Re: 1 KVA 240 v transformer output 6v,8v,12 v AC how to test if it can supply 83 amps?

On Tue, 12 Apr 2005 12:32:51 +0100, Rob <Rob@xxxxxxxxx> wrote:

> Ok let me explain a little bit more.. on the front panel i have 2
>control knobs each one controls voltage and the second one is used to
>set a limit on the current...the digital display, displays the actual
>voltage and thel current in use..
>
>Today what i have done is the following .. The rear Positive and
>negative terminals i connected a piece of copper welding rod about a
>foot long its about 2 mm thick.. To me this is a dead short and i
>expected to be able to turn up the current so as to melt the copper
>rod... However all that happend was that at about 30 odd amps it
>glowed red and then the current gradually dropped to about 25 amps

---
Yes. what happened is that the resistance of the rod was greater than
you thought, initially, and then as it got hotter and hotter its
resistance got greater and greater and increased to the point where
with 5 volts across it the power supply could only push 25 amps
through it.
---

>...I admit i am a total novice at electrics but i was sure it would have
>took more current than 30 amps...i was sure it would glow white hot
>and pop like a fuse?...I then cut the welding rod and tried it at
>about 3 inch long this time it glowed whiter at about 80 amps...at 5
>volts..Now i have to say im getting very pissed off!!!..

---
Why? You're starting to get at the root of the problem! A shorter
piece of rod would have less resistance than than a longer one so, for
the same voltage across it, the PSU would push more current through
it, exactly what happened!

Since you cut the rod to 1/4 the length of the one foot one you stared
out with, the resistance would drop to 1/4 the resitance of the first
one, so with the same voltage across the short one you could expect
four times the current through it. Since you got 25 amps through the
long one, that means you should have been able to get 100 amps through
the short one, but since the short one got hotter than the first one
its resistance would increase to the point where you'd get something
less than 100 amps, which is what happened; you got 80 amps. Pretty
close, though!
---



>So next i added to the rear terminals a 10mm bolt about 3 inch long ..
>turned up the current and i stopped as it went over 150 amps???....
>didnt want to break anything so i stopped increasing the current...
>Now i am totaly baffled

---
The bolt had less resistance than the copper rod, so the PSU pushed
more current through it.
---

>the PSU can supply at least 150 amps to a
>big bolt?... so why isnt it giveing that type of amperage thru
>my electrolyitic cell?...

---
Could be lots of reasons... The resistance of your electrolytic cell
is too high, the cables connecting it to the PSU are too long or have
a diameter that is too small, the connections from the PSU to the
cables and from the cables to the PSU have a resistance that is too
high...
---

>My final test was to use a car battery tester.. this consists of a
>large strip of bent metal in between 2 metal prongs.. which has a amp
>meter in between it can test 12 v and 6 v car batterys...you normaly
>hold the prongs on the battery terminal and the heavy metal strip
>gets very hot you can read the ammeter readings to see how good the
>battery is..the bent metal strip is coverd with asbestos *** to stop
>getting burnt..
>
>Well i connected this battery tester to the positive and negative
>connection while my Dad turned up the current control..we stopped the
>experiment at about 250 amps.. it was obvious the PSU was working?....
>
>So i am totally gutted have i just wasted a lot of money... on a PSU..
>
>Why ? wont the PSU supply lots of amps to my Cell?....I have read
>several patents...the Voltage on the cell ranged between 3.9 v and 4.5
>v? at several 100 amps?.. the electrolytic solution is a super
>saturated Salt solution..?... I have replicated the exact experiment
>yet it wont work?..So surley the PSU is faulty?... but yet if it is
>why did it supply 250 + amps in the battery test?.. is there some way
>i can force the amps out of the unit?...

---
Lower the resistance of whatever is between the PSU and the cell or
get a PSU with a higher voltage output.
---

>If the answer is yes I have wasted even more money .. what power
>supply do i actualy need?...I have had several knock backs with this
>project.. but this one as knocked me of feet..My friend's say i have
>to stop wasting money...but i will get another power supply if i have
>to...i wont be beaten...i will just go bankcrupt :)..

---
You refuse to quit, huh? EXCELLENT!!!
---

>Hope you can advise?...

---
If you crank it to the max with no load on it, what kind of voltage
can you get out of it?

Your PSU obviously works, so the trick is going to be to get as much
of that 5V it can put out to the cell.

If we look at the worst case input voltage for the cell, you say that
that's 4.5V, so that means that if the most you can get out of the
power supply is 5.0V and the cell needs 4.5V to work, you can only
lose 0.5V in the wiring between the supply and the cell. If you want
to put 250 amps into the cell, that means that 250 amps also has to go
through the wiring, so the maximum resistance you're allowed is:

E 0.5V
R = --- = ------ = 0.002 ohms
I 250A

which isn't a lot of resistance, so the first order of business will
be to select the size of cable which will pass 250 amps without
dropping 0.5V. If we arbitrarily say that the PSU will be located so
that there will need to be ten feet of cable from the PSU to the cell,
and then ten feet of cable from the cell back to the PSU, then that
means that the cable we choose will have to have a resistance of less
than 2 milliohms per 20 feet, or 100µohms per foot.

Consulting a wire table, we find that #0 AWG has a resistance of 0.098
ohms per thousand feet, so that will work. With a diameter of 0.325",
it won't be that hard to work with, either. To get the lowest
resistance connections you should solder lugs on the ends of the
cables and make sure the connections to the power supply and the cell
are good and tight.

One final trick, your power supply has remote sensing, which means
that instead of keeping the voltage regulated at the output of the
supply, you can set it up to keep the voltage AT THE CELL regulated
and compensate for the cable resistance, within limits. I'd still go
with the #0 and the soldered lugs and all, but I'd use remote sensing
to compensate for cable heating and changes in the cell resistance
when it's running, etc. Check 3.5 on page 36 of the manual for how to
do it.

--
John Fields
Professional Circuit Designer
.

Loading