Dual 7660s - Keeps blowing capacitors!!

Thread Starter

danielantonic

Joined Sep 22, 2019
26
Hello all

I am making an Arduino Controlled FSM Device (medical device) that uses multiple analog multipliers to create specific waveforms in the +/-18V range. All the programming/digital stuff is fine, the issue is my power design keeps blowing capacitors to a dead short (specifically C1 in the attached schematic).schematic.png

Power comes in to the device via a 12V/1Amp wall wart, through to a DC-DC converter that boosts the voltage up to 23V. This leads the header H1 to supply 12V on pin 1, ground on pin 2, ~23V on pin 3 (which goes stright to the 7818 and associated capacitor C1).DC-DC.jpg

On all low power tests, my power supply will provide +/-18V, and on a test of a single op amp/analog multiplier, everything worked. But as soon as I leave it for a day and turn it back on, C1 overheats and shorts out. At first I thought it was the 7660s getting locked, so I added D2 and D3, but it just happened again now.
board.jpg
The capacitors are Tantalum rated at 25V, and the DC-DC converter measures 22.7V out, so everything should be in range. I have noticed that it only seems to be when there is a load on the +/-18V lines that this will happen (never on tests with just the 12V side of things connected) so could the extra draw make the voltage on C1 greater than 25V momentarily? Any help will be appreciated :)
 

Sensacell

Joined Jun 19, 2012
2,903
Tantalums are prone to doing this when rated voltage is exceeded. ( I don't use them anymore - ceramics are less trouble)
Check to see what the actual voltage is across the cap. The capacitor should be rated for DOUBLE this voltage.
Check the polarity as well.
 

Papabravo

Joined Feb 24, 2006
16,938
Hello all

I am making an Arduino Controlled FSM Device (medical device) that uses multiple analog multipliers to create specific waveforms in the +/-18V range. All the programming/digital stuff is fine, the issue is my power design keeps blowing capacitors to a dead short (specifically C1 in the attached schematic).View attachment 235425

Power comes in to the device via a 12V/1Amp wall wart, through to a DC-DC converter that boosts the voltage up to 23V. This leads the header H1 to supply 12V on pin 1, ground on pin 2, ~23V on pin 3 (which goes stright to the 7818 and associated capacitor C1).View attachment 235429

On all low power tests, my power supply will provide +/-18V, and on a test of a single op amp/analog multiplier, everything worked. But as soon as I leave it for a day and turn it back on, C1 overheats and shorts out. At first I thought it was the 7660s getting locked, so I added D2 and D3, but it just happened again now.
View attachment 235426
The capacitors are Tantalum rated at 25V, and the DC-DC converter measures 22.7V out, so everything should be in range. I have noticed that it only seems to be when there is a load on the +/-18V lines that this will happen (never on tests with just the 12V side of things connected) so could the extra draw make the voltage on C1 greater than 25V momentarily? Any help will be appreciated :)
Polarized capacitors get very cranky when subjected to revers bias and current surges. They don't like turn on transients at all. 22.7 is far to close to 25 for my taste. I would use capacitor with a 50 Volt rating minimum
 

bertus

Joined Apr 5, 2008
21,588
Hello,

You are using the 7660 out of spec.
icl7660 max ratings.png
They may be used until 12 Volts and NOT the 18 volts you are using.
Better use a LT8471 for the application.

Bertus
 

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Thread Starter

danielantonic

Joined Sep 22, 2019
26
Hello,

You are using the 7660 out of spec.
They may be used until 12 Volts and NOT the 18 volts you are using.
Better use a LT8471 for the application.

Bertus
My Schematic is a bit misleading - the voltage in to the 7660 is actually 12V minus the voltage drop over the diode. The 18V part is separate from the 7660s :)
 
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