Capacitors

[12dstring]

A I understand it, the purpose of the capacitor in a driver circuit is to absorb any start-up voltage spikes. Is there any reason that it can't be put across the input voltage before the current regulating components? Surely this would still absorb any spikes, but if the diode was accidentally disconnected and reconnected then the capacitor charge would still have to go through the current regulator and wouldn't get fried.

I suspect I'm wrong, but would be interested to know why

Also, is there a need for a capacitor at all if you're using a voltage regulator (eg 7805) before the current regulator?


Thanks

 

[rkcstr]

Most datasheets will suggest an input capacitor for these regulator ICs, and they can be important. For mine, it does have a small cap on the input to improve stability. It's not necessary to be a large capacitance unless it will improve operation.

 

[12dstring]

Thanks rkcstr, I should have remembered about them on voltage regulators.

I'm still wondering about my first question though. I am sure there must be a reason or else people would be doing it, but I still can't work out why the capacitor in the DDL driver for example can't just go across the input instead. It's more curiosity than anything

 

[Tw15t3r]

I think it's a personal preference. For me, I'm not intending to put a cap next to the LD (I'm worried about the disconnection frying problem)

If you put a cap parallel to the battery, then you'd smooth the current entering the regulator which will result in smoother output. If you put the cap near the diode, since there wasn't a cap before the regulator, I assume that the regulator will still produce ripples so the cap helps to smoothen that out.

Either way, I believe that it doesn't matter. One thing's for sure though: caps smooth current.