Author Topic: Rigol DP811 (may be DP8XX) short current feature (may be bug)  (Read 593 times)

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Online MegaVolt

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Rigol DP811 (may be DP8XX) short current feature (may be bug)
« on: January 05, 2018, 07:38:56 pm »
I found the following feature on my new power supply.

To repeat the problem, follow these steps:

1. Connect the battery (Li 18650) to the switched off source through the multimeter connected to the current measurement.
2. Turn on the power supply.

The battery is discharged through an internal resistance of about 150 mOhm.

Enabling or disabling the output does not affect the current level. The voltage is measured and displayed correctly. The current level is displayed and measured incorrectly.

If the circuit is broken for a short time, the power supply returns to normal operation and can charge the battery without problems.

I know that Rigol does not recommend connecting active loads. But I would like to understand what caused this unexpected behavior? What is hardware protection? Calibration error?

Is there a similar problem on other sources of this family? For example, the DP832x?
« Last Edit: January 05, 2018, 07:56:46 pm by MegaVolt »
 

Offline thmjpr

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Re: Rigol DP811 (may be DP8XX) short current feature (may be bug)
« Reply #1 on: January 06, 2018, 06:24:47 pm »
Is there a similar problem on other sources of this family? For example, the DP832x?

I didn't see it, at least when supplying 0-10V with another power supply. Either the topology is slightly different, or I need to try again with a Li-ion.
It could be some kind of crowbar protection, I believe the only major protection on DP832 output was a reverse diode.
I have not seen a teardown of the DP811.

DP832 behavior:
- 0-3mA draw when the power supply is off and power is injected.
- 35mA (or 50mA on the 5V output) draw when the power supply is on (its some kind of current sink, perhaps to discharge the output caps).

If you look at the specs, DP811 has a 3x slower fall time over the DP832, could mean the current sink circuit is different.
 
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Offline TurboTom

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Re: Rigol DP811 (may be DP8XX) short current feature (may be bug)
« Reply #2 on: January 07, 2018, 01:08:00 am »
I can confirm this behavior of the DP811. I just tested it with a second PSU instead of a Li chemistry battery. Actually it seems to be even more weird because this shorting out / latch-up of the input appears to occur only at certain voltage ranges.

The sequence to induce the latch-up condition is as follows:

1. With the mains power to the DP811 switched off, apply a voltage (current limited supply between 1 and 4V or above 8V) of correct polarity to the DP811's output. At that time, there won't be any considerable current observable.
2. Power on the DP811 -- as soon as the switch is flipped, even before the DP811's display shows anything, the externally applied voltage gets shorted or pulled down to a residual voltage of 0.95V @ 3.5A.
3. The DP811 apparently boots normally and doesn't show anything abnormal about its output condition on its display.
4. If now the DP811's output is enabled, the instrument directly goes into CC mode and the full, selected current is read on the display. The voltage displayed matches what can be measured across the output terminals. This voltage is now slightly higher than when only the external PSU is fed into the DP811, in my case it was approx. 1.25V @ 3.5A externally supplied and the DP811 set to 10A CL and enabled. The load stays in this condition also if the external PSU is switched off as long as the DP811's output stays enabled.
5. If now mains power to the DP811 is switched off again, the current supplied by the external PSU keeps on flowing and the DP811 only resets itself if the external supply is momentarily disabled.

So I guess this exactly matches MegaVolt's findings.

I assume there's some kind of crowbar SCR device installed across the DP811's output that for some reason gets triggered when the power supply is switched on. Apparently this is only related to the output since all the other circuitry appears to be working normally. Considering that there doesn't seem to be any current limiting device present between the output and the supposed SCR, it would be a very bad idea to try to charge a powerful battery with the DP811. If there's mains power cycle for whatever reason, the whole setup may end up in flames.

Maybe I'll tear down my DP811 to check if this shortcoming could be revised.

Cheers,
Tom
« Last Edit: January 07, 2018, 01:24:04 am by TurboTom »
 
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Offline JDubU

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Re: Rigol DP811 (may be DP8XX) short current feature (may be bug)
« Reply #3 on: January 07, 2018, 02:30:17 am »
 
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Online MegaVolt

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Re: Rigol DP811 (may be DP8XX) short current feature (may be bug)
« Reply #4 on: January 07, 2018, 10:06:51 am »
latch-up of the input appears to occur only at certain voltage ranges.
It is a good idea to test this problem from another regulated source. I did not reach such a point. And I do not have a second source :(

The fact that the problem exists only in a limited range of voltages is very interesting. Although the range does not seem to be strict. I'm triggered by 4,2V
Quote
Maybe I'll tear down my DP811 to check if this shortcoming could be revised.
Oh that would be very cool !!! My power supply is still under warranty.

I will be very grateful for pictures of the entrails.
 


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