[TriEmbed] Power Supply rail current

Wed Jul 9 20:40:50 CDT 2014

While reading through the latest online catalog today, I ran across this:

http://www.sciplus.com/p/12-VDC-RELAY_53625

12v relay, with 40A contacts for $2.50 each and $1.25 for an outlet - I
think it will fit your application fine.

On Wed, Jul 9, 2014 at 2:57 PM, Robert Mackie <rob at mackies.org> wrote:

> I am way out of my depth in this whole discussion, but I did convert a
> couple of gas vehicles to 100% electric drive trains a few years ago.  In
> both vehicles, I installed relays to cut out in case of impact (driven by
> inertial switch de-energizing them). One relay was at the back of the
> vehicle and another at the front so that power would be cut even if one
> relay was crushed in the closed position in an accident.
>
> I mention it because these relays, while not cheap, were rated beyond 500
> amps at around 150VDC, which was the full drive of the PWM controller
> running the DC motor that activated the drive train. They controlled arcing
> by having permanent magnets installed near the contacts to "bend" the arc
> away from the far contact.  One of the relays activated and deactivated
> each time I touched or released the accelerator pedal.  Although, that
> implies that under normal conditions it was at low current when it opened.
>  But it also worked if I turned the car off at the ignition under full
> acceleration, or pulled the gear shift to neutral.
>
> Rob.
>
>
> On Wed, Jul 9, 2014 at 11:42 AM, Shane Trent <shanedtrent at gmail.com>
> wrote:
>
>> If you are using Remote Sense you may want to connect the on-board sense
>> terminal to the output terminal with a 100 ohm resistor at the power supply
>> terminals to provide feedback if your remote sense wire becomes
>> disconnected. The pdf below has more on remote sense, Fig 6b.
>>
>> I once had a developer at Borg Warner melt a 24V clutch unit under test
>> when the remote sense of their power supply line became disconnected. The
>> power supply also toasted my PWM FET module but I melted the unit under
>> test before my module failed. They were impress at how much power the PWM
>> module delivered before they it died!
>>
>> http://www.astrodyne.com/wp/wp-content/uploads/AP_Remote_Sensing_AP6.pdf
>>
>> Shane
>>
>>
>> On Wed, Jul 9, 2014 at 9:24 AM, Rodney Radford <ncgadgetry at gmail.com>
>> wrote:
>>
>>> Relays are rated at a specific amperage and as long as that amperage is
>>> not exceeded, you can open and close the contacts while current is flowing
>>> so that is really the best option
>>>
>>> Since this was a 1-off private build, for me, I would have checked the
>>> price of the larger capacity relays and if they were too high, I would have
>>> gone for a cheaper relay and just be careful that the current through the
>>> contacts is low at the time they are opened/closed.  That is a pain to do
>>> manually and I was just checking the supply specs to see if it has an
>>> inhibit on the output that can be used to control the supply to prevent it
>>> from supplying current until the relay is energized and it does not.
>>>
>>> Given that, just look for a high enough amperage relay - I would push
>>> beyond the 30A - perhaps to 50A for each of the legs.
>>>
>>> The supply does have remote sensing and I would recommend running that
>>> to the front panel.  That allows you to connect small wires to the load and
>>> it will measure the voltage at that point and control it to meet the
>>> desired voltage - so any voltage drop across the wires, relay, binding
>>> posts, etc will not be a factor.  Usually these are connected internally to
>>> the power supply outputs already, so you would need to remove that wire
>>> (specs don't say how to do that, but it is usually just a small wire that
>>> is cut or unscrewed) and then connect it to a set of binding posts on the
>>> front panel.  To make life easier for you in cases where you don't really
>>> care that much about exact monitoring, you can provide a front panel switch
>>> that connects the remote sensing back to the outputs on the front panel.
>>> Just turn that to off and provide the wires to the load when you do care
>>> about exact voltae monitoring in high current situations.
>>>
>>>
>>>
>>> On Wed, Jul 9, 2014 at 8:08 AM, Jeffrey Crews <cruzetti at gmail.com>
>>> wrote:
>>>
>>>> Asking more for my own education than as a suggestion: could a large
>>>> enough capacitor prevent the arcing across the contacts as the relay opens
>>>> and closes? And if so, how large a capacitance would it take to handle this
>>>> much current?
>>>>
>>>> jsc
>>>>
>>>>
>>>> On Wed, Jul 9, 2014 at 8:01 AM, Adam Haile <email at adamhaile.net> wrote:
>>>>
>>>>> Ok, yeah, that's what I was thinking. I wanted it to be normally open
>>>>> and then energize the relay to close and provide power. That way when I
>>>>> turn the supply on, nothing is connected by default. But I assume I still
>>>>> have to be worried about the contact welding in that scenario since I might
>>>>> already have the device drawing power hooked up (I would rather not have to
>>>>> disconnect it every time I start up the supply).
>>>>>
>>>>> Also, I think I found my power rails:
>>>>> http://www.amazon.com/dp/B000H9JRI0/ref=biss_dp_t_asn
>>>>> 1/8" x 1/4" copper bar stock. My calculations show this should be able
>>>>> to handle WAY more power than I need. And it's pretty cheap. Figure I'd
>>>>> just do a slightly wider than 1/4" trace and then solder this on top.
>>>>>
>>>>>
>>>>> On Wed, Jul 9, 2014 at 4:19 AM, Scott Hall <scottghall1 at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>  On 07/08/2014 05:50 PM, Adam Haile wrote:
>>>>>>
>>>>>> So,  let me make sure I understand this...
>>>>>> When you say have the relays energized, I know you mean energize the
>>>>>> solenoid but do you mean by that that the relay contacts are closed (power
>>>>>> flowing) or open (no power)? I'm guessing you mean don't go from open to
>>>>>> closed while there's power flowing since their will be a temporary arc?
>>>>>> Could I just use a normally closed relay and then use the microcontroller
>>>>>> to open the relay and cut the power if I need to.
>>>>>>
>>>>>>
>>>>>> For safety reasons, you want the relay to disconnect if energizing
>>>>>> power is removed.  An NC contacts will not break the circuit if you have
>>>>>> power control problems to the energizing coil.
>>>>>>
>>>>>>
>>>>>>  Do you have any links to those higher cost relays? I would really
>>>>>> love to actually make it so that I could have a button on the front (hooked
>>>>>> to the uC) that would allow me to, using the relay, turn the power on or
>>>>>> off to any of the outputs. So having one of these nicer relays, if not too
>>>>>> expensive, might be nice.
>>>>>>
>>>>>>
>>>>>> Look up "contactors" for relays designed for high currents and
>>>>>> regular switching.
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Scott G. Hall
>>>>>> Raleigh, NC, USAScottGHall1 at GMail.Com
>>>>>>
>>>>>>
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