(Need help) repairing a faulty Velleman PS3005D (Korad KA3005P) power supply

[henken]

Hello,

I have a Velleman branded equivalent of the Korad KA3005P power supply, which isn't performing to spec (30V, 5A). Also, I am a beginner when it comes to servicing and troubleshooting electronics. This power supply has only seen very limited use, and usually at the low end of both voltage and current.

With no load attached, the maximum voltage I can get on the output is ~18.3V, not nearly the rated 30V. At settings above this, there is no change on the output. As for current capability, it reaches the rated 5A, at least when tested at lower voltages. I don't believe the unit had this problem back when it was purchased, since I have a weak memory of testing it out at the specified limits a couple years ago.

Seemingly broken:
- Voltage regulation up to 30V.

The following things appear to work fine:
- Voltage measurement and display (tested with external multimeter)
- Current measurement and display (tested with external multimeter)
- Voltage limit (CV), regulation seems spot on until ~18.3V.

Possibly not working fine:
- Current limit (CC). For example, when set to a maximum output current of 20mA, it often delivers only 15mA, even though there is voltage headroom (calibration issue?). I don't know if this is normal.

Tools available:
- Multimeter
- Digital oscilloscope
- Small home made constant current load
- ..

Schematics (provided by forum member pomonabill221 in this thread: https://www.eevblog.com/forum/repair/korad-ka3005p-power-supply-calibration/30/:
https://www.eevblog.com/forum/repair/korad-ka3005p-power-supply-calibration/?action=dlattach;attach=77401
https://www.eevblog.com/forum/repair/korad-ka3005p-power-supply-calibration/?action=dlattach;attach=77402

I have done a cursory inspection of the PCBs of the power supply, and haven't found anything obviously broken or damaged.

Does anybody have an idea where to start troubleshooting this issue? What faults are likely to cause a limited voltage output?

(Note: a small modification has been made to the unit. I have replaced the original noisy fan with a more quiet, similarly rated fan. I don't think this has anything to do with my problem.)

Thanks in advance.

(Edit: updated subject)

[Kevin.D]

Hi . Confirm the main transformer taps and tap switching relay's are working.
That's  schematic he did and looks like it's got mistakes, why don't you redraw it for other's to use but use some logical flow. Inputs to left, Outputs to right, Positive rail top, Gnd rails down where possible but don't break it up into  unconnected  bit's of schematic with  node labels like this one.
Regards

[henken]

Unfortunately I do not know how to properly verify the taps and the relays. As I mentioned, I am a beginner when it comes to this kind of troubleshooting.

The secondary side of the transformer has the following wires coming out:

Four thick ones going to the rear power board: black, white, yellow and red.
Three thinner ones going to the logic board in the front: red, red, black.
Two going to the rear comms board (USB, serial): black black.

I can see the relays back on the power board, but I also do not know the procedure of confirming that they work.

Thanks

[Andy Watson]

Unfortunately I do not know how to properly verify the taps and the relays.

Can you hear a relay go CLUNK as you turn the voltage up? You should hear it operating as you change the output voltage past the half-way point - i.e. around 15V.

[henken]

Unfortunately I do not know how to properly verify the taps and the relays.

Can you hear a relay go CLUNK as you turn the voltage up? You should hear it operating as you change the output voltage past the half-way point - i.e. around 15V.

There is no audible sound when I increase the output voltage. So I suppose the next step is to figure out whether the relays are broken, or if the problem is in the circuitry driving the relays..

[Kevin.D]

Ok sorry, didn't know what level of beginner you where. Since you own a scope I assumed you must have some level of competency  above complete beginner right ? . Ok to test the taps/relays turn your P.supply Voltage to max and measure the Voltage from the collector to emitter of one of the main regulation transistors, if this is  low ~.2V-.7V and your main output V is still only 18V then the relays/taps are prob not switching.
If the transistor is not fully on the problem is likely elsewhere.
I am off to bed now so good luck.

[henken]

Ok sorry, didn't know what level of beginner you where. Since you own a scope I assumed you must have some level of competency  above complete beginner right ? . Ok to test the taps/relays turn your P.supply Voltage to max and measure the Voltage from the collector to emitter of one of the main regulation transistors, if this is  low ~.2V-.7V and your main output V is still only 18V then the relays/taps are prob not switching.
If the transistor is not fully on the problem is likely elsewhere.
I am off to bed now so good luck.

No problem. I appreciate any help I can get. I am just a low level hobbyist who dabble with low voltage DC stuff, microcontrollers, etc.

The collector to emitter voltage of the main regulation transistors, Q1 and Q2 in the hand-drawn schematic, are both 0.58V. No relay sounds heard. The supply was set to output 30V.

