| Electronics > Beginners |
| LM358 in Consant Current Circuit Problem |
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| Yansi:
No it is not, as it will complicate the current sensing, as you can not then simply use a shunt in the emittor. You would be measuring emitter current, not the collector one. Emitter current is Ic+Ib and Ib being significant (remember that high power BJTs have low current gain typical) compared to the Ic. |
| TheInfernoMan:
Ok, thanks for the tip regarding to the offset voltage. Would the following schematic would work now ? There are a lot of things I had not known, but I learned a lot now. But I want to build the PCB now, but there a lot of mistakes in my schematic |O sorry |
| Yansi:
Add a cap accross R10. Value not critical. 100nF for example. Will work as a small noise filter. Will come handy, if you wire the potentiometer on long leads, that would pick up RF noise. Also, when making the layout, make sure that the you know signal wire/trace from pin2 (inverting input) is a high impedance and can accept interference - keep it short as possible, place both C1 and R12 close to the pin2. Place another 100nF decoupling cap close between pins 4 and 8 of the opamp. (The 10uF on the input already might be enough, but aswell might not, depending on the layout. ) |
| Richard Head:
Yansi Yes, you are correct. sorry. |
| Yansi:
But to be fair, it can be done with the BJT too, even with similar simplicity: but the constant current load won't be ground terminal referenced then. But that is really no problem, if the supply for the control circuitry is floating. Note: TheInfernoMan: Now for this circuit, the buffer will come handy, as the output current is proportional to the current flowing through R1. The circuit also can be designed so that the effect of offset voltage of the buffer will play insignificant role, as the offset voltage of the buffer is effectively divided by the ratio of R1 and R2. Also good to note, that the formula for Iload works only when R2 is significantly higher, than Rsense, which for most cases is fulfilled (Rsense some tenths of ohm, R2 hundreds ohms or few Kohms) |
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