Swany - 29-10-2006 at 16:03
I need some quick help: I need to create a 12v supply that gives 1.5 amps maximum, but can drop down to about 0.8 amps. It will not be in continuous
use and will be used to power a portable HV transformer to show some interesting HV phenomena. I can use any common cells; AAA, AA, C, D etc. I am
trying to work this problem myself, but I could use some help. I hate to ask like this, but I put it off for too long.
Thanks in advance.
Magpie - 29-10-2006 at 16:17
This is going to be quick and dirty, and probably doesn't really address what you need. But it may be a start.
Take 8 ea D cells (1.5v ea) and connect in series = 12vdc. Then by ohm's law if you want the current limited to 1.5a max, then R= 12v/1.5a = 8 ohms.
Place an 8 ohm resistor in series with the cells.
Now the unknown in this is the cell internal resistances at 0.8 to 1.5 amps. I don't know anything about that. 
Oops - don't you need ac current for a transformer? I think so.
[Edited on 30-10-2006 by Magpie]
not_important - 29-10-2006 at 21:43
How precise does the 12 V need to be?
Without having more detail, I would say either a pair of lantern batteries in series, see
http://www.duracell.com/procell/products/lantern.asp
for one set of possibilities and some data sheets.
Another is a small lead-acid battery, such is used on motorcycles.
And, yes, aren't you goingto need AC for that transformer? Or do you already have an inverter?
woelen - 30-10-2006 at 00:13
I assume you have some oscillator circuit, which does the HV generation by means of a (flyback) transformer.
For this purpose, I would go for a miniature car racing pack, consisting of NiMH batteries. These packs are failry cheap, and can easily be recharged.
They can provide good peak current and also have good continuous load properties.
Such a pack is available in 7.2 volts, 12 volts and 14.4 volts (6, 10, and 12 NiMH batteries in series).
YT2095 - 30-10-2006 at 00:35
SLA type leisure batteries would be my choice, the sort used as backup power in house alarm panels.
IIRC, `C` size cells only deliver 1000mah, so you`ll need at least a `D` cell and 8 of.
edit: this sort: http://www.maplin.co.uk/search.aspx?MenuNo=1259&MenuName...
[Edited on 30-10-2006 by YT2095]
Elawr - 30-10-2006 at 06:36
The newer NiMH batteries have droppped in price and are quite robust. I recently bought a 4 pack of AA size with charger at Walmart for under $20.
These 1.2V beauties are rated at 1200 miliampere hours. From this, it should be possible to get 12V at 1 amp easily from 10 of these in series. Of
course there are size C and D NiMH cells with higher capacity.
I'm interested in details about your HV dervice. It sounds very similar to a kit named "Plasma Generator" from Ramsey Electronics I once put
together. Based around an astable oscillator driving a ferrite-core transformer at around 20 kHz, the little device outputs up to around 15 kv. Fun
for playing with all kinds of HV tricks. Mine is set up for 12 to 15v input, but you can feed it up to 24 v and get proportionately higher voltage
output. Not for very long though or the power transistors will overheat! 
Swany - 30-10-2006 at 07:11
Yes, I have some of the NiMH packs, and usually I use those for this portable demonstration. However, my charger just died.
I need 12v DC, thanks for the help. Silly me.
YT2095 - 30-10-2006 at 23:59
you could use an ordinary car batt charger for them then all you`ll need in-line is a suitable reactive load, like a 12w car indicator/brake light
bulb. when you start charging, the bulb will be lit brightly, and get dimmer as the charge comes towards the end, when the bulb is out it should be
nicely charged 
not_important - 31-10-2006 at 05:48
Um, a tungsten lamp is resistive, if non-linear. A solonoid or motor would be reactive. But yes, the basic idea works; the lamp needs to be picked to
match the charging characteristics of the battery so as to properly limit current into the discharged battery. Might be a little tricky for NiMH, I've
never done the maths.
anterloperless - 12-1-2007 at 23:08
easy way.......
12V bat and a light dimmer switch from any hardware store. $11.00, 5 mins 
Quick, cheap, and easy solutions to HV phenomena !
Lambda - 13-1-2007 at 17:25
Hi @Swany,
Here you have few good links to get in gear:
PowerLabs Flyback Driver:
http://www.powerlabs.org/flybackdriver.htm
Snock's High Voltage Page:
http://www.geocities.com/CapeCanaveral/lab/5322/hv2.html
HV stuff:
http://www.kronjaeger.com/hv/
High Voltage Generating Circuits:
http://members.misty.com/don/hv.html
For most of these designs, with 1.5 Amps you will be sucking a little bit on the low side. With about 3 to 6 Amps, you will be able to generate some
interesting HV phenomena,....and that exactly what you want to do.
Secondhand small Lead Batteries of 12 volts and sufficient Power, go for about 5 to 10 Euro. You can buy them in just about any Electronics store.
This is what I mean (See Battery photo's):
http://www.batteriesamerica.com/newpage4.htm
You can also use the Battery Pack of just about any Rechargeable Power Tool for your experiment. They are usually about 9.6 to 18 Volts and deliver
more Current than you had bargained for (Quick and easy solution !!).
Remark:
The supply Voltage will usually determine how much Current is needed, independent of how much your Power Source can deliver. A Power Source that
delivers more Current than is needed, is usually no problem. If however you want to limit the Current for safety reasons, then you need a Current
Source Regulator, or just a plain Power Resistor in series with your HV Power Supply. A Light Bulb works just fine, for its a PTC (Positive
Temperature Coefficient), and the Resistance becomes higher, the hotter the Filament becomes.
Good luck !
Regards,
Lambda.
[Edited on 14-1-2007 by Lambda]