Battery Powered System, Overvoltage Possible!? 1

[cmamich]

Hey everyone, first of all --- great website, long time lurker, first time poster. Thanks to all who contribute.

I have a National Instruments data acquisition device and a 900Mhz serial modem being powered by two 12V gel batteries in series. The DAQ and RF modem are in a NEMA rated enclosure and the power is brought in with a marine grade 3 prong plug/receptacle. There is a switch between the receptacle and the devices themselves. The issue I have been having is the RF modem has been intermittently failing. Inspection of the devices shows no evidence of physical damage or burned smell. The failures do not occur at any specific time (ie during battery change) or seem to follow any pattern. The OEM inspection of the failed device claims overvoltage destroyed the device. The device accepts a range of 7-28VDC, two 12V batteries at full charge should check in at around 25.5V. There has never been a failure immediately following a battery change which would stand out to me as the only way to potentially ever have an overvoltage condition but still unlikely. The DAQ accepts 11-30V and has never failed. The application has substantial intermittent impact and vibration. Both devices receive power in similar way with the ground at the receptacle and the positive coming off the fuse block. The RF device is a typical size N radio plug for power while the DAQ is a screw type terminal. What could possibly be happening to cause an overvoltage condition for that device? Is it purely happenstance that the DAQ has never failed in that same manner or perhaps the circuitry inside the device has more protection for such conditions? Is it a bad assumption to think that the voltage of a gel battery is stable even under harsh conditions? My plan at this point is to power the system via 12V to give the device a bit more of a cushion but I'm not answering the true question nor do I think that is really the cause. Perhaps you electrical gurus can help a mechanical guy stumble through this?

 

[cmamich]

Furthermore, wouldn't the easy answer here be diode protection on both wires? That would protect me from reverse polarity and back current yes? If there was enough current to blow zero ohm resistors wouldn't a diode get smoked too?

 

[VEBill]

A series diode will 'block' reverse current thereby protecting the downstream circuit.

Another method sometimes used is a parallel diode (normally reversed biased) that will conduct heavily if the power is reversed and will then blow a series fuse to open the circuit.

...

Zero ohm resistors should not fail under moderate over-voltage, unless there's something funny going on. The funny business might be some other upstream component failing as a dead short due to the over-voltage, and that other failure then causing high currents that damage the zero ohm resistors. But you'd have to mention the other failure. Resistors (zero ohm or otherwise) aren't going to fail directly from application of a bit more than 28 VDC.


At this point it's all guess work. If you can find a schematic diagram for the RF Modem's PS circuit then we could probably identify the design limitation.

...Or simply back away from the 28 VDC upper voltage limit, thereby leaving more time for lunch/beer.

 

[cmamich]

The system is extremely simple, only RF Modem, NI DAQ, 2 batteries. No item in the system has ever failed except the RF modem.

The OEM recommended that I connect ground of battery connection to the frame as their theory is somehow potential is being built up in the frame and finding its way into my system --- I just don't see how this is possible and I have some reservations about doing that because then if during battery change somehow a tool were to bridge the frame and the positive we'll have fireworks.

The zero ohm resistor was described as .020" x .040" in size, what sort of voltage/current can these handle?

 

[MacGyverS2000]

An 0204 is usually rated for 0.25-0.5W

Dan - Owner

http://www.Hi-TecDesigns.com

 

[zappedagain]

What resistor? You aren't giving us the whole story here...

That is commonly referred to as a 0402 resistor. The ERJ series from Panasonic is rated for 1A and has a max overload current of 2A.

John D

 

[Warpspeed]

I think Opera House is on the right track with his suggestion of inductive spikes. Many of these black box devices now have LC low pass filter circuits fitted in the dc input lines to meet conducted noise immunity standards.

Switching off the power abruptly might be causing one device to inductively kick the other into a momentary over voltage.

If the input voltage range is 7 -28 volts, why are the batteries being connected in series ?
I would put them in parallel. As the battery end voltage is going to be around 10v, that will make far better use of the total battery capacity. Especially if linear voltage regulators are being used. It would effectively double the amp hour capacity.

Another suggestion might be to connect a really large electrolytic capacitor across the dc line, between the on/off switch and your two devices.
When power is switched off, the voltage on the capacitor will die away gracefully, without any sudden vicious step changes in current. A very low value resistor in the battery line could limit inrush into the electrolytic. Another non violent way charge up the big cap, might be a three position on off switch, with a suitable starting resistor in the middle position.

