Help needed implementing max current pot

[LaserAdct]

Hi all.

As the title sais I hope someone will be able to help me modify a driver schematic originally created by Karol (aka C4R0 - ..:: c4r0 homepage ::..)

Unfortunately my own electronics knowledge only let me make stuff from schematics, measuring and those sort of things. I am not able to modify or invent things, yet.

What I am hoping someone can help me with is to implement a MAX current pot, so that one pot sets the MAX current, and the other can be adjusted from 0-MAX current.

The schematics i need modified can be seen here: http://c4r0.skrzynka.org/_hv/lasers/LD_drv_2ch_smd_sch.png The current pot is R5 and it is for this i need another pot that sets the max current.

Any help would be highly appreciated.

Best regards

Nicki

 

[Bionic-Badger]

This LED light dimmer essentially does what you're asking if you replace the resistor that is trimmed for max current with a pot. You should study the circuit to learn how it works though.

 

[LaserAdct]

Thx for your reply.

A pot would have to be connected otherwise than the resistor that is trimmed, and with my current knowledge of these things it is not something I can do myself, hence this post. And I want the max current trim pot implemented in the schematic I posted since I already have 3 of these drivers made.

 

[Bionic-Badger]

Better break out the books and start reading and learning.

 

[LaserAdct]

Apparently yes, however I havent got the time to do that so will seek an answer elsewhere, maybe in an electronics forum. I really need the TTL thats why that dimmer wont do me any good. Thought this was trivial for those who know electronics, but I guess not. Oh well...

 

[Bionic-Badger]

Well it's not always straightforward to retrofit something into a design. It'd probably be easier to retrofit the LED dimmer with the TTL input than the other way around with the circuit you linked to, but you've already built those.

You could try putting a pot in parallel with that 1.3 ohm resistor between the source of the mosfet and ground, but you'll have to scale the resistor too. You can do the calculations for that (for parallel resistors) yourself though.

 

[LaserAdct]

Thanks, I can see what you mean and will try that to see if it works

 

[LaserAdct]

I am soo confused, with my skills I am just not capable of making this. Does anyone know of a good driver schematic with ttl and two pots - one for max current and another for adjusting between 0 and max?

 

[HIMNL9]

Well, i can't read the text (Czeck ? ) , but the schematic is easy to understand (it almost resemble one of my first trials, LOL).

If you look at it, you can see that it's just a common "column" (or serial) current regulator, with a modification that give it a stable reference for have the possibility to change the current with a normal trimmer, instead to have to change the Rsense (R6, in that schematic) ..... it work with a "negative feedback" (or whatever you call it in English) from the Rsense to the base of the transistor that control the conduction of the mosfet .....

In poor words, the loopback control works in this way:

when load current increase -> more voltage is present on R6 (and to Q2 base) -> more Q2 conduct to GND -> this cause less current go from R3 to Q3 (Q2 draw to GND some) -> less Q3 conduct -> load current decrease

And for the opposite, ofcourse:

When load current decrease -> less voltage on R6 -> less Q2 conduct -> more current go from R3 to Q3 -> more Q3 conduct -> load current increase

The TTL modulation works in the same way, when you put any voltage over 0.6V on the TTL input, Q1 close Q3 to GND and turn it off (or "open" it, if you prefer it said in this way) ..... BTW, this system is unstable if left floating, so it's better that you add a 10Kohm resistor from Q1 base and GND, for avoid that random charges turns off your driver with nothing connected to TTL input ..... NEVER left a transistor base floating, it's one of the first rules

The main difference from a normal fixed driver, is that part of the voltage to Q2 is given from the voltage reference source LM431A, this cause both the need of a lower Rsense, than the possibility to change the current regulation changing the reference current ..... ofcourse, with this reversed configuration, for DECREASE the current on the load you need to INCREASE the voltage of Q2 base (and for derivation, for set a lower maximum current, you need to increase the reference voltage).

So you have basically 2 ways, for "set" a maximum current with this type of schematic, without have to change R6 ..... the first one is to change the output of the reference voltage IC (LM431A, using its reg pin, that in the original schematic is connected to cathode ..... detach it from cathode and connect it to GND with a fixed resistor, say 1K, and to cathode with a trimmer of 1K, gives you the possibility to regulate it from 2,5 to 5V) .....

The second system is to modify the schematic in this way: detach the components R2, R4 and U1 where they are connected together, then connect an 1K trimmer between U1 and R2 (one side at U1 cathode and the other side at R2), then connect the slider of the trimmer to R4 (this system is a bit less stable, but still work)

An also less stable system (but more easy), is to try to substitute R4 with a 470 ohm or 500 ohm trimmer with a 100 ohm resistor in serie ..... i'm not 100% sure if it can end being stable enough, anyway, for high currents, but it's an easy modification, and can worth a try .....


EDIT: i have a decent schematic in my album, but it involve op-amp circuit .....

 

[LaserAdct]

HIMNL9 - Thanks a lot for that very thorough post! I actually think you tought me something I will try one or more of your suggestions when I come home from work later today.

//If possible I would very much like to see the schematic you have in your album. Im not set on a specific driver, its a learning curve and I want to use the best I can DIY for my projector project.

 

[HIMNL9]

LOL, it's all in my public album on LPF, not a secret album ..... no permission needed.

(stil need to clean up some mess and organize my schematics in readable form, when i find some time, then probably i made an album dedicated to schematics)

 

[LaserAdct]

Nice treasure chest you got there :bowdown:

 

[LaserAdct]

I didnt get it to work on a breadboard. Im not good with these things without a schematic hehe. Its more of a nice to have than a need to have thing, so for now, and at least till i start using a different driver I will just live without it. Thanks for your replys though.

//Think ill play with some tec instead

 

[jbord39]

I would not trust this circuit in sensitive applications. As the current increases the gains will change. Try using an op-amp instead.

 

[LaserAdct]

Plus as the current increases the gains will change.

How do you suggest the current increases? Its a constant current driver. The voltage goes up as the lasers heat up, but the current stays the same.

 

[jbord39]

How do you suggest the current increases? Its a constant current driver. The voltage goes up as the lasers heat up, but the current stays the same.

I didn't realize you had 3 of them built already. That said it is probably not worth rebuilding. However, as transistors heat up their parameter values (beta and forward voltage for BJT's and turn on, etc for MOSFETS) will change.

A lot of that circuit is basically making an op-amp for use in a feedback loop. However, pre-built opamps will do this much better design, like short circuit protection and internal compensation for parameter variations.

An op-amp circuit will also make the circuit look a lot simpler and easier to breadboard, since half of the circuit you linked would be included inside one chip.

What I would recommend is downloading PSpice student (or even searching circuit simulator and using that flash program) to build this circuit. You can try modifications out a lot quicker and see where to add a potentiometer to change the max current.

-John