General > General Technical Chat
Wayward DC currents
paulca:
I keep doing these things, getting bizarre readings and then failing to explain it properly. I have another problem.
I have a single source of 24V DC which is running 2 DC/DC bench PSUs.
One is set to 13V and is floating a lead acid. The lead acid is powering the network switch and the internet/wifi router (and 2 USB hubs).
The other is set to 19.50V and is running the work laptop docking station.
The trouble is, the currents are very suspicious. The lead acid's loads have been pulling pretty much exactly 1.3 Amps for the past 2 weeks. This morning the current out of the battery fell to 1.0 Amps.
The PSU powering the laptop started out at 4.5 Amps, topping up the laptop battery and fallen to around 1 Amp. Which is "about right", 20 Watts or so.
So where did the other 300mA go?
As a "finger in the air" I connected the multimeter to the "-" pole of the lead acid and via the "Amp" range to the copper radiator pipe beside it. Hmmm. 0.5 Amps, falling over a few seconds to.... 300mA.
At the weekend I tried to power a monitor from the PSU which is powering the work laptop and I ended up disconnecting it pretty quickly.
The monitor came on fine. Worked perfectly, but when I look at the PSU running it it said "0.00A" and the other PSU was showing 2 Amps more. I could even unplug the "-" from the PSU and the monitor continued to run.
So 2 amps which was not returning to the PSU that made it. The only other path from the monitor to anywhere was via the HDMI and DP cables, from there t the USB hub and back down the "-" rail via the battery and back untimately to the 24VDC that provided. I didn't like the sounds of 2 Amps on the lowside of 19.50V "leaking" that way, so I shut it down.
This mornings oddity is different. Those 300mA appear to being "added" to the circuit, not re-routed.
ebastler:
Could you please draw and share a wiring diagram first? You have described your setup pretty clearly -- but it is still easy for readers to make implicit assumptions and maybe misinterpret your description. And drawing all sources, loads and connections might also help you understand what is going on.
paulca:
This is not a schematic, I just used a scematic editor to diagram it. I added the "para-paths" as un-connected lines, they are just suspicions. (The DCDC converter outputs are swapped, please ignore that :))
The path out of the laptop docking station I have confirmed is the HDMI cable. When removed the 300mA appears coming out of the battery again.
In the case of the monitor, the only change was replacng the laptop dockign station with the monitor and it showed no current and everything coming from (or returning via the 13V PSU)
EDIT: This might show how I can lose 300mA, sort off via the other PSU, which will be making both PSU's current readings completely b*ll*cks.
It's still very curious as to how can I also produce a return path for that current from the local earth!
paulca:
I remembered I have a current clamp meter. So I was able to bunch all 4 wires from the PSUs into the clamp and there was a stray 20mA (neglibible, noise or error margin).
The lower voltage circuit appears to be providing/sourcing 0.5A to the higher voltage circuit. One shows -0.5A total current and the other shows +0.5A total current.
I don't think I need to be that concerned with 0.5A over HDMI etc, all of the cables/interfaces involved support about that current or more for power transfer.
I suppose the only way to stop this is use isolated DC/DC converters?
madires:
Many possibilities: shielded ethernet cables, PCs (grounded via mains PE), audio cables and all the other stuff.
I have a somewhat similiar setup with a central DC power supply for powering small devices and have grounded the negative output to PE to minimize ground currents (which create noise) through the tons of cable between all devices.
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