Author Topic: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike  (Read 9407 times)

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Offline RogersanTopic starter

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This power supply can produce a 19 volt spike at the output terminals as it is powered on. This is in spite of the supply having been set previously to 5 volts, and the On/Off switch not having been operated. The On/Off switch is not isolating the output terminals at power up

Any sensitive circuitry connected to this supply can be destroyed at power on. I tried a 1 watt 5.1 Zener diode across the output terminals and this was shorted out by the surge so there is a fair bit on energy available in the start up spike.

                 Roger Sanderson
 
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Offline nrxnrx

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #1 on: July 23, 2016, 06:42:18 am »
Thank you for the warning!

I just checked mine and it also happens when you turn it off.
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #2 on: July 23, 2016, 07:38:47 am »
The On/Off switch is not isolating the output terminals at power up


Are you shure that the relay for the output is not defective (hanging).
I usually have no spikes with a KA3005 as the relay switches off during power off.
Only if I actuate the power on switch very fast from on to off to on (before the relay can switch off) I can generate some overshoot.

With best regards

Andreas
 

Offline nrxnrx

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #3 on: July 23, 2016, 07:54:14 am »
I can only find two relays inside the unit and none seem to be used for the output.
I have't traced the circuit, but I hear no click at power-up or when turning the output on or off.

--
Mihai
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #4 on: July 23, 2016, 08:17:13 am »
Ok,

so it seems they only have a relay on the KA3005.
I hear it clicking on switching the output on/off.
Default on power on is output off.

With best regards

Andreas
 

Offline tautech

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #5 on: July 23, 2016, 08:40:50 am »
This power supply can produce a 19 volt spike at the output terminals as it is powered on. This is in spite of the supply having been set previously to 5 volts, and the On/Off switch not having been operated. The On/Off switch is not isolating the output terminals at power up

Any sensitive circuitry connected to this supply can be destroyed at power on. I tried a 1 watt 5.1 Zener diode across the output terminals and this was shorted out by the surge so there is a fair bit on energy available in the start up spike.

                 Roger Sanderson
Welcome to the forum.

You really need to offer a screenshot for proof of your claims.
Not that I don't believe you but something like this would be in order:
Borrowed from this thread:
https://www.eevblog.com/forum/testgear/cheap-variable-switching-power-supply/msg666082/#msg666082



Appropriate trigger settings, timebase and input coupling required.  ;)
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Offline nrxnrx

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #6 on: July 23, 2016, 09:06:50 am »
Attached are two screenshots of probing across a 10ohm resistor and a 100ohm one - when powering off.
(at the end of 2 ~30cm leads)

DC coupling for the signal & trigger.
« Last Edit: July 23, 2016, 09:17:20 am by nrxnrx »
 

Online Kleinstein

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #7 on: July 23, 2016, 09:34:47 am »
The spikes look like noise from a switcher - many pulses at constant frequency with massive ringing. It so high in frequency that it may get to the scope through the air as well. The shown waveform may not fully show the true voltage though.
This has not much to do with the normal power on-power / power-off spikes seen with some supplies. It more like a massive EMI problem.

A first test would be to turn the supply of / on by pulling the plug. It might be the switch is bad, producing excessive arcing - though it should be rather bad for this.

I would have a good look at the capacitors at the output.

Is there this type of RF interference also present during normal operation ?
 

Offline tautech

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #8 on: July 23, 2016, 10:35:54 am »
@ nrxnrx
Use single shot triggering.
Have a fiddle to find the right V/div and timebase settings.
You'll notice my screenshot shows Stop, that's the single shot doing it's job.....my finger isn't that fast.  ;)
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Offline nrxnrx

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #9 on: July 23, 2016, 10:43:42 am »
@ nrxnrx
Use single shot triggering.
Have a fiddle to find the right V/div and timebase settings.
You'll notice my screenshot shows Stop, that's the single shot doing it's job.....my finger isn't that fast.  ;)

I can't trigger manually for microsecond events either :) . I did use single shot for all the screenshots I posted here (top left of the pictures, to the right of "RIGOL").
As for the V/div settings, what do you want to see?

It's a linear supply and during normal operation the output is nice and clean.

