Advice on a failed power supply

Thread Starter

austin944

Joined Mar 25, 2023
9
I bought this B&K Precision 1550 power supply new back in February 2023 from a reputable and authorized distributor, used it a few times, and it worked great until recently. Now it always shows 0V and 0.16A Current Controlled (CC) no matter what I do with the controls. This is my first power supply as a hobbyist so I could be doing something wrong with it. AFAIK I've not abused it -- no shorts, no high temperatures, not dropped or hit, nothing spilled on it. Just one day I powered it on and it decided not to work. I checked the fuse and it's good. The USB port still provides 5.2V.

Should I send it in for repairs? What could cause such a problem? I'm a little confused how it could apparently fail(?) so early and so easily from such a reputable manufacturer. I bought it specifically because of the brand and because it's what I could afford.

The first pic shows the output, the second has me setting the current limit, and the third shows me setting the voltage, all with no load.
 

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ApacheKid

Joined Jan 12, 2015
1,658
I bought this B&K Precision 1550 power supply new back in February 2023 from a reputable and authorized distributor, used it a few times, and it worked great until recently. Now it always shows 0V and 0.16A Current Controlled (CC) no matter what I do with the controls. This is my first power supply as a hobbyist so I could be doing something wrong with it. AFAIK I've not abused it -- no shorts, no high temperatures, not dropped or hit, nothing spilled on it. Just one day I powered it on and it decided not to work. I checked the fuse and it's good. The USB port still provides 5.2V.

Should I send it in for repairs? What could cause such a problem? I'm a little confused how it could apparently fail(?) so early and so easily from such a reputable manufacturer. I bought it specifically because of the brand and because it's what I could afford.

The first pic shows the output, the second has me setting the current limit, and the third shows me setting the voltage, all with no load.
Do you have a multimeter handy? if so measure the output voltage of this supply. Then put a resistor across the supply in series with the same multimeter in current mode, does a current flow?

Does the measured voltage in any way, correspond to the displayed voltage? there's also talk in the manual about firmware, is that up to date (that's a long shot but worth eliminating at least).

I have one of these and shortly after getting it I became confused, no matter what voltage I set the current through my test circuit was always the same, then someone pointed out that's expected - it had a constant current mode and I'd completely overlooked that!

1682016723412.png
 

Thread Starter

austin944

Joined Mar 25, 2023
9
Do you have a multimeter handy? if so measure the output voltage of this supply. Then put a resistor across the supply in series with the same multimeter in current mode, does a current flow?

Does the measured voltage in any way, correspond to the displayed voltage? there's also talk in the manual about firmware, is that up to date (that's a long shot but worth eliminating at least).
With no load and it powered on, I get 0.045V on the terminals with a multimeter, despite setting the voltage way above that, like 12V.
With a 100ohm resistor across the terminals, I get 0.003V on the terminals.
With an ammeter in series with the 100ohm resistor connected to the terminals, I get 0.000A. I confirmed my ammeter is working with a different power source.
During all this time, I always see 0V on the PS display.
When the PS "boots", it briefly shows a screen with a "1.3" on it, which I presume is the FW version.

I have definitely played with the Current Limit setting when it was working, and recognize the CC mode, but it does not appear to be in a real CC mode. Maybe some kind of internal short?
 

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ApacheKid

Joined Jan 12, 2015
1,658
With no load and it powered on, I get 0.045V on the terminals with a multimeter, despite setting the voltage way above that, like 12V.
With a 100ohm resistor across the terminals, I get 0.003V on the terminals.
With an ammeter in series with the 100ohm resistor connected to the terminals, I get 0.000A. I confirmed my ammeter is working with a different power source.
During all this time, I always see 0V on the PS display.
When the PS "boots", it briefly shows a screen with a "1.3" on it, which I presume is the FW version.

I have definitely played with the Current Limit setting when it was working, and recognize the CC mode, but it does not appear to be in a real CC mode. Maybe some kind of internal short?
It is intriguing, but other than opening her up and visually exploring I can't think of any way forward. I'd personally take a look inside, look for some obvious oddity like a connector that's come loose or a crack in a PCB track, perhaps one of the output terminals has become disconnected inside the cabinet and your seeing some sort of tiny float voltage to ground.

Be sure to disconnect from the power before opening the unit (I'm sure you're not a fool, but anyone of us can make mistakes). Then only apply power if you are sure it is safe, not tools fallen inside and no kids or pets that can get to the unit.

I really don't know how these modern PSUs are designed (I built a non-digital bench supply in my teens from a magazine using 2N3055s and so on), they likely have all kinds of digital devices in them, the weird voltage you're seeing though does suggest some kind of floating output connector...perhaps you can find two points inside the device where the voltage does vary as you adjust the controls, might give you some ideas...
 

MrChips

Joined Oct 2, 2009
30,946
First things first.

1682022149775.png

The B+K Precision 1550 is rated for 3A @ 36Vmax.

Can you set it for 1A and 15V or something similar?
The output should read 0A and 15V.

VERY IMPORTANT NOTE
You cannot get a current output unless you supply an external load.
You cannot expect current and voltage output to match your setting unless you have an external load R.
The current output is dictated by Ohm's Law, not by the PSU setting.

I = V / R

The power supply will always be in constant current mode or constant voltage mode, never both at the same time.
The current and voltage settings are limits. The PSU will always hit one of the two limits, whichever comes first.

To adjust the voltage limit, remove any external load and set the voltage limit.
To adjust the current limit, short the output terminals and set the current limit.

Now you should have a properly working PSU with set current and voltage limits.
 

MrChips

Joined Oct 2, 2009
30,946
Have you read the User Manual ?
B+K Precision 1550 PSU.jpg

The three photos posted in post #1 indicate that the output terminals are internally shorted as for when setting the current limit.
Press the V button for Voltage setting.
 

KeithWalker

Joined Jul 10, 2017
3,141
I don't know what the circuit of this supply is like but I had a similar problem with a HP supply years ago. There was a diode connected across the output to protect it from applied reverse voltage. It had gone short circuit and put the supply into overload shutdown. Just a loing shot!.
 

ApacheKid

Joined Jan 12, 2015
1,658
I want to say too, that I'm regarding B&K Precision as an inferior product, if this unit is dead then I'd suggest you step back and consider carefully how to replace it. I'm "just" a hobbyist (though I did study and earn various qualifications in electronics in 1979/1980) but I always try to get stuff a little better than I think I'll need, punch above my weight so to speak.

I've had excellent experiences with Siglent and for what I do I am always comfortable that my equipment is always good enough for almost anything I'm likely to do.

The Siglent SPD3303X-E (or even SPD3303X which is higher precision but 150 bucks more) is excellent for my modest needs, certainly superior to B&K (which I did consider a few years ago when equipping my workshop).

Of course any of my Siglents could fail but overall they do seem very well built, excellent quality for the price and that goes for the PSU, sig gens, scopes and digital multimeters.
 
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