts
183 photos

    I think i 'd use the constant current circuit  to adjust the plating / anodising current
 
      John

Edited By john swift 1 on 05/10/2010 14:42:57

macmarch05/10/2010 20:14:49
147 forum posts
1 photos
Thank you John, I'll take all that on board.  I have sufficient knowledge of electronics but power supplies are a different kettle of fish. 
 
My first priority is to get a power supply up and running to get going on whilst I build a 12 - 24v 30A variable voltage, variable current PS.
 
This small unit will supply up to about 6Amps and the rheostat, (variable resistor ex RS) will enable me to cope with small numbers of parts.
 
I have a transformer for the heavy power supply  and suitable caps. I just need to get 8 off 2n3055's and 4 off rectifiers. these will come from RS.
 
The anodising current is not critical, a working figure is 150ma per sq inch.  Calculating the actual area to be anodised, especially when there are screw threads, is to say the least hit and miss. So within reason wire up the parts, connect PS and let them take what they want.  The thing to watch is letting it take too much. The current/time affects the size of the surface holes. Too small and the dye won't go in and too big and the dye won't stay in.  The actual current only needs to be near enough. I might actually use a bank of 1 ohm HSA type resistors wired in series parallel and switch them in to achieve a supply rising in 3A steps.
 
Thank you all for the help.  I'll get it built in the next week and put up  a pic of the finished unit.
 
cheers
 
Ray
 
 
WALLACE08/10/2010 13:43:29
304 forum posts
17 photos
Variacs (variable autotransformers) are cheap enough on ebay - and won't heat the room up either !
 
Need a transformer in the way as well though to give isolation from the mains - this is important !!!!!
 
Seem to remember from a magazine article years ago that a pulsing current is better for plating - I think a crude way to do it is use a bridge rectifier with  resitors across two of the diodes so there's about 30% reverse current if that makes sense.  I don't think you need any smoothing either - but I've no idea if this is the same for anodising.
 
Dare say Google will give a lot more info !
 
W.
 
 
Les Jones 108/10/2010 20:21:55
2292 forum posts
159 photos
Hi Wallace,
                       I agree with your suggestion of using a Variac but resistors across two of the diodes of a bridge would not produce a current pulsing in the reverse direction.   A resistor in parallel with a single diode would produce this effect.
Les.
Les Jones 108/10/2010 20:24:35
2292 forum posts
159 photos
Just a clarification to my last post. I do not mean a resistor across one diode of a bridge.
I mean as a single diode as a half wave rectifier.
Les.
Billy Mills08/10/2010 23:13:10
377 forum posts
Ray
Would you be better off running a constant current supply instead of a constant voltage with dropper resistors? One way of arranging this is to have a number of -say- 1A current sources, you then switch in as many as you need to get the current desired. Or if you wanted to be digital have 1A,2A,4A, 8A sources seperatly switchable.
 
 For a 1A module you might use  something like a TIP120, output from the collector , emitter to -ve supply via 0.68 ohms  1W . BC184 or any other small NPN , emitter to negative supply, base to TIP120, collector to TIP120 base. A 4k7 from the last node to + supply completes the job. You can mount all TIP120's without insulators on a common heatsink. Wire a switch between -ve supply and the TIP120 base to turn off that current source. Dirt cheap, simple and reliable.
As you might be warming a heatsink a fan might be good news. Nowadays instead of spending a fortune on finned ali extrusions,  people are using smaller ali heatsinks and blowing air over them ( the heatsink should get the airflow FROM the fan for best cooling as the flow is turbulent).
Regards
Alan.
 

Stub Mandrel10/10/2010 22:24:38
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4318 forum posts
291 photos
1 articles
Mac,
 
Don't fear an old fire element - at 1/20 of normal volts it will only run warn, not red hot.
 
It's exactly the same as a big rheostat just without the wiper.
 
Neil
Ian S C11/10/2010 09:58:49
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7468 forum posts
230 photos
Neil, it's actually quite simple to set up a wiperon a fire element. A couple of rods mounted parallel to the element with a sliding block on them, with a wiper attached to the block. Ian S C
Roger Brealey11/10/2010 11:54:12
4 forum posts
If you are using a mains power source, it may be easier to control the mains input to the power supply by using something like a light dimmer.  Easy to get hold of, ready made (just needs an enclosure) and reasonably cheap
 
This probably won't work if it's a switched mode power supply.
Ian S C12/10/2010 01:55:23
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7468 forum posts
230 photos
Roger, I tryed that for the power supply that I use for the power feed on my mill, fed the imput from a 1Kw dimmer, did'nt work to well with the windscreen wiper motor(via DC), so using bits of nichrome wire made up 3 resistors, made a rotary switch, put the lot in a metal box, and its been OK ever since. The dimmer, and outlet are still on the control pannel,but so far have not been used, perhaps if I put a universal (brush type) motor on my Super Adept, instead of the induction motor it has now I could have variable speed! Ian S C  ps The non sine wave form may be OK for anodising, once rectified.

Edited By Ian S C on 12/10/2010 01:57:25

macmarch12/10/2010 20:49:18
147 forum posts
1 photos
Wow, so much help. Thanks everyone.  I have now solved the problem.
30 amp transformer + ham radio high current rectifer block, (multi finned and about an 8" cube + 10 2n3055's + a max current limiter. Output controlled by switching in as many as needed.  Working a treat..........so far!
 
cheers all
 
 
John Olsen12/10/2010 21:33:55
1294 forum posts
108 photos
1 articles
I would not suggest using a light dimmer in front of a power supply, as mentioned above by Roger. Light dimmers are not necessarily designed to maintain a symmetrical waveform at lower power, so you can end up with a DC component going into your transformer. Even if you don't, the jagged waveform will cause the transformer to make unhappy noises.
 
I have seen this done in a commercial supply, by Hewlett Packard no less, back when that name meant quality. But their preregulator was designed for the job, so would have maintained a symmetrical waveform. Simple light dimmer circuits don't, since the triggering thresholds for triacs are not necessarily symetrical. When I tried it myself with a light dimmer, it worked to a degree, nothing blew up but the transformer was quite evidently not very happy.
 
regards
John
Axel15/10/2010 11:05:26
126 forum posts
1 photos
Lights dimmers are not legal to install for the DIY'er in my area, check yours.
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