Edited to add:
So, this is my newbie understanding, correct me where I am going wrong:

The transistors are in saturation, so the supply is trying to give as much as it can to the output. The problem is that the wrong taps of the transformer are connected (by the relays) to the collectors of the transistors, so the output voltage is limited by the selection of the lower voltage taps.

Now to figure out why the relays aren't switching.

[henken]

Well..

I disassembled the unit to get at the power board where the relays sit, in order to inspect things closer. The relays turned out to work fine when manually applying an external 12V supply. Not finding any visual fault, I put everything back together. Lo and behold, it worked again. It now clickety-clacks when increasing the output voltage. Every connector had looked well seated prior to disassembly,  but there must have been some loose connection between the logic board driving the relays and the relays themselves.. Perhaps from the time when I replaced the fan. I rarely use the higher part of the voltage range, so I never noticed any missing sound from the relays.

So the problem turned out to be embarrassingly trivial this time.. Go figure.

In any case, thanks for the help! I learned a few simple things in the process anyway (for example how NOT to probe a live, mains connected supply).

Edit:

It turns out all is not well after all.. The relays do click when scrolling through the voltage range, but here are some further observations. I can reach the full 30V now with no load, which wasn't possible before.

When changing the output voltage in integer intervals, the relays change at 8V, 15V and 22V.

Problems:
 - If I set it to 21V, and attach NO load, it drops down to 18.6V by itself after a couple seconds, showing a draw of 2 mA. At 22V and above it stays stable with no load.
 - If I set it to 22-30V with a current limit of 100mA, and attach a load, it ramps down from 20V to 18.5V, only giving ~27mA. If I then remove the load, it stays around 18.5V showing a draw of 2mA.

So there is something going on still.. Hmm. I'll have to check the lower end of the voltage range too now.

[henken]

Testing with a better load (constant current, 0.4A), it seems to act like this:

- Below 17V it seems stable, loaded or not.
- At ~18-21V it drops down to ~18V when loaded or not.
- At 22-30V it seems stable, loaded or not.

If I increase the output voltage after it's "stuck" at ~18V, it won't click into a higher tap even if I configure a higher output voltage. But if I disable the output, and re-enable it at a higher level, it stays there just fine.

Might the relays get configured incorrectly for this particular misbehaving range?

[Kevin.D]

Still sounds like the tap responsible for the 15-22V range is not connecting, or maybe contacting briefly but not holding due to a low V+unreg supply to it or an intermittent/poor connection from inverter/driver ic. An explanation of why the output voltage may behave like it is here is that the still lower connected tap can eventually charge the smoothing cap to almost it's full AC peak value when there is no load current(since no winding loss and almost no Vf drop on the diodes when no current through them etc) this may just be high enough to activate the highest tap (~22V) thus the intermediate tap is effectivley skipped, but lower tap cannot reach that 22V level with even a few mA load current present. You need to identify the correct tap/relay that SHOULD be active for that range and confirm it is closing and holding .

[henken]

I measured the C-E voltages at both gates of the darlington array (ULN2003) which are concerned with the relays:

Both relay coil resistances are measured to be the nominal 400Ohm.

Results:

0-8V output configuration: Relay 1: 21V, Relay 2: 21V
8-15V output configuration: Relay 1: 19V, Relay 2: 12V
15-22V output configuration: Relay 1: 21V, Relay 2: 21V
22-30V output configuration: Relay 1: 19V, Relay 2: 12V

That should give relay voltages across the relay coils: 21V(supply) minus the C-E voltage.

When I measure across the relays, that is of course what I get:

0-8V output configuration: Relay 1: 0V, Relay 2: 0V
8-15V output configuration: Relay 1: 2V, Relay 2: 9V
15-22V output configuration: Relay 1: 0V, Relay 2: 0V
22-30V output configuration: Relay 1: 2V, Relay 2: 9V

Relay 1 only gets 2V across the coil, which isn't enough to toggle it. It does go between 0 to 2 volts though, so there must be a signal from the microcontroller.

Is this likely a fault in the darlington array, or the pin driving the gates?

What confuses me is that it seems to want to switch both relays simultaneously (even though one is not driven enough). That would only yield 2 alternating relay configurations.

Relays in binary:
0-8V: 0 0
8-15V: 1 1
15-22V: 0 0
22-30V: 1 1

I would have expected the following pattern, yielding 4 voltage ranges:
0-8V: 0 0
8-15V: 0 1
15-22V: 1 0
22-30V: 1 1

Hmm.

[Kevin.D]

Keep at it you'l get there in the end   . Your right about confusing configrations I also would have expected the relay taps to switch in the four binary configurations you later gave since  apparently you are currently showing that the 8-15V and the 22-30V output's have the same output's but this does not fit with your current 'stable ranges'.
 Anyway it looks like the darlinton driver array is toasted (you would get 1 Vce(sat)+Vbe drop = ~ .8V Vce across a  working one in saturation (where u measuring at ?), the relays recieve the full V+ unreg, and looks like there's some unintended crosstalk between drivers ). But before replacing it I would check this ic's GND to both the 12V GND and uController GND for good  continuity. Also check input's from uController to these two drivers are good and matched to expected configuration. You could then confirm proper operation of relays and all power supply ranges by temporarly shorting the output of the darlinton driver to the IC GND with a probe .  You said earlier that your fitted a new fan driven by this ic, double check rating to be sure the new fan did not overload the driver array .

[henken]

I measured the voltages between the driver IC's pins going to the relays and the ground pin of the driver.

V+ unreg is ~21V, and the relays are rated for 12V, how does this get along with a small voltage drop across the driver to ground?

The data sheet suggests a maximum voltage of 15.6V for the relays.

[henken]

I measured the input to the driver from the microcontroller vs ground for both relays. Only one of them toggled from 0 to 3V. One seems dead.. Broken micro?

Edit:

Both 8-15V and the 22-30V do get the same voltage from the relays. There's only one relay switching between two different taps. The other relay isn't moving. Only the range inbetween is unstable. Since it gets the same voltage from the taps as the 0-8V currently does. It does line up with the stable ranges, unless I misstated them somehow.

[Kevin.D]

Nowt on that schematic but there maybe  a series resistor ~180 or so knocking about then .

I measured the input to the driver from the microcontroller vs ground for both relays. Only one of them toggled from 0 to 3V. One seems dead.. Broken micro?

.

Maybe or the darlington  could be s/c and pulling it down .

[henken]

Indeed. Only a couple of ohms between the inputs of the array to ground. Except for the one going to the working relay and one more. 5 out of 7 are shorted.

Time to desolder the array to see if the microcontroller output still works?

[Kevin.D]

Just before you do, you can confirm all p.s ranges now work ok when the static relay is manually operated by shorting driver collector to I.C  gnd (use 100 ohm series just to be safe ).

[henken]

What's up with 21V driving the 12V relays though? I don't see any extra series resistors or something else to drop the voltage.

[henken]

Well, crap.

The driver input pins (well, pads) are still shorted even when the driver IC is removed. I guess the short is in the microcontroller

[henken]

New approach.

I have found a convenient point to read out a reference voltage in the circuit (TP1 in the hand drawn schematic), which changes depending on the set voltage. I might draw up a circuit which controls the relays from there, ignoring the seemingly broken microcontroller output.

[Kevin.D]

Hi 
You said other uC output's to array where also s/c so where these all due to uController or U12 s/c's . If so was your your fan drive not working ok before ?
Yes you could make your own array driver signal to replace uC output but you need to check some things that would have been easily checked before you removed ic .You can still check them though  1/ Make sure
that the supposed working relay drive from uC was actually working properly and it was a damaged U12(and not uC inability to source enough current) that was limiting full driver turn on (12Vce is not on).(confirm u12 driver or uC output prob with a simple Darlington test circuit).
2/ You need to confirm relay sequence (because in an earlier post you stated that the highest range was stable and working then your later V test result post showed output for this range was actually 0-1 which either means it wasn't stable up to 30V before or 0-1 actually is the correct highest range sequence and not 1-1 which is what I think it should be ) this can be confirmed easiest by measuring Vac of the other 3 taps relative to the Black one.
This is important now since if the correct sequence is 00-01-10-11  then it's easier to drive the first relay with a single comparator.

[Kevin.D]

Here's an example replacement driver I just did,  here you would normally have say ~1.2 V
Hysteresis to stop relay chatter, but here I take some reinforcing feedback from the good uC relay drive, I used it to make this relay switch at the same time as the good one (but only around the ~15V range) .The reason they need to switch synchronized is that if the  point of switching was diff then between these points the incorrect range would  actually be selected (like it is now), Once relay 1 is on then it will remain on up to your Max Vout. I assumed a V(off) of 15V for the good relay in order to calculate the summing node resistor values used for this relay, if it's > .5V different then you may need to tweak them.
How much is this model new ? about £45 
PS. If your going to use this dont use lm193 (expensive wide operating temp version of lm293) use much cheaper lm293 or lm393 .

[henken]

Hi 
You said other uC output's to array where also s/c so where these all due to uController or U12 s/c's . If so was your your fan drive not working ok before ?

Well, that's the confusing part.

The fan was working when I noticed the problem at certain voltage ranges. However, the fan has been disconnected during the troubleshooting. Now with the driver IC removed, I still measure a 2 Ohm short circuit from each driver input pad (except to one relay and one more), to the ground pad of the driver. So I don't see how the fan could have been working.

