Doubt at early step

Hi everybody, I need to better understand why the optional C31 cap is… optional. What if i mount it and what if I don’t?

And I have a question on soldering too: once done, the soldering points on the bottom of the pcb are dirty of fluxant. Do I have to clean them or leave the thing as it is?


It’s 2 AM, the roads are desert, you are at a road junction and the light is red. It’s very unlikely that another car is going to come from the other road but still, you stop. Maybe because following the rules of the road has become unconscious/automatic to you, or maybe you want to show the way to your kids staying on the back seat. Or maybe you’re scared of the 0.00001% chance of another car coming from the other road…

According to the LT1054 datasheet, having a high ESR cap as the charge capacitor (eg vanilla electrolytic without tantalum in parallel) is going to cause voltage drops at high currents. The SMR-4 circuit doesn’t draw a lot of current on the negative rail, so you don’t have to bother much about this. I had no tantalum cap when I tested the circuit and nothing wrong happened. But maybe people are going to reuse this schematic for other filter boards, or someone will just do a random Google search on the LT1054 and will end up on this circuit etc etc etc - in which case I think it’s a better thing to show the “correct” circuit (at least according to the datasheet).

I personally scrub fluxant with a toothbrush. It’s not pretty but I haven’t found the “miracle” cleaning product yet.

muldee: pick up a little round tin of tip cleaner/tinner from wherever you got your iron from. It helps prevent excess flux from building up on the iron and also aids heat conduction - plus it saves you from wasting a lot of your solder by ‘manually’ tinning your iron and coats the tip more evenly anyways. Also wipe the tip across a damp sponge occasionally (before tinning) to clear off any residue.

@ pichenettes: I’ve tried to scrub fluxant with a toothbrush but it’s too hard to remove and I’m afraid to scratch the pcb or to screw up something. I’m leaving it as it is.

@ WillyyDavidK: yes, tinning and coating the iron with the solder is a big waste. I’ll go and buy the cleaner.

Together with your advice I followed this tutorial (very helpful indeed):

Ok, I got the answers here: