Author Topic: New power supply PS305D not regulating and running over voltage and current  (Read 25254 times)

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Offline TrontoTopic starter

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Hi EEVBlog;

I could use some help if you can spare some time.   Not really sure what the process on this forum is, so apologies if I get this wrong.

I have a brand new Longwei branded PS-305D-M 30V 5A bench power supply. 

So when the PS started at first, it was ok for a few seconds then the smell of magic smoke and then it showed 49.3V and 3.5 or 6.5A depending on the Hi/Low switch.

Now on startup, it is ok for a second then ramps right up to the 49V 6.5A.

There is a burnt out resistor at R144 who's value I do not know (68?, thanks gman4925).   This resistor is one of two that feed non inverted input 1 to an LM324N. 

There is a resistor adjacent to the LM324N in a spot marked D107.  Not sure if label or resistor is wrong.  The resistor is 150K.  Suppersready has a diode at D107 on their board, but gman4925's board has a resistor like mine.

I've been working through the system slowly trying to understand each part.  I'm also using photos to make an image of the traces with components. 

Power to the board comes in at the bottom left and gets turned into +27 and --27V at the rectifier.  Power then runs through an L7815CV and L7915CV which should regulate to +15V and -15V. From there power goes to power the LM324N quad op amp and HA17741 op amp.  There is a short in between +15v and -15v somewhere.

The power pins 4 and 11 show a short (0.5 ohms) on the LM324N.  When I test a different LM324N out of circuit there is no connection between the power pins.

I have replaced R144, L7915CV, D107 and the LM324N.  Before powering on, the 15v system had a healthy resistance between + and -.  Powering on produced the same symptoms as brand new and for 30 seconds everything worked perfectly until R144 burned out again and a short appeared in the 15 volt system.

Thanks for reading and I appreciate any comments.

EDIT:  Higher Res images here  http://imgur.com/a/YUEIG

EDIT2:  The big power transistors 2N3055s never get warm.  R106 150? is starting to burn up.     I've added to the Imgur album an image that I have been working on for my use understanding the circuit. 

EDIT3:  Schematics from Longwei are attached. 

EDIT4: Found a PNP A1015 in a spot for an NPN transistor at Q104.  Anyone know what is supposed to go in Q104?   Schematics don't have values on them.   
« Last Edit: June 18, 2015, 12:07:58 am by Tronto »
 

Offline gman4925

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I have the same power supply.
R144 is 68?

 

Offline TrontoTopic starter

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Thanks for posting a pic of your control board.

I see you also have a resistor in D107 (just under the LM324). 

I'm still having a heck of a time figuring out the problem.
 

Online IanB

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If there was smoke when you turned it on it suggests a short somewhere. Presumably the unit passed a smoke test in the factory before it was shipped, so it would be an idea to check for shipping damage. Did anything come loose or get bumped out of position during shipping? Did any component leads touch something they shouldn't touch?

(Note: too late now, but it is a good idea to open up and inspect items received before turning them on just in case there has been anything bumped or disturbed during shipping, especially if the the item was not carefully packaged.)
« Last Edit: June 13, 2015, 06:38:47 pm by IanB »
 

Offline TrontoTopic starter

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Was packed well, no damage to box or power supply. 

I did not find anything loose or shorted inside.  Went over the pcb carefully looking for bridges or leeds touching and there were none.

There are no missing or extra screws. 

Thanks for the suggestion.

If there was smoke when you turned it on it suggests a short somewhere. Presumably the unit passed a smoke test in the factory before it was shipped, so it would be an idea to check for shipping damage. Did anything come loose or get bumped out of position during shipping? Did any component leads touch something they shouldn't touch?

(Note: too late now, but it is a good idea to disassemble and inspect items received before turning them on just in case there has been anything bumped or disturbed during shipping, especially if the the item was not carefully packaged.)
 

Offline mikerj

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The soldering on the TO220 voltage regulator closest to the edge of the PCB looks very poor,  check this for dry/broken joints.
 

Offline TrontoTopic starter

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Yeah, it looks poor in the picture, but it's actually ok. 



The soldering on the TO220 voltage regulator closest to the edge of the PCB looks very poor,  check this for dry/broken joints.
 

Offline jaycee

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I take it you checked that the pass transistors are not shorted ?
 

Offline TrontoTopic starter

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I take it you checked that the pass transistors are not shorted ?

Well, I'm narrowing it down.

Power to the board comes in at the bottom left and gets turned into +27 and --27V at the rectifier.  Following the positive side through the L7815CV gives +15V out, but following the negative through the L7915CV and I get +15, rather than -15.   

