Burnt resistor in BT-305 lab suppy

[Jeroen3]

For the second time a BT-305 lab supply burnt a resistor inside. R38 to be precise, it's a 5W 10 Ohm resistor.

Schematic, second page 13A.

I can't find the purpose of the resistor. It looks to be there to prevent common mode on the shunt sensing. But why do you need a resistor there?
Anyways, during some testing I found that it sometimes failed to enter current limiting mode when the output is shorted, and then 10V is present over R38.
No other parts become very hot, only N1 ref is ~45C, which is warm, but not hot. ^{(thanks flir one)}

What causes this?

I did notice CA,CB and C15 to be typical 0805 ceramics, which is wrong. C15 is missing, exploded or something. CA was cracked, yet, that might have other causes and should be very easy to do.

[schmitt trigger]

All I can think of: is that K2 fails to switch states when the supply output is turned on, and then all the output current flows thru R38.

Mind you, the K2's contact may actually move....but it may fail to make a good electrical contact.

[Toasty]

Can you please identify what K1 is?  I'm looking at an online picture of the supply and only see:

- Power - which I assume is S0-A/B
- Output - which is K2 ?

- What & where are K401, K402, K403

- What & where is K1a/b - "Current" control?

K1a shunts R38 when its position is as shown in the schematic.  When is it in that condition?

Is the resistor burned open or just "cooked"?

T

[Toasty]

I did notice CA,CB and C15 to be typical 0805 ceramics, which is wrong. C15 is missing, exploded or something. CA was cracked, yet, that might have other causes and should be very easy to do.

Picture(s) please?

Thanks!

T

[Jeroen3]

Picture(s) please?

https://imgur.com/a/M6lje

The C's have been removed. What purpose have broken C's 

The bridge rectifier is added at the output terminals to prevent current from going back in. More robust, at the cost of the voltage reading.

I have decided to remove K1 and K2 to make it always-on. I'll see how it goes.

[Armadillo]

Quite a nice power supply. You should repair it than doing funny stuff.

Check if R32 or R33 is opened?

turn it off;

Continuity Check Anode of V3 to N402 [7815] Centre Pin 2.

Hope it helps.

[Jeroen3]

Quite a nice power supply

They are.

Shunt is good, I've checked. K1 and K2 switch together on S1 Output Enable switch.

I could not reproduce to fault the next day. Yet, later that day it went bad again. I noticed because of the smoke again.
I've removed the relays and bypassed them. It appears the the fault current has now damaged some regulation part since it's not stable anymore.

So I call beyond economic repair, it's not mine anyways.
We use loads of these cheap ones at work, to power large inductive loads, as a trial we bought a few with output on/off. Better not do that again.

These we have for ages. Indestructible.

[Jeroen3]

Another supply with the same failure mode just happened on my desk. It showed wrong current, and moments later a whole lot of smoke. Burnt resistor.
Don't buy Basetech BT-305.

[DevMan095]

Hi,

my Display just stopped showing the voltage, i disassembled the display and saw there is a resistor missing (idk how, but it got scratched away).

could anyone who owns the same device (Basetech Bt-305) upload a picture of the display pcb unit? the part i mean is this:

https://photos.app.goo.gl/kQMSq2LabecT2412A


the broken resistor is in the upper middle area, can't even read the number of the resistor as it is also scratched away..

Thanks in advance for any help

[Jeroen3]

Looks like the voltage regulator on the right has a hole.

Also, I should mention that all the basetech supplies we bought have self-destructed and have been scrapped.

[coromonadalix]

Definitly a hole, this is a 5 volts regulator ??   you can check  icl 7107 datasheets voltmeter ic, pretty common, you will be able to find the resistor value.   too test the display  put regulated 5 volt on c13 side going to the busted regulator pins,  youll see if the chips and the leds are still good.
Theses regulators normally go up to 40volts    so something bad really happened.


Do you have an complete schematic link ??


The first op thread about r38

Normally  this psu has a floating regulator section over the main supply,   the common point pass thru R38 ?? but i find it slighlty strange.

I begin to doubt they arent well designed in some way if you have them all failing at some point.