Best way to get 0.7V ~1.5A

[large_ghostman]

Hi
While my HP power supplies can do this with ease I need a portable solution on board. I need anywhere between0.4V and 0.7V, depending on the voltage it will need to be able to handle upto a max of 1.A.

I cant go above 0.7V under any circumstances, so how best to do this and keep the voltage stable? Its a add on for my project so I cant say yet why I need it (Not helpful I know), what I can tell you is the voltage is for a chemical process hence why the upper limit, if I go above this limit then the process wont work.

I have had it working perfectly with a decent power supply, but it isnt mobile! Its only now that I am assembling the final project that it suddenly hit me that 0.7V might not be so easy to do.

 

[JimB]

A couple of ideas come to mind, neither are very energy efficient but far better than nothing.

Use a typical series regulator circuit with a zener for reference, an op-amp for control and a nice chunky transistor as the pass element to control the voltage.
I suggest either a rail to rail op-amp, or use a +/- supply for the op amp so that it can control down to zero volts.
Use a big low voltage battery (portable ?) say 3v for the supply to the pass transistor, and a separate higher voltage supply (9v battery) for the op-amp.

Or, you could use a shunt regulator circuit with maybe a power MOSFET as the shunt controlling element. Again using an op-amp etc for the controls.

As I have been writing, I vaguely remember a variation of the LM317 circuit which can control down to near zero volts. I am not sure without going searching whether it could be made to handle 1.5 amp.

JimB

 

[large_ghostman]

Weight wise I can use maybe upto 4 P9 batteries, the high current isnt constant, its more the peak limit. I didnt think of opamps, maybe because the rest of the project is all micro etc and I overlooked the obvious!

So finally I cant avoid it any more and I need to learn LT Spice! I have tried so many time to get to grips with that! I guess this is a good time to do it, thanks for the ideas Jim its appreciated, and sorry for the confusion over Tim and co. For a kid its truly been awe inspiring to follow the progress of the station, if only they would now build a Bio dome on the moon I would be happy
Off topic but apparently on the news they were saying that another astranaught nearly drowned in his suit during a walk! The clip they showed of what happened today shows a guy in mission control asking them to try and taste the liquid, so it seems they are unsure where the water comes from.
14 layers in the suit and it can go from -100C to 120C. The fact you can even see the space station now with the naked is just amazing, I wouldnt want to do it but the technology is something else!

 

[large_ghostman]

Thinking about it some more, I could maybe drop the current to 1.2A. But that would be the absolute bottom as I have to allow for the initial spike, the spike lasts around 100ms worse case.

 

[JimB]

I quickly scribbled this to show the idea for a series regulator.

JimB

 

[large_ghostman]

Thanks Jim I will sim that , where would it be best to put a ten turn pot for fine adjustment? I might need to think about some way to monitor the battery voltage, at the moment I dont know if its needed as I dont know for sure how long the runs will be.

Before I build anything else I need to find out why my ATX bench supply has just started throwing bad ripple! That or I might just add some caps for now .

 

[JimB]

R2 is a variable for adjusting the output.
If my quick calcs are correct, with the values shown it will adjust from about 0v to about 0.7v

JimB

 

[large_ghostman]

Cheers Jim, I am on the ipad so the screen isnt picking up the detail well, I will fire up the laptop so I can read it better.

 

[JimB]

Sorry it is a quick pencil sketch which does not give a very good pic.

JimB

 

[ronsimpson]

Look at the LT3081. It will go to "0" volts.

 

[large_ghostman]

Thanks Ron. Its fine on the laptop Jim, the ipad is alot brighter and it can be hard to see detail on it. All I need to do now is try and sim it, I am determined to learn this program!

 

[Externet]

A 'D' size cell (carbon or NimH) and a series rectifier to drop 0.7V would be very portable. Stable for how many minutes/hours ?
A single biggie or paralleled solar cells and good stable light source ?
Galvanic cell of choice ?

 

[Les Jones]

Hi LG,
There is a higher current version of the LM317 It is an LM338. If you have room for an extra battery to supply a low current negative rail you could connect the bottom end of the voltage setting potential divider to a regulated negative voltage thus subtracting that voltage from the output. A zener may be good enough to regulate this negative voltage or you could use the negative voltage version of the LM317 ( LM337 or LM337LZ (100 mA version.))

Les.

 

[alec_t]

2V SLA batteries are available if you need several Ahr capacity using JimB's circuit.

 

[large_ghostman]

2 hours at a time is ball park. The other thing I need to watch for is stability over temperature, the device will start at around 10C but the temp will rise ~30-40C.

So I am wondering if a ref voltage would be an idea? I start to get Chlorine of NOx or other nasty fumes as soon as the voltage rises above 0.7V, maybe if I set the voltage a little lower and let it rise to 0.7V once the ambient temp has reached its maximum.
Sorry just thinking out loud! This is another reason to learn spice!! If I get this working well then I might build a slightly different one for use in a Chlorate cell.

 

[alec_t]

Not sure how stable the voltage needs to be, but even a silicon diode might be good enough as a sub-0.7V reference?
Is this too simple?

 

[large_ghostman]

Not sure how stable the voltage needs to be, but even a silicon diode might be good enough as a sub-0.7V reference?
Is this too simple?
View attachment 96841

Thanks

I think as long as max voltage isnt exceeded then apart from a loss in efficiency (I can live with that) all is good. My main concern is the 0.7V is not exceeded as its gets really nasty very quickly. I am slowly going through LT spice tutorials, got its boring!! And having trouble finding many parts to sim!!
I am in the chem lab tomorrow to test out a couple of things, I have swapped one the electrodes to a MMO (Mixed Metal Oxide) electrode. It looks like I will have to choose a specific electrode and stick with it, the carbon ones dont seem to be very stable (no idea why).

 

[alec_t]

And having trouble finding many parts to sim!

If you want more parts (models) there are free lbraries of them available from the Yahoo LTspice User Group.

 

[large_ghostman]

If you want more parts (models) there are free lbraries of them available from the Yahoo LTspice User Group.

I joined that a while back and got totally lost!! I will rejoin though, maybe now I have a slightly better idea how to use LT spice I might do better on the user group.
The one issue I did face was my internet connection, we dont have normal broadband because of where we live, so we use mobile phone tethering.

On some Yahoo groups the phone company (3) block the site as unsuitable material! It can be really frustrating, I have found a model conversion program so it might be worth using that.

 

[large_ghostman]

I picked a random rail to rail opamp in spice, but I am unsure where to put the earth for the sim? I was going to put it between the two batteries but somehow that dosnt seem right!

The other question is, is there a variable resistor in spice or do you just tell spice to alter one of the divider values by x increments during the sim? Sorry for noob questions but spice is new to me although it appears pretty good fun