--
Mihai
« Last Edit: July 23, 2016, 10:50:53 am by nrxnrx »
 

Offline tautech

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #10 on: July 23, 2016, 11:15:18 am »
@ nrxnrx
Use single shot triggering.
Have a fiddle to find the right V/div and timebase settings.
You'll notice my screenshot shows Stop, that's the single shot doing it's job.....my finger isn't that fast.  ;)

I can't trigger manually for microsecond events either :) . I did use single shot for all the screenshots I posted here (top left of the pictures, to the right of "RIGOL").
As for the V/div settings, what do you want to see?

It's a linear supply and during normal operation the output is nice and clean.

--
Mihai
The thread is about power on spikes.  ;)

Then try normal triggering and single shot, set the trigger to a value the signal will cross at power on and you should have a power on spike capture.
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Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #11 on: July 23, 2016, 12:58:57 pm »
Hello,

first I have to correct myself. The clicking that I heard is only at higher output voltages than 5V.
(So it is the relay for the transformer winding at higher voltages).

I repeated the measurements with 5V (usually I do not use a 30V supply for a 5V load).

On switching mains on I see 0.8V (low energy pulse < 1us) at the output. (I guess capacitive coupling from the mains transformer to the output).

Switching the output on gives up to 0.4V overshoot to the 5V for around 5 ms on my device.

Mains off gives some contact bounce at the mains switch together with the transformer inductivity (burst impulses).
Zooming in shows that they are low energy. (although up to 4V above the 5V)
I guess that most of those pulses travel by the mains line or capacitive coupling into the scope.
(The scope may be disturbed by those pulses).

Finally especially for Kleinstein the noise in normal operation (AC coupled)
first with 20 Mhz the 2nd with 100kHz bandwidth.

So there are no switching noise spikes from the linear power supply.

with best regards

Andreas


 

Offline nrxnrx

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #12 on: July 23, 2016, 03:20:49 pm »
The thread is about power on spikes.  ;)

Then try normal triggering and single shot, set the trigger to a value the signal will cross at power on and you should have a power on spike capture.

The problem as I understand it is that the thing outputs highish energy pulses when the unit is turned on and off, even though the output is nominally "off".

Quote
The On/Off switch is not isolating the output terminals at power up

Perhaps I misunderstand?
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #13 on: July 23, 2016, 04:31:15 pm »

The problem as I understand it is that the thing outputs highish energy pulses when the unit is turned on and off, even though the output is nominally "off".

Perhaps I misunderstand?

The question is high energy pulses. (19V (how long?) instead of 5V).

On my unit it is max overshoot to 5.4V for 5ms with light loads.

On your unit it is most probably a measurement problem of your scope with switch bouncing.
Disconnect the cable on the Korad side and leave it connected to the scope and measure again.

With best regards

Andreas

 

Offline Mr. Scram

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #14 on: December 04, 2017, 03:36:46 pm »
I've recently received one of these power supplies, a two channel model, and noticed the same issue along with a few others.

  • When the unit powers on, you often hear a loud "twang" sound, like you used to hear in CRT monitors. A vibration coinciding with the sound can be felt. Sometimes dimmed lights nearby flicker slightly.
  • There's 3,3 mV on one output and -5 mV on the other when the outputs are off. The supply display shows nothing.
  • There seems to be a far amount of voltage spiking when turning the unit on and off. Voltages as high as 80 Vpp can be seen. When loaded down with a 20 ohm resistor, the spikes drop to about 10 Vpp. Turning channels on and off isn't a problem. There's very minor overshoot when turning on a channel, but otherwise the unit is well behaved.

The spiking isn't always present and doesn't necessarily coincide with the twang noise. I've repeated the measurement with the leads disconnected from the power supply but with them next to it, with the leads still attached to the oscilloscope. Even though there is a signal induced, it's negligible in duration and amplitude compared to the signal when the leads are connected. It's not a muted mirror image either, so the signal definitely seems conducted.

