How long can cap leads be?

larrymoencurly · 04-29-2006, 04:05 AM

I have a PSU where some of the output filter caps are a tight fit, and it would help the crowding a lot if I could install a couple of the new caps about 1" above the board. Would this make the leads too long? I can solder 16 gauge wire to each lead to reduce the inductance, if it will help. Of course, each lead will be covered with heatshrink to prevent shorts.

**kc8adu** · 04-29-2006, 04:45 AM

i have done 1/2 in when 12.5 mm caps wouldnt clear stuff on the board.some psu use 10 or 8 mm caps.no harm done.

linuxguru · 04-29-2006, 05:03 AM

The switching frequency is only ~100 KHz, and there's already inductance (either stray or part of the circuit) in series at most locations on the secondary side where large caps are usually found. A few pH/nH additional in series won't make a difference.

Exception(s) to this are the small bypass caps next to the switching controller IC or on the output side of 78xx/79xx 3-terminal regulator(s) - these have to have very short leads to avoid instability/oscillations due to high loop-gain linear circuits.

04-29-2006, 04:05 AM	#1
larrymoencurly Badcaps Veteran Join Date: Oct 2004 Posts: 711	How long can cap leads be? I have a PSU where some of the output filter caps are a tight fit, and it would help the crowding a lot if I could install a couple of the new caps about 1" above the board. Would this make the leads too long? I can solder 16 gauge wire to each lead to reduce the inductance, if it will help. Of course, each lead will be covered with heatshrink to prevent shorts.

04-29-2006, 04:45 AM	#2
kc8adu Super Moderator Join Date: Nov 2003 City & State: dayton ohio Posts: 6,437	Re: How long can cap leads be? i have done 1/2 in when 12.5 mm caps wouldnt clear stuff on the board.some psu use 10 or 8 mm caps.no harm done.

04-29-2006, 05:03 AM	#3
linuxguru Badcaps Veteran Join Date: Apr 2005 Posts: 1,002	Re: How long can cap leads be? The switching frequency is only ~100 KHz, and there's already inductance (either stray or part of the circuit) in series at most locations on the secondary side where large caps are usually found. A few pH/nH additional in series won't make a difference. Exception(s) to this are the small bypass caps next to the switching controller IC or on the output side of 78xx/79xx 3-terminal regulator(s) - these have to have very short leads to avoid instability/oscillations due to high loop-gain linear circuits.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode