Automotive power supply reset design - Page 1
Forum section: electronics / projects

author: Opticalworm
author_rank: Contributor
timestamp: June 13, 2011, 04:05:03 pm
content: Hello everybody.Recently I've found a truck exhibiting some odd behaviour from it supply. when I turn the power on (i.e the battery isolating switch is turned on) the supply voltage ramps up to around about 12v in 50ms and then its deeps to 9v and back up to 24v in another 50ms. (See the attached image). I belive its due too the vehicle battery not providing enough current on start-up to maintain the voltage.the real problem happens randomly when some of the equipments connected to that supply fail to start-up correctly. It could be that the controllers built in POR/ BOR are failing to set/hold the device reset long enough for the supply to stabilise which could randomly cause the device to reset in a unknown state.Currently I have a separate controller that waits for 2 seconds before driving a relay which connects the truck battery to my system. this solution works because its waits long enough for the battery to recover from the system initial current load. usually cause by smoothing capacitors charging up.Does anyone know of a better way to solve this problem?

author: Simon
author_rank: Unknown
timestamp: June 13, 2011, 04:34:33 pm
content: well really you need to find out why it does this even before connecting your own stuff to it

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 13, 2011, 04:42:56 pm
content: I would highly doubt that the battery can't supply enough current. You can melt a screw driver with the amount of current a typical auto battery can put out. The cause of the dip is the components past the battery. Voltage drop across too small of conductors or arcing in overloaded relays, etc.How close to the battery are you connecting? The closer the better. I'm guessing that you are back far enough that some part of the circuitry is dropping out voltage. It is also possible that the 24V you are seeing is a relay that trips with startup and then releases and dumps some back EMF onto the power line.I would try a fused tap directly of the battery for your power and then buffer your relay driver with a cap so that it will hold power during the noise. But this is just dealing with the occurrence, not finding out out why it is happening. I think you have a bad connection or worn out relay somewhere in your power chain.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 13, 2011, 05:08:46 pm
content: I do not see any attached image ..Even so the electrical automotive issues, never was easy to deal with.Especially in the modern cars.My opinion are to look for a professional electrician so to deal with it.

author: PsI-On
author_rank: Contributor
timestamp: June 13, 2011, 05:27:23 pm
content: Having worked as an Auto-Electrician for more than 17 years I agree 100% with sacherjj's diagnosis and suggestions.Just one thing to add, bad earths are also a huge problem! As sacherjj suggests, run a new power and earth directly from the battery (please do add an inline fuse as close to the battery as possible, to prevent fire incase of a short).-PsI

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 13, 2011, 06:08:39 pm
content: Quote from: PsI-On on June 13, 2011, 05:27:23 pmVery, very good point about bad ground (earth). So many problems when you don't realize that there is a weird variable resistance back to the battery that you thought was just a good ground.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2011, 08:11:17 pm
content: What voltage is the electrical system?I agree, a bad earth sounds likely, if the batter had gone bad, you wouldn't be able to start the engine.

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 13, 2011, 08:23:21 pm
content: Quote from: Hero999 on June 13, 2011, 08:11:17 pmI'm assuming based on the post that this is standard 12V US style auto system, with body grounded to - terminal.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2011, 09:36:56 pm
content: But he mentions a truck (a lorry?) which in the UK often uses 24V (I don't know about the US) which he mentions in his post.

author: Opticalworm
author_rank: Contributor
timestamp: June 13, 2011, 09:52:53 pm
content: Sorry guys I Forgot to attach the image.Thank for the advice. next time I get a chance to look at the truck I'll wire my equipment directly (or as close as possible) to the truck battery. but its going to be impractical to take the vehicle of the road and on top of that, Unfortunately, its quite a distance away from me (around 200miles).Shouldn't I be designing my equipment to handle this sort of issue? (i.e truck proff?) especially when my equipment isn't starting up correctly?@Hero999 Yeap your right its UK truck (volvo) running 24v.

author: KeithBrown
author_rank: Contributor
timestamp: June 16, 2011, 09:57:47 pm
content: The curve you supply is actually quite benign! Try to get your hands on IEC 7632-2 (Road vehicles-Electrical Disturbances from conduction and coupling - Part2: Electrical transient conduction along suply lines only), or don't if you want to sleep at night! The specified test pulses are quite scary,but based on real world conditions. If your equipment can work (or just survive!) under these conditions you are getting somewhere.