Best way to get 0.7V ~1.5A

[alec_t]

For a dual supply it's usual just to stack two voltage sources, ground being the intermediate point.
Here are some pots for you to play with:

 

[Les Jones]

Hi LG,
If my understanding of JimB's circuit is correct then I would consider the 0 volt rail as earth. The 9 volt battery connects to the OP amp + and - power pins. (It does not connect to the 0 volt rail.) The 4.7 volt zenner makes the negative of the 9 volt battery sit at -4.7 volts an the + of the battery +4.3 volts. (With respect to the 0 volt rail.) The 9 volt battery only supplies the OP amp and the base drive current to the transistor. (Via the OP amp output pin.) The negative of the 3 volt battery will connect to the 0 volt rail. This battery will supply the output current. I suspect the transistor may need to be a darlington transistor unless the OP amp can supply enough base drive for an normal transistor. If you consider the 150 ohm resistor a fixed resistor and connect the - input of the OP amp the the junction of the 150 ohm resistor and the 1K resistor then the output should be almost exactly 0.7 volts. I have never used any of the simulator programs so I can't help with that,

Les.

 

[JimB]

If my understanding of JimB's circuit is correct then I would consider the 0 volt rail as earth. The 9 volt battery connects to the OP amp + and - power pins. (It does not connect to the 0 volt rail.) The 4.7 volt zenner makes the negative of the 9 volt battery sit at -4.7 volts an the + of the battery +4.3 volts. (With respect to the 0 volt rail.) The 9 volt battery only supplies the OP amp and the base drive current to the transistor. (Via the OP amp output pin.) The negative of the 3 volt battery will connect to the 0 volt rail. This battery will supply the output current. I suspect the transistor may need to be a darlington transistor unless the OP amp can supply enough base drive for an normal transistor. If you consider the 150 ohm resistor a fixed resistor and connect the - input of the OP amp the the junction of the 150 ohm resistor and the 1K resistor then the output should be almost exactly 0.7 volts.

Correct!

LG, if you ever build this thing for real, not just play the computer game, don't forget to add a few decoupling capacitors in appropriate places.

JimB

 

[large_ghostman]

Thanks Guys. Its built!! I was using the sim and the tutorial said all sims have to have a ground, it wasnt that obvious where to put it in the sim.

I have some problems with the pot I am using and the transistor is getting warm, but I think its one the ones I got that ended up having the pins the other way around to the datasheet.

Despite it being a ten turn precision pot the voltage jumps about alot, I have some more in the shed so will replace it. I know its the pot because its played up before.

The sim side is just because it looked like a good circuit to try and learn spice on as I can measure the real circuit.

 

[large_ghostman]

Seems to be working fine now I have replaced the defunct parts! The issues now are all based around the electrodes. Nexy step for a slightly different problem is to build a comparator circuit for a Chlorate cell.

I might do this as analogue as well.

 

[Les Jones]

Hi LG,
I would expect the transistor to get warm without a heat sink. It will be passing 1.5 amps and have a voltage drop of 2.3 volts (3.0 - 0.7) across it that is 3.45 watts. (2.3 volts x 1.5 amps)

Les.

 

[large_ghostman]

Hi Les, it took me ages to work out the obvious!!! I now have one of those hat heat sinks on it, not really needed but you never know. I will get a decent opamp and rebuild it properly at the weekend.

 

[large_ghostman]

Ok all working fine or rather will be when I use a better opamp! Using one from Noahs ark control panel and dont want to use the over priced ones I have, so I have some others coming. But the point is it does what it says on the tin!!

However......... I still cant get it to sim! I have messed with the ground that apparently spice needs and still not getting it grrr, I am playing with the sim because now I want to take things in a different direction and add in a comparator etc. If I can sort the basic sim out I hope I can design the other circuit by myself.
So where in the circuit for sim purposes would you place a ground?

Also any suggestions on opamps suitable for this kind of circuit? I seem to have mainly the ones that send AG off into a rage at the mention of the number lol.
Not done much with op amps, I did play with them for a while but not much. I am actually enjoying using them for this kind of thing instead of the normal reach for a micro that I normally do.

Its so nice to just put a meter on a line and set a voltage by the pot. The sort of things I am using this for seem real overkill to use a micro, but I have always struggled with spice and learning it! For the next stage I have no problem designing it for building, but it would help alot if I could sim it and see the behavior over temp and voltage range,

OR
Anyone got a link to a decent resource that has a video tutorial of sim ming opamp circuits with spice? Most seem too basic and dont have opamps, or are complex circuits but no real guidence on building the sim and explaining what to set up.

I tried to join the yahoo group but (3) our network provider wont let me use yahoo mail signup!! I am using mobile tethering and sometimes sites get blocked for reasons I doubt even 3 knows! I get told its a age related site and I need to contact them and get the filter lifted. Actually they want us to do that so they can monitor our tethring limit! we have eat all you want data but they changed the rules to 4 GB data if tethered.

On my phone for some reason they cant normally tell what is tethered data and what is phone data, so tempted to leave filter on


sorry ended up rambling! must be time for bed

 

[alec_t]

If you post your asc file perhaps we can make suggestions.

Edit: Here's the asc file for Jim's post #5 circuit. Note the ground point is the zener cathode. The sim will enable plots for v(out) etc over a temperature range of -10C to +50C.

 

[large_ghostman]

Thanks Alec, I think this should give me the answer i was looking for. I will post circuits as I go and comments welcome.

LT spice is a handy tool to master, would save alot of wasted build time if used correctly. spice and MATlab are the two things I intend to learn this year, MATLAB isnt so bad now I have got off the start line, like most things the bottom of the learning curve is the worse bit lol.

As a side note, alot of companies like R PI provide MATLAB blocks FOC for student version, I believe even Arduino do, so why on earth does Microchip charge almost £1,000 for the same! seems insane to me to charge so much and gives the competition the edge.

Lately I have to admit MC seems to be behind the others, its a shame as I love pics but I think they need to rethink alot of things.

 

[large_ghostman]

Thanks Alec, its proving a good start point, once I get a few basics under my belt I will post the sim I am trying to work on. Wish I had learnt spice when I was doing incubators!!! Still next generation of incubators will be even better now