Originally Posted by
The_Doctor
Indeed. The Die4drive has a MOSFET, then a current sense resistor on the ground side of the diode. It would actually be fine in a positive ground system, which is odd given that most diodes driven with it are likely to be in negative ground cans.
Not odd, actually.
There is a very good reason why Robin and I and many other manufacturers choose this approach.
First, it has to do with modulation speed..it is mostly the choice of devices used. using a P channel FET may fix that problem but I'm not sure it will.
First you have to find a good P channel FET that will do the job.
Second, When I first started making drivers, and I've made many . I started with the opposite polarity cans as this was the norm for a long time so this was the way to go, then these new cans came out changing everything, but no matter what circuit I used I never could get the switching speed up high enough to suite my needs or at least work up to a flat 30khz..I was very disappointed by the results altho I did get close it was not good enough.
Finally, After much digging and experimentation I stumbled on the culprit.
I found that input capacitance of the switching device was way too high and that the op-amps liked switching to ground better rather to rail.
Bi-polar devices could not do this properly so FETS were the answer.
However, Finding suitable devices were easy to find but were always out of stock for some strange reason. Then I found what I was looking for in a device which I had to commit to. both in op-amp and FET I found that they had to work together in concert rather than just throwing a circuit using off the shelf devices in order to achieve the desired results. And with some added work I found what I was looking for.
Yes, You can make a driver that just gets the job done but when you are catering to lasershow people who use TOP end equipment you cannot scrimp on the driver as this will effect the speed and quality of the projected image. These things show-up as lack of blank spots, delay problems and dim edges to bright spots. Not to mention overspikes that kill diodes or the reverse in which I have seen from many others as lack of a clean edges caused by smoothing or cheating by over dampening to try to cure a fault or problem with the circuit.
I have tried the P channel FETS that are available but so far I do not like the results, Perhaps thats why you seen others using this opposite methode,
For a long time I often wondered why manufacturers used this approach until I found out for myself. Because it is easier and it works. and rather well I might add.
One thing that I did find funny was I just seen on another forum where someone decided to change from NPN bipolar darlington to a PNP darlington output driver so that they may use the grounded cans, He quoted that the capacitance was lower this way and would increase the speed of the circuit and also eliminate static damage to the diode..this is a false statement.
Static will kill the diode just the same. Capacitance is still high and speed is sacrificed.
But I remember doing the same thing when I started..so.
yes, it would make mounting much easier as the machining and assembly would be easier and less prone to accidents but so far the best methode is the isolated approach.
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