Yes you could make your own array driver signal to replace uC output but you need to check some things that would have been easily checked before you removed ic .You can still check them though  1/ Make sure
that the supposed working relay drive from uC was actually working properly and it was a damaged U12(and not uC inability to source enough current) that was limiting full driver turn on (12Vce is not on).(confirm u12 driver or uC output prob with a simple Darlington test circuit).

I will probably order a few new driver IC's together with another batch of components. They are cheap. That would allow me to test with a known good one.

2/ You need to confirm relay sequence (because in an earlier post you stated that the highest range was stable and working then your later V test result post showed output for this range was actually 0-1 which either means it wasn't stable up to 30V before or 0-1 actually is the correct highest range sequence and not 1-1 which is what I think it should be ) this can be confirmed easiest by measuring Vac of the other 3 taps relative to the Black one.
This is important now since if the correct sequence is 00-01-10-11  then it's easier to drive the first relay with a single comparator.

I will look into that.

Here's an example replacement driver I just did,  here you would normally have say ~1.2 V
Hysteresis to stop relay chatter, but here I take some reinforcing feedback from the good uC relay drive, I used it to make this relay switch at the same time as the good one (but only around the ~15V range) .The reason they need to switch synchronized is that if the  point of switching was diff then between these points the incorrect range would  actually be selected (like it is now), Once relay 1 is on then it will remain on up to your Max Vout. I assumed a V(off) of 15V for the good relay in order to calculate the summing node resistor values used for this relay, if it's > .5V different then you may need to tweak them.
How much is this model new ? about £45 

Heh. It's not a high end unit.., but I probably paid closer to £100 at the time of purchase. Maybe you consider it too cheap to warrant a repair, but if I can make it work again with some cheap parts, and learn something in the process, I will. It was serving my needs just well.

Actually I have been looking into getting a new unit as well, but I hate throwing away something that hasn't even seen much use. I'd try a repair anyway, as a learning experience.


Regarding your example driver. What do you mean by "-Vout", the negative set output voltage? So far, measuring the TP1 node I have found a voltage that goes up when I adjust the output voltage, but isn't the same. It starts out quite close to the set output, but levels off at 21V. In the end it takes 0.1V steps for each 1V change to the output, while in the beginning it takes 1V steps. It's not increasing linearly.

Thanks for the example circuit though!

[Kevin.D]

Regarding your example driver. What do you mean by "-Vout", the negative set output voltage? So far, measuring the TP1 node I have found a voltage that goes up when I adjust the output voltage, but isn't the same. It starts out quite close to the set output, but levels off at 21V. In the end it takes 0.1V steps for each 1V change to the output, while in the beginning it takes 1V steps. It's not increasing linearly.

Yes -Vout is the neg output rail. Note that -Vout  is not aux gnd as mistakenly shown on page one of that schematic. Your aux supplies gnd should be tied to +Vout (continuity from +vout to 12V gnd will show you that)
The problem with using TP 1 to set relays is that because this is Vset and not the actual Vout it MAY NOT reflect Vout when in CC mode ( Vout is now controlled by the CC error op-amp and not Vset error amp )thus an inappropriate Input range will be selected in CC mode. ( TP1 would be useful in output disabled mode where since Vout=0 you then set relays according to Vset,this is what the uC does but using my example scheme I don't think it's a necessity as the relays would get set correctly as soon as output is re-enabled).

[henken]

Regarding your example driver. What do you mean by "-Vout", the negative set output voltage? So far, measuring the TP1 node I have found a voltage that goes up when I adjust the output voltage, but isn't the same. It starts out quite close to the set output, but levels off at 21V. In the end it takes 0.1V steps for each 1V change to the output, while in the beginning it takes 1V steps. It's not increasing linearly.

Yes -Vout is the neg output rail. Note that -Vout  is not aux gnd as mistakenly shown on page one of that schematic. Your aux supplies gnd should be tied to +Vout (continuity from +vout to 12V gnd will show you that)
The problem with using TP 1 to set relays is that because this is Vset and not the actual Vout it MAY NOT reflect Vout when in CC mode ( Vout is now controlled by the CC error op-amp and not Vset error amp )thus an inappropriate Input range will be selected in CC mode. ( TP1 would be useful in output disabled mode where since Vout=0 you then set relays according to Vset,this is what the uC does but using my example scheme I don't think it's a necessity as the relays would get set correctly as soon as output is re-enabled).

The supply actually only sets the relays when output is enabled.

Isn't there a chicken and egg problem when it comes to using Vout for setting the relays? The relays are needed to get the higher voltages at the output, so how can you use it to set the relays in the first place?

Edit:

The Vset actually does change when the output is limited in CC operation, so I think it is a candidate for driving the relays in the same way they were originally. I'm not saying it's a better way than what you proposed, I just still have the question posed above when it comes to using the Vout measurement for driving the relays.