Maker Pro
Maker Pro

In Rush Current

R

Richard

Jan 1, 1970
0
I posted about a SBC P4 that randomly failed to boot (Power up) and want
some ideas how to get accurate current measurements. You kind folks helped
out by explaining the Shunt deal.

Since then I have taken numerous measurements and I want to share my setup
in case someone has so ideas.

First, the spec of the SBC is Max. 4.5 amps on Start up, Normal 3.2 amps.
I am using a 5 vdc DC COnverter located 14" away using 18 ga Instrument wire
14" long to carry the power. I "thought" is was large enough, but now I
have second thoughts.

When I first power up the DC Converter, I see a small spike which is right
at 10 amps, and it only last for a few ms. Then the power drops to about
1.5 amps. When the computer beeps, I jump up to 3.5 amps then to 7.5 amps,
then immediately back to 3.5, then it slowly settles down to around 2 amps.
Now I only have 10% Processor Utilization at any given time.

I take it, the 10 amp jump is the initial in rush current? then the 7.5
amps is a very breif spike from the SBC kinking off?

Does this mean my 18 ga is border line and I would really need 16 ga for the
power distribution wires? I did note that with the 18 ga wire there is 90
millivolt drop from the DC Converter terminals to the SBC Power Terminals so
I think that is a good indication I need to step up a size.

Any thoughts,
Richard
 
L

Larry Brasfield

Jan 1, 1970
0
Richard said:
I posted about a SBC P4 that randomly failed to boot (Power up) and want some ideas how to get accurate current measurements. You
kind folks helped out by explaining the Shunt deal.

Since then I have taken numerous measurements and I want to share my setup in case someone has so ideas.

First, the spec of the SBC is Max. 4.5 amps on Start up, Normal 3.2 amps.
I am using a 5 vdc DC COnverter located 14" away using 18 ga Instrument wire 14" long to carry the power. I "thought" is was
large enough, but now I have second thoughts.

When I first power up the DC Converter, I see a small spike which is right at 10 amps, and it only last for a few ms. Then the
power drops to about 1.5 amps. When the computer beeps, I jump up to 3.5 amps then to 7.5 amps, then immediately back to 3.5,
then it slowly settles down to around 2 amps. Now I only have 10% Processor Utilization at any given time.

I suspect your current spike corresponds to some
extra activity. Is the internal clock speed varying,
as uP's intended for mobile use often do?
I take it, the 10 amp jump is the initial in rush current?

Yes. You are charging the bulk bypass caps with it.
then the 7.5 amps is a very breif spike from the SBC kinking off?

Hard to say (even if I wished to guess what "kinking off"
is, but I don't want to know ;-).
Does this mean my 18 ga is border line and I would really need 16 ga for the power distribution wires? I did note that with the
18 ga wire there is 90 millivolt drop from the DC Converter terminals to the SBC Power Terminals so I think that is a good
indication I need to step up a size.

That's a little much for a 14 inch wire run. At those
currents, heating is not really an issue, but drop may
be depending on your supply tolerance and how the
SBC is specified. 16 guage should be plenty big.
Any thoughts,

Is there a reason to go for the smaller wire?
 
R

Richard

Jan 1, 1970
0
Yes. You are charging the bulk bypass caps with it.


Hard to say (even if I wished to guess what "kinking off"
is, but I don't want to know ;-).

Sorry, typo, I meant that was the CPU kicking off or Starting up. The SBC
has it's on DC Converter inside so it could be it starting up.
That's a little much for a 14 inch wire run. At those
currents, heating is not really an issue, but drop may
be depending on your supply tolerance and how the
SBC is specified. 16 guage should be plenty big.


Is there a reason to go for the smaller wire?

No, we just had a 100 premade harness kits made because I looked at the
original motherboard specs and then looked up the 18 ga. and it seemed to be
sized ok, and our other harnesses were 18 ga.

I made up a 16 ga harness to see if it helps. Besides the fact that the
voltage drop will be much less, will I see differences with the inrush
currents as well? I think the wire being too small and the high inrush is
what keeps the computer from booting sometimes.

Richard
 
L

Larry Brasfield

Jan 1, 1970
0
Richard said:
Sorry, typo, I meant that was the CPU kicking off or Starting up. The SBC has it's on DC Converter inside so it could be it
starting up.
Whew!


No, we just had a 100 premade harness kits made because I looked at the original motherboard specs and then looked up the 18 ga.
and it seemed to be sized ok, and our other harnesses were 18 ga.

If the spec on your supply output voltage is enough tighter
than the spec on your SBC supply requirement so that
the 90 mV drop is not a problem, then I would use those
100 harnesses. And have more made up with bigger wire.
I made up a 16 ga harness to see if it helps. Besides the fact that the voltage drop will be much less, will I see differences
with the inrush currents as well?

The inrush current is most likely limited by the supply
rather than the wire. I would expect no real change.
I think the wire being too small and the high inrush is what keeps the computer from booting sometimes.

That seems unlikely to me. Your SBC probably has
a spec on how fast the supply should come up. Be
sure you meet it. Also, be sure there is no overshoot
just after that initial current spike driving the supply
voltage above the SBC voltage spec.

You might want to look carefully at the supply voltage
waveform as it comes up. Maybe the DC-DC converter
is initially putting the supply into current limit and causing
a wobbly turn-on waveform rather than what the SBC
designers assumed (and should have specified). You
might try a higher current limit supply, experimentally,
to see if that reduces the misbooting incidence.
 
L

Lord Garth

Jan 1, 1970
0
Any thoughts,
Richard

It occurs to me that the modern ATX power supply has extra power supply
lines. These
are intended as a parallel path to supply the CPU with the extra power it
needs. You
may have noted the (usually) two odd power connectors coming from the power
supply.

Are you using these?
 
R

Richard

Jan 1, 1970
0
No Sir,

This is an Industrial, Embedded Motherboard running a Via 667mhz on a single
5 vdc supply. We are using a Datel 18-36vdc input to 5 vdc output rated at
5.0 amps. It's 14" away from the SBC.

Richard
 
Top