Last suggestion would be to fit a simple pre regulator, but I still think the most efficient solution would be to put the batteries in parallel.

 

[cmamich]

The input range of the NI DAQ is 11-30 VDC. The DAQ pulses the load cells at 10VDC, therefore I wanted to ensure proper operation throughout the range of charge hence the 24V system selection.

Warp,

Could you, in layman's terms, briefly explain what you are asserting when power is switched? I do not understand this concept of momentary overvoltage via induction. This has me thinking though, when I switch the power off --- what happens in the circuit physically from a current standpoint?

I am switching 2 of the 4 systems to parallel and hence 12V to monitor performance/life versus the two.

 

[Warpspeed]

In simple terms, when you have direct current flowing through any inductor, and you suddenly break the source of current, a high back EMF voltage can be produced.

Now if there is only one piece of equipment with an inductor in the dc path, then the voltage spike will most likely all appear across the opening switch contact.

But if you then connect a second piece of equipment controlled by that same switch, then the sudden inductive voltage kick across the switch might be enough to cause a problem.

The worst possible case would be one heavy load having a lot of series inductance, and a weak but voltage sensitive second load connected together to the same switch.
As the total load current is broken, the weak load could possibly receive a fair kick in the guts from the resultant inductive overvoltage spike.

 

[Warpspeed]

I believe you will be surprised at the extra life you will get from the parallel connection.
The load current per battery will be half, but at the halved discharge rate the total amp hours available to the voltage end point will more than double.

An eleven volt end point is fairly high, but because of the "S" shape of the voltage discharge curve, and it's more than doubling in width, I believe you should still gain significantly in available total equipment running hours per charge.

 

[cmamich]

So definitely going to go 12V system, but want to provide all the extra protection I can. So would a TVS diode on the incoming PWR lines after the fuse make sense as well as a Zener diode on the GND lines? Is this a comprehensive way to protect my devices?

http://www.nxp.com/documents/data_sheet/PESDXS1UL_SER.pdf

This is the TVS diode I have preliminarily selected, I was going to go with the a reverse standoff voltage of 15V. The application notes that this is for data lines...which mine is not --- what am I missing here? The specifications of the device looks like it would work nicely in my application.

I'm sort of annoyed these expensive devices don't have such protections already built in --- now I have to worry about more solder joints and durability of construction of what I make. /grumble

 

[cmamich]

I don't know how the word Zener got in there....just a normal diode on the GND lines to prevent any sort of current flow in that direction. That alone should provide reverse polarity protection or any sort of induced current.

 

[zappedagain]

That TVS looks fine for your application (transients).

You'll be better off with a reverse-polarity-protection diode wired between the +12V supply and the load, not between the load and ground. The diode forward voltage drop could introduce enough ground shift to mess up your logic levels, and introduce ground bounce problems as the current changes (Tx to Rx, for example).

John D

 

[cmamich]

zapped,

Thanks for the tip. So could go battery, rpp diode, tvs diode, fuse, load --- all in series on the +12V supply line?

This diode should do the trick yes? Enough forward current capacity to handle device requirements while minimizing foward voltage drop....

http://www.mouser.com/ProductDetail...GAEpiMZZMuIUjt4yeP9c0eSDJ8%2bdf4YGwmLkj5HLPY=

Thanks again...now I just have to put all this crap a piece of board nicely and mount it in the enclosure.

 

[Warpspeed]

A series diode is a workable solution, but the resulting forward voltage drop is certainly not going to be an advantage down near the battery end point.

My preferred option would be a big ugly electrolytic capacitor placed right across the supply.
No voltage drop, and it will gobble up any voltage spikes of either polarity without raising a sweat.

 

[Warpspeed]

Thinking a bit more about this.
An electrolytic will cause the load voltage to slowly decay when switching off, which should eliminate the voltage spike problem at sudden turn off altogether.
So the electrolytic might not need to be very large at all to do the job.

 

[zappedagain]

"So could go battery, rpp diode, tvs diode, fuse, load --- all in series on the +12V supply line?" - Not quite, the TVS shunts to ground:

[tt]
---|>|-------|~~~~|----
| | |
--- | \
- --- /
--- ^ \
- / \ |
| --- |
| | |
----------------GND------
Batt RPD TVS Fuse Load

[/tt]
Z

 

[LionelHutz]

The fuse should be the first thing in the chain coming off the battery, not the last.

 

[zappedagain]

Good point.