Either the L7915CV is dead or I have a short somewhere.

UPDATE:  the power pins 4 and 11 show a short (0.5 ohms) on the quad op amp LM324N.  When I test a different LM324N out of circuit there is no connection between the power pins.

« Last Edit: June 14, 2015, 06:27:34 pm by Tronto »
 

Offline suppersready

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Hi Tronto,

I can confirm that in my PS305D D107 is a diode and is a 1N4148.



 

Offline TrontoTopic starter

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Hi Tronto,

I can confirm that in my PS305D D107 is a diode and is a 1N4148.

Thanks suppersrady.   
 

Offline TrontoTopic starter

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This morning I replaced a 4.7 uF cap that looked off.  Actually replaced all the 4.7 uF caps. 

Also replaced both L7815 and L7915 and put a proper diode in D107. 

The system ran fine but the output voltage ranged from 0.1 to 0.3 volts rather than 0-30 V.  Also, R144 was getting really hot, so still haven't found the problem.

Reference voltage off of pin 1 of the LM741 is good (compared to the schematic) as is the voltage at C108.

 

Offline TrontoTopic starter

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I just confirmed that U103 which is a TL431A is good by pulling it and using this circuit on a breadboard.

http://www.electro-tech-online.com/attachments/tl431_voltageregulator-01-gif.74473/
 

Offline TrontoTopic starter

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I have the same power supply.
R144 is 68?

Hey, could you please confirm the value of R144.  The manufacturer told me 680 ohms so I just want to double check. 

Thanks
 

Offline gman4925

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Quote from: Tronto on Yesterday at 20:44:49

>Quote from: gman4925 on 13 June 2015, 09:58:09
I have the same power supply.
R144 is 68?

Hey, could you please confirm the value of R144.  The manufacturer told me 680 ohms so I just want to double check. 

Thanks


I'm 90% sure it was 68 ohm, though I measured it in circuit.
I read it as BLUE GREY BLACK GOLD GOLD off the markings as well, which matched.
If you look at the pic the fourth band is definitely not black.
If I remember I will take another look tonight.

« Last Edit: June 16, 2015, 03:22:51 pm by gman4925 »
 

Offline gman4925

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Checked again, it's 68 ohm in my one.
 

Offline TrontoTopic starter

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Checked again, it's 68 ohm in my one.

Is your PS working?

I'm now a bit confused.  Guess I'll ask Lonwei to confirm the value as they told me 680 ohms in email.
 

Offline gman4925

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Mine works fine, though I've not really tested it much past 1A.
 

Offline TrontoTopic starter

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I found a PNP A1015 in Q104, which according to the schematic should be a NPN transistor.  Hmm
 

Offline gman4925

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Q104 is A1015 in mine too
 

Offline TrontoTopic starter

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Q104 is A1015 in mine too

thanks so much for checking.   The schematic sure looks like it should be a NPN.

Edit:  I think the schematic just has the wrong symbol  Looking at the connections PNP looks right.   
« Last Edit: June 18, 2015, 03:06:27 am by Tronto »
 

Offline broxsoft

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Re: New power supply PS305D not regulating and running over voltage and current
« Reply #21 on: December 24, 2017, 01:23:49 am »
Tronto, It has been awhile since there was a post here. Did you fix the power supply? Moreover, someone says they have a schematic for it? Any chance I can get a copy? I have just started having problems with my PS and it has the Longwei PS-303D-M board. Thanks.
 

Offline jaycee

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Re: New power supply PS305D not regulating and running over voltage and current
« Reply #22 on: December 24, 2017, 12:00:45 pm »
Looking back at this thread, and having seen many similar PSU's around.. I would only ever be tempted to buy one of these for around the £40 mark - simply to get the case and the transformer. The rest I would gut and replace with my own control circuitry, and a proper heatsink on the back!!
 

Offline higopi

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Re: New power supply PS305D not regulating and running over voltage and current
« Reply #23 on: February 24, 2018, 08:56:52 am »
Hi Tronto

I have EXACTLY the same issues with a new power supply that I purchased. Did you manage to resolve it ?

Thanks
 

Offline smooth_jamie

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Hi All. For anyone still looking, I had the same problem and I fixed it very easily after testing/probing a few diodes.

I did the obligatory visual check. No blown caps or obvious signs of burning so started checking components starting at the DC output. Used the diode checker to test all the diodes and found a 1N5399 rectifier diode failed to short circuit.

Replaced with a 1N4007 and this solved my problem. From memory it was D108. It is right next to the two output wires going from the main PCB to the output binding posts
 


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