Andy said:
Hi all,
I'm looking at building a Voltage Controlled Current Source; approx 15V
output, with up to 70A available - I need to be able to control the
current.
I've found this schematic (
http://www.neillans.co.uk/VCCS1.pdf), and if
I were to uprate all the components (e.g. from 1/4W resistors to 1W or
higher), and replaced the IRFZ48Vs with the IRFZ48Ns which have a
higher current load, would it work?
I tried to build + simulate this circuit in EWB, but it didn't work out
- but that could be down to my VHDL
I've been working with electronics etc for years, but never ventured
far from digital solutions, although I have knocked up power supplies
in the past, they have never been VCCS.
Please Help
Andy
I've made a couple of VCCS type plating power supplies capable of reverse
pulse waveforms (or other waveforms if you wanted...), but none as big as
what you're suggesting. The first one I did used a TDA2030 audio op-amp
(very cheap opamp, puts out at least an amp but has large DC offset, and
not unity gain stable). The circuit ran off split supplies, probably about
+/-10V. I connected the supply pins of the op-amp to the bases of 2N3055
and 2N2955 transistors to boost the output current, so when the op-amp
would start to struggle with the output current, it gets a bit of 'help'
from the power transistor. I think it would do about 10 Amps comfortably
but I can't remember and didn't need to push it too far anyway because the
object being plated was small. (The emitter of the PNP 2N2955 goes to the
positive supply, the collector goes to the op-amp output, the positive
supply pin of the TDA2030 goes to the base of the 2N2955, and there is a
resistor maybe 10 Ohms from the base to emitter. Same for the NPN 2N3055,
but the emitter goes to the negative rail and the base goes to the negative
supply pin of the TDA2030. The output of my boosted power op-amp went
(through a small resistor or inductor to help stability I think...) to one
electrode of the plating tank, and the other electrode of the plating tank
went through a current sensing resistor to the common of the power supply.
I say common rather than ground, because I think I actually floated the
power supply itself to allow one of the electrodes to be grounded if so
desired. Of course the current sense resistor develops a voltage which is
fed back to the inverting input of the TDA2030, and the non-inverting input
is driven with the waveform which you want the current to follow, that
waveform being with respect to the common of the power supply, (not
necessarily ground). I think I used a 555 timer and a 4066 chip and
several pots, to make the reverse pulse waveform. I think I needed some
fancy things like Zobel networks to make it all stable, most of which I
copied out of the TDA2030 datasheet (which contains only audio circuits),
and tweaked by trial and error, with various test loads and looking for
ringing on a scope.
By the way, the TDA2030A and 2040 are probably better, I think one of them
includes protection diodes to the supplies which are absent from the 2030,
and the current rating might be better.
The second plating supply was much fancier, it had 16 channels to drive 16
independent plating outputs, and the current sources were proper ones which
could drive a real ground referenced load - I used a current sensing
resistor in series with the output pin of the power op-amp. I think each
channel would drive 1Amp and it was all software controlled using DACs and
a PIC microcontroller connected to a PC through a serial cable. If you're
really interested I could try to find the old schematics, but it's in
protel format so it'd take me a while to fix up a PC to convert it to
anything else.
Chris