« Last Edit: December 04, 2017, 03:39:00 pm by Mr. Scram »
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #15 on: December 04, 2017, 06:32:02 pm »
Hello,

did you use proper shielded cables/connectors?
E.g. similar to this connector togeter with a BNC-cable?

https://www.digikey.com/products/en?keywords=BKCT2939-ND

That what you show (50ns pulses) is nothing that can be generated by the (slow) power transistors of the power supply.
These are common mode pulses generated by bouncing of the mains switch together with the inductivity of the transformer.

with best regards

Andreas


 

Offline Mr. Scram

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #16 on: December 04, 2017, 08:03:35 pm »
Hello,

did you use proper shielded cables/connectors?
E.g. similar to this connector togeter with a BNC-cable?

https://www.digikey.com/products/en?keywords=BKCT2939-ND

That what you show (50ns pulses) is nothing that can be generated by the (slow) power transistors of the power supply.
These are common mode pulses generated by bouncing of the mains switch together with the inductivity of the transformer.

with best regards

Andreas
I didn't use those connectors. I've ordered them, but haven't received them yet. However, instead of user two leads, I've now probed the binding posts directly with the scope probe. I think I'll repeat the test later, as results haven't been completely consistent in regards to the voltages generated, but this seems to be a representative sample of spikes during turn on. Turning the power supply off generally generates a lot more spikes. Probe at 1x, DC coupling on channel and trigger.

Either way, it seems the voltages could be problematic for certain parts and devices attached to the power supply.

« Last Edit: December 04, 2017, 08:06:31 pm by Mr. Scram »
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #17 on: December 04, 2017, 09:35:43 pm »
Either way, it seems the voltages could be problematic for certain parts and devices attached to the power supply.

Hello,

these short pulses even if they should be differential mode (between + and - pole of supply) do not carry much energy.

But they are most probably common mode (between poles and earth ground).
Thats why you see large differences when connecting different ways.

with best regards

Andreas

 

Offline Mr. Scram

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #18 on: December 10, 2017, 07:22:11 am »
Hello,

these short pulses even if they should be differential mode (between + and - pole of supply) do not carry much energy.

But they are most probably common mode (between poles and earth ground).
Thats why you see large differences when connecting different ways.

with best regards

Andreas
What would be a relevant method to test the nature and energy of the spikes?

I guess I could add a resistor across the leads. With a known voltage and resistance, current and energy could be deduced.
« Last Edit: December 10, 2017, 08:36:00 am by Mr. Scram »
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #19 on: December 10, 2017, 04:00:44 pm »
I guess I could add a resistor across the leads. With a known voltage and resistance, current and energy could be deduced.

Hello,

the measurement at these frequencies is not trivial.
You need a coaxial resistor suitable for some 100 MHz.
Scope or power supply has to be in a shielded cabinet.
(the coaxial resistor through the wall).

With best regards

Andreas
 
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Offline ziplock9000

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #20 on: August 28, 2018, 09:48:20 pm »
I just bought one of these from eBay that is supposed to be in excellent condition. This version has the binding posts, which I understand is the older of the two models (although that are both Tenma 72-10480)
If I find I get the spikes outlined above when I check it with my scope, is there anything I can do to mitigate the effects of this? Maybe replace a component?
 

Offline Andreas

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #21 on: August 29, 2018, 05:17:19 am »
If you mean the high frequency pulses from arcing of the mains switch:
Try to convince that he should not arc. (e.g. with a snubber).

With best regards

Andreas
 
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Offline ziplock9000

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #22 on: August 29, 2018, 10:13:03 am »
If you mean the high frequency pulses from arcing of the mains switch:
Try to convince that he should not arc. (e.g. with a snubber).

With best regards

Andreas

What type & values of snubber would work well for this PSU?
 

Offline ziplock9000

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #23 on: August 30, 2018, 12:41:28 pm »
It arrived. I did a power on (via hardware switch) test when set to 5v / 200mA and no load and I don't get a spike >5v, although it's close. I've tried this in various modes (peak, normal, hi-res, 20m BW limit).

I'll do some more testing later with different voltages and loads. So far I'm very happy with it and it's a great upgrade to my Farnell L30-2
 

Offline Mr. Scram

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Re: Korad KA3003D (Tenma 72-10480) power supply - start up voltage spike
« Reply #24 on: August 30, 2018, 01:08:11 pm »
What did you end up installing?
 


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