I really like my Siglent bench supply, it’s been excellent. But recently I learned about a “feature” the hard way and maybe I can save someone the trouble I had. Nothing terrible happened, though I’d rather nothing at all did...

I was working on diagnosing a problem with a UPS where it suddenly stops charing the battery and somehow eventually discharges it. I needed to get the SLA 12V 7Ah battery charged for testing so I hooked it to the SPD3303X-E and set it to current limit at 1A, with the voltage at 13.8V—so far, so good. Everything worked as expected and the battery charged just fine, then went into constant voltage mode and floated. The 3033 makes a fine if outrageously expensive SLA charger.

Then, I accidentally hit the power switch on the supply and turned it back on. BLAM
OK, that might be a bit of an exaggeration, more like—snap—but it felt like BLAM to me!

Well, the snap and sparks were the minigrabber leads on the battery. One had its hook blown off and housing melted, the other just the melted housing.

Why? You might ask. I know I did. And I even answered it, and here is the answer any way of a warning for users of the SPD33-3X-E:

AT STARTUP THE OUTPUTS OF THE SUPPLY ARE A DEAD SHORT FOR SOME FRACTIONS OF A SECOND
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So, when I turned the supply back on my poor minigrabber took on the role of a fuse. I don’t know what the peak output of the battery ism but I can tell you that the hook on the minigrabber can handle much less current than the test lead.

As an aside, I had just replaced those leads, and they aren’t cheap. I need to get better ones (I had trouble finding Pomonas for a reasonable price) but I needed them to work, so I printed little sleeves on my new 3D printer in PET-G, and voila—a new life for the leads.The part I printed is only 4mm high, with a 2.4mm hole in it. The 0.2 nozzle with 0.06 layer height did it. I put a drop of UV adhesive on there to keep the notch for the hook indexed and it’s all cool.

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AT STARTUP THE OUTPUTS OF THE SUPPLY ARE A DEAD SHORT FOR SOME FRACTIONS OF A SECOND

That's certainly unexpected.
Seems like a rather odd thing for a power supply to do.
Another fine print gotcha.

 

That's certainly unexpected.
Seems like a rather odd thing for a power supply to do.
Another fine print gotcha.

Surprised the heck out of me. I had to test it several times to convince myself it was happening.

 

I have the exact same bench power supply. How did you prove that it does that? also since the supply has current limiting how is it even possible for a current to flow that exceeds that limit?

I'm not saying you're wrong, but I am curious how you proved that it does and that it isn't a fault.

 

I have the exact same bench power supply. How did you prove that it does that? also since the supply has current limiting how is it even possible for a current to flow that exceeds that limit?

I put a DMM on the output in resistance mode and watched it short. Being a dead short, there is no current limiting involved.

 

@tautech - is this true? is this by design?

 

I put a DMM on the output in resistance mode and watched it short. Being a dead short, there is no current limiting involved.

So, if you shut the DC output off, connected a battery (as you did, to charge it) it would briefly short circuit the battery?

 

So, if you shut the DC output off, connected a battery (as you did, to charge it) it would briefly short circuit the battery?

The output switches don't do the same thing. It is only at power up.

 

The output switches don't do the same thing. It is only at power up.

The only reason I can think of to do that, is to prevent any voltage overshoot on power-up.

 

You may be right, Cruts.
Or this “feature” may be there to discharge any capacitors.
But I fully agree, there should have been a warning note on the manual.

 

Think this through. It's a power supply. It supplies a voltage at very low output impedance. At startup, when the power supply voltage is low, that low output impedance means that the power supply was effectively a dead short across the battery. I would actually expect many power supply models to do this.

Only a power supply specifically designed to start at high impedance would avoid this.

 

Its true, I verified. At power on (before one has actually set any of the output terminal pairs to "on") the DC resistance is indeed, zero.

It then, after about six seconds (as the unit initializes) it goes to about 450 ohms and stays there, naturally we must disconnect the ohm-meter before activating any of the DC outputs.

Curiously it does not have this "auto short" behavior on the fixed voltage output:

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At power on that has a DC resistance of about 880 ohms, across each of the four range options.

 

Think this through. It's a power supply. It supplies a voltage at very low output impedance. At startup, when the power supply voltage is low, that low output impedance means that the power supply was effectively a dead short across the battery.

Ok, I thought is through.
That low impedance is only as a source, not a sink.
Most power supplies will not sink any appreciable amount of current during normal operation.
This, on the other hand, appears to be a deliberate low sink impedance

 

Even still, it's a source that starts at a very low voltage, i.e. significantly lower than the 12 volt SLA battery.

 

There is no problem with the battery connected to the supply and then lowering its output voltage to below the battery's voltage. In fact, it properly displays the battery's voltage on the output display.

 

Thinking even further, a simple Schottky diode on the output of the supply would fix the issue. As crutschow notes, it would ensure that the supply is a source and not a sink.

 

Even still, it's a source that starts at a very low voltage,

Yes, but it normally starts with a high sink-impedance, so it will go to the output battery voltage, even if set at 0V

 

Surely the output should be open circuit until the user presses the "On/Off" switch for that channel. What would happen if a car battery were placed across the output terminals (with the intention of say trickle charging it), with thick wires, and then the unit powered up...

 

So, when I turned the supply back on my poor minigrabber took on the role of a fuse.

As AK mentioned, I wonder how much current that momentary short can handle, say if you had a more robust connection to the battery.

 

As AK mentioned, I wonder how much current that momentary short can handle, say if you had a more robust connection to the battery.

Also the short circuit state, lasts around six seconds Zapper, quite a long time.