Problem - how to discharge a capacitor ONLY after the voltage has dropped to around 6VDC.

John Robertson · 2017-05-09T06:03:23+00:00

Interesting problem (at least I think so). We have a situation where a tiny (5VDC @ 5A) switching supply that takes roughly 8 to 15VDC in and converts it to 5VDC out, however the regulator won't boot unless the input voltage falls below 4.xxVDC prior to power back on, otherwise the device just sits there and plays dead/off. When the input power is shut off the voltage across the input filter cap (raw DC filtered with 10K @ 25VDC cap) drops quickly to around 5.5 volta as the switching supply discharges into the circuit, then the voltage sits and drains too slowly from 5.5 to below 4.00 where the device will again behave as a voltage regulator instead of an off switch - minutes perhaps. A 220R 1W resistor across the input cap does work, but it is not elegant in that it draws around 60ma at the 12VDC (average) input. Had considered a constant current device, and that would need to pass about 30ma (at 6V this will discharge the cap in a second or so to below 4.00VDC) to do the job, but even that seems inefficient. Is there an obvious (cheap) way to discharge electrolytic caps (up to say 25K @ 25V) when the voltage has dropped to around 6VDC and retires when the voltage is down to around 3VDC? Thanks! John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 (604)872-5757 (Pinballs, Jukes, Video Games) "Old pinballers never die, they just flip out."

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John Robertson 9 years ago

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Indeed, the 5VDC switcher draws the Cin down to around 7VDC at which point it shuts down. It won't start up until Vin is under 4.0VDC. So, really, one just needs to knock off 3VDC off Cin as quickly as reasonable (1 second perhaps?) but not loading down Cin too much during the power up cycle.

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If I wasn't in the midst of moving my business which consists of a shop that is 3,300 sq ft and 1500 sq ft storage into a larger warehouse I'd be all over that.

I'll be taking a fresh look at this in a couple of weeks once I have my bench set up again.

Thanks!

John :-#)#

(Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."

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rickman 9 years ago

1 second is gravy. The only question is, how much is too much load? Actually, if you are driving 5 amps at 5 volts, even with a 15 volt into a buck converter you are talking about well over 1 amp to bring up the PSU into a load. So 40 mA won't be a big issue. That's what the simulation shows at 1 second.

Heck, I don't get why the 220 ohm resistor is a concern. You could always use an LED with a series current limiter and have a pilot light!

Rick C

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Jim Thompson 9 years ago

[snip]

Reliability score, -1000. ...Jim Thompson

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John Robertson 9 years ago

I appreciate the thought, however, that product is fine if you are designing a game from scratch, but here I am trying to find a simple solution (that requires a minimum of customer work) that doesn't introduce the element of electrical shock risk where people are trying to connect to the AC line.

The replacement part I would like to use requires unsoldering one part that has two solder connections. Hard (but not impossible of course) for the average person to screw up, and possibly risk personal harm.

John

(Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."

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John Robertson 9 years ago

Too much additional stuff to add a 555 timer.

John :-#)#

(Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."

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Problem - how to discharge a capacitor ONLY after the voltage has dropped to around 6VDC.

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