Simple, curious problem... SLA supply ? - Page 1
Forum section: electronics / projects

author: SL4P
author_rank: Super Contributor
timestamp: February 04, 2017, 09:19:27 am
content: I'm working on a small project that is self-sufficient using a 12V SLA battery, simple regulator (the diode is there in event an exterrnal charger is connected up wrongly) and a controlled 12V load.Powered from a bench supply it doesn't miss a beat, but when on the SLA battery-ONLYthefirsttime it tries to drive the output load - the micro resets. The VR doesn't get more than a little warm under continuous operation. Not hot by any stretch of the imagination.There is more critical info - the micro also controls a 3G modem on the 5V rail - which operates flawlessly in all circumstances - except the first event which drives the load.Brown-Out reset is set low (2V7) - so I doubt that's the primary cause.Subsequent trigger - modem send / output events have no issues at all (or I'm lucky so far).I suspect a capacitve load in trhe driven device is demanding a momentary spike of current when first driven on, and never seen again if the load holds it charge.What's the most direct / simplest solution to this - I suspect a ferrite of some sort in series with the load to soften the initial 'first-charge' current..Any suggestions?

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2017, 09:29:47 am
content: Try increasing the 330 uF to 1k uF and/or adding a gate resistor, say ~3.Have you put a scope on the 5V to see if it's being momentarily dragged down ?EditFurther thoughts...has the micro got sufficient gate drive capability without a dedicated gate driver doing the job ?Fet specs ?Micro ?We're only guessing in the dark without more info.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 04, 2017, 09:53:42 am
content: Add a freewheeling (antiparallel) diode over the load. This diode must be physically placed very next to the FET - not at the remote load. So basically you create a half-bridge, always providing a way for the current to go uninterrupted, when there is stored energy in inductances (happens with resistive load due to wiring inductance, too).Then, add a combination of an electrolytic cap (e.g. 330 uF 25V, or several in parallel if the load is big) and a small ceramic (e.g. 100nF 25V X7R in 0603 or 0805; or a few in parallel) over the supply of the said half bridge - between the FET source pin (ground) and +12V going to the diode. Imagine the loop area; the capacitor wires/traces must be short, and because the FET and the diode are right next to each other, the caps can be connected without adding much wiring.Now you have a proper DC load switch which shouldn't cause huge amounts of voltage dips, spikes and EMI, and which shouldn't burn your FET when your load is too inductive.Then, as a bonus, add a series gate resistor approximately equal to the IC CMOS output stage resistance, which is about 20 ohms. So something between 22R...56R to prevent ringing and MCU supply dip, but without slowing down switching too much. If the FET is too slow to switch, you need a FET driver, but it will make your problem probably worse unless you solve it first. And add a gate pulldown resistor (something around 20k-200k works) so that you won't burn your FET when the MCU is initializing and the port is still hi-Z.

author: SL4P
author_rank: Super Contributor
timestamp: February 04, 2017, 09:58:53 am
content: EDIT: As the load is polarised - I did omit the snubbing diode - but that may be worth consideringQuote from: tautech on February 04, 2017, 09:29:47 amAll fair questions...I was considering all these ideas, but was torn because the unit has zero other issues, and the load is from the 12V battery rail - not the micro's supply with the 1000uF in and 330uF out of the regulator.TBH, I haven't put the scope on it yet - just being lazy, as the scope isn't always up on my desk!The FET is a 2N7000, and the quoted load is tiny - 50mA @ 12VI could have almost as easily driven it from paralleled pins on the micro (ATMEGA 1284P)This is why I was considering the initial charge of any circuitry inside the load.The load is this -http://www.altronics.com.au/p/s5445b-12v-1w-flashing-blue-led-strobe/

author: nali
author_rank: Frequent Contributor
timestamp: February 04, 2017, 10:48:30 am
content: Does your bench supply have a current limit? If so does the CC indicator flash when the load is first powered on?2N7000 isn't a good choice of FET BTW, as it's not logic level and at the 4.5-5v gate level it'll still be in its linear region.

author: SL4P
author_rank: Super Contributor
timestamp: February 04, 2017, 11:06:48 am
content: Actually i just realised a critical part of the explanation is missing...!Sorry about that.This has been built over a previous project, and i have two FETs following each other.. i need to undo that first.Funny how you become so familiar with a project, that you don't see the obvious issues on the lash-up.