Anyone know how to make a 12v PS variable?

[Garyh]

Hi people,
Just wondering how I can make my 12 volt PS variable from say 5v to the full 12v and with a load of 5A or so.
I am using a old ATX power supply at the moment but want to vary the output on the 12v via a pot. This is to fine tune the cooling on my modded 40D.
At the moment it is fed a full 12v through a switching regulator to keep the temp stable but I find that it`s on and off ever few minutes and I have read that a TEC`s lifespan is greatly reduced with all this on and off use.
Have seen on the net the use of the LM-338k to do this but I would loose the full 12v? and will it handle the full 5A current?

Anyone that is electronic savy and can help or have some info would be greatly appreciated!. :thumbsup:
cheers Gary

[peter_4059]

How about pulse width modulation? You can get a couple of different 3A unit from RS components:
http://australia.rs-online.com/web/search/searchBrowseAction.html?method=getP roduct&R=2389816

http://australia.rs-online.com/web/search/searchBrowseAction.html?method=getP roduct&R=4179728&cm_vc=prev_au

Jaycar has 10A kits
http://www.jaycar.com.au/productView.asp?ID=KC5225&keywords=speed+control&form=KEYWORD

[Garyh]

Thanks Peter, know nothing about pulse width modulation.
The main thing is that the power has to be quiet clean with little noise as will cause interference with the camera. I have tried a 12v 5a jaycar supply and it played havoc with my camera causing heaps of interference so went back to the PC PS.
Maybe the motor controller might do it?

[peter_4059]

PWM is what is used in most dew heater controllers. I'm not sure if these basic units provide "clean" power as I have heard people complain about pwm causing electrical interferience. I know the more expensive commercial dew heater units like Kendrick boast of their filtering to avoid this however I have no experience with this problem.

I have the first of the two RS units I've used as a dew heater controller for a couple of years and it has worked fine in that application.

[Barrykgerdes]

Hi Gary

Using an old ATX power supply is not a very practical way of sourcing a regulated 5-12volt 5A supply. By the time you make a 5A regulator you may as well have started with a proper purpose built supply.

To run a camera using a PWM supply you will most likely run into noise from the switching. This type of supply is good for heaters, lights and motors but not more sophisticated electronic equipment. The most satisfactory method is a series regulator that will drop the voltage but of course waste the extra dropped voltage power as heat.

My son Trevor built a multi output regulator to drive his camera (Canon 1000D) and dew heaters using the output of a regulated 13.8v 5A supply that also drives the G11 with parts obtained from Jaycar. He can probably give you the design details. If your ATX has a 12v 5A output it will probably be suitable to drive the regulator but be careful those supplies are meant for indoor use protected by a computer case. Any ingress of moisture from outside use could be disastrous or even fatal.

Barry

[Garyh]

Thanks Barry,
The power supply sits in the observatory so theres little problem with moisture etc. The supply handles 5A ok at just over 11v.
The design details sound great Barry, just a rough circuit diagram should get me out of trouble! If I can adjust the voltage between the 11v to 7.5v I would be very happy! :)
Gary

[Barrykgerdes]

Hi Gary

PM Tlgerdes and ask him for the drawings and photos of his PS regulator. He made a very neet little PS that should suit your purpose. It worked great at Wiruna last time I was there.

Barry

[robzy]

Is the camera operating off the same power supply as the TEC?

What if you had two power supplies, one clean constant one for the camera, and one not-so-clean variable one for the TEC?

12v -> 5v at 5A, with a linear regular, will be throwing out 35W of heat, which is a tricky figure to deal with.

Rob.

[Jeffkop]

HI Gary
http://www.elecfree.com/circuit/power-supply/lm338k-adjustable-power-supply/ here is a basic circuit for a linear regulator with values. It uses a T03 case regulator and minimal parts.
You could replace the lm338K with a newer 338t T0 220 package as well.
You could very well get by with different capacitor values as you only require it to feed a TEC device. If its going to be a permanent power supply then by a torroidal transformer that has at least 9v secondary capable of the current.
PM me if you want or email me at jeffkop@internode.on.net ... We can sort something out no problems

[kustard]

I'm sure most technical guys will know this but here are a few tips when designing PSU's.

1) For a linear regulator to work there needs too be a certain voltage level before the device will start to regulate. For devices like the LM78XX series, this is roughly 2 volts. This means that you need to have a minimum of 7 volts input voltage for a 5 volt LM7805 reg to work.

2) On both sides of the linear regulator (input and output) you should place both ceramic and electrolytic capacitors. The electrolytic caps act like a storage tank, keeping the voltage at the desired level when load changes. The ceramic caps help to filter out high frequency waveforms.

3) Placing a power diode from the output to the input (i.e. the cathode on the input side) helps in protecting the linear regulator from high voltages flowing through the reg during power up/down times.

PWM (Pulse Width Modulation)/switching regulators use a generated frequency either fixed by discrete components or variable via a potentiometer or processor to control the output level. The generated frequency helps to make these PSU's efficient but they also suffer from EMF issues if not designed correctly. They should be shielded and if you are laying out the circuit yourself then invest some time into the correct layout of parts to help avoid RF interference.

Hope this helps and isn't just me ranting :)

[Garyh]

Hi Rob, the camera is on a separate supply at the moment, this one shall be just for the TEC, but this needs to be fairly clean due to the cold finger going internally into the camera. Don`t mind dumping heat as long as the power is clean, Would mostly operate from 12v down to say 7.5v at the lowest.
thanks Rob.



Thanks for that info Jeff, might take you up on it yet! Don`t quiet understand the terroidal tranny , but if I get intouch you shall set me straight!
:thumbsup:



Thanks for that info too!
I know I will loose 1.5-2v due to overhead but I can live with that.
What I have read about cheap PWM PS, I think they will not be suitable due to interference....
I shall put in diode and other protection to keep things safe and hopefully good value caps to smooth things over..
Thanks guys, shall get there soon! :thumbsup: