power supply timing - Page 1
Forum section: electronics / projects

author: houghe9
author_rank: Newbie
timestamp: September 13, 2013, 09:48:31 pm
content: would like to start off saying thank you to Dave and to community. I regularly watch Dave's shows and I am new to this community. from what I know so far everybody keeps this forum positive. i subscribe to the idea that no matter how stupid an idea or suggestion it could spark new ideas that someone has not considered. which brings me to my question. during an engineering investigation someone brought up powersupply timing as a possible cause for inconsistency upon power up. something I never would have looked at before. it was the cause. there are 2 power supplies providing +15volts dc and the other provides -15v dc.upon power up it is random on which one comes up first. depending which one hit 15 first the system locks up.I am asking for opinions on what what would be the best way to ensure that the correct power supply hits first.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 13, 2013, 10:19:06 pm
content: Best way? I'm not sure what constraints need to be optimized.Regardless, you've first got to decide which rail needs to come up first. Then, figure out how much delay is needed between the first rail and the second rail. And maybe refine your criteria of what it means for the rail to "come up" -- is there a certain trigger voltage that must be exceeded?One brute force way to make sure things happen in sequence is to switch the second rail through a relay, with the coil of the relay connected to the first rail. If needed, you could use a little RC circuit (a 555 timer?) to provide a bit of delay between the first rail and the second rail. There are perhaps more elegant solutions using semiconductors instead of a relay.But the basic idea is, use a device as a switch to keep the second rail disconnected until you've detected that the first rail has power.Depending on just how the rails come up, you might want to trigger the switch by the output of a comparator, with one input being a resistor divider based on the rail that's supposed to come up, and the other being a voltage reference -- a voltage divider consisting of a zener and resistor. That way, the comparator will only switch states after the voltage has exceeded whatever target you've decided upon.

author: dannyf
author_rank: Super Contributor
timestamp: September 13, 2013, 10:39:18 pm
content: QuoteDesign an appropriate reset circuitry for the mcu.

author: Richard_Fry
author_rank: Contributor
timestamp: September 14, 2013, 04:02:24 am
content: Think AG has it covered, you need to determine which power supply you want to come up first.Then you can switch the second one on (either a relay, discrete FET or power control switch) after a suitable delay using a simple RC network or similar.How are you generating the + and - 15 voltages?Some voltage regulators have enable pins on that you could attach an RC network to, with different rise times for each supply to stagger which one comes up first.

author: fcb
author_rank: Super Contributor
timestamp: September 14, 2013, 10:27:17 am
content: Insufficient info really. Personally I would set out to cure the sensitivity before sequencing supplies.That said - perhaps you are using 7815 & 7915 type regulators? Why not replace one of them with a 317/337 and build a hold down circuit on the ADJ of the delayed rail, it could be time triggered, or enabled from your processor, or triggered when the other supply is stable, etc..