Question about using two SMPS.

[HellasTechn]

Hello !!!

I am going to post a silly question but since i am not 100% sure i would like your advice.

Can i use two identical smps 28V each to get symmetrical output (+28V) - (common) - (-28V) ?

what should i consider ?

I will explain further what i am trying to do and show pictures but i dont have them today...

THANKS

 

[Minder]

I don't see any problem, the output is typically isolated from each other, more info on the application would help.
M.

 

[HellasTechn]

i took the pictures now for you to see.
On my bench there is a tape drive from an old computer. this tape drive is powered from a 5v smps and two 28v smps. one 28v smps provides the +28v and the other -28V.

both 28V smps are damaged and i am not sure how to fix them. also the ic's inside are obsolet.

Here is the thing... there are two wires feeding 110V to each one, then as you may see in the picture on each smps the lead used as "negative" is connected to the chasis. On psu 1 terminal 9 is connected to chasis and on psu 2 terminal 7 is connected to chasis.
Note that terminals 7-8 are bridged and 9-10 also on both SMPS units.

im sure there are more going on than what i can see...

So i was thinking to save me the trouble of repairing and just replace the units with two new 28V smps.

 

[HellasTechn]

Could it be because they are using the chasis as common pin for the +28V and -28V ?

 

[Minder]

Something does not look right, the output should Not be connected over to the AC input, the S terminals are for remote sense, if used, If unused, they should each be connected to their respective + & - output terminals.
If using two supplies, simply connect the -ve output on one to the +ve output of the other for
-ve 0 +ve supply.
M.

 

[HellasTechn]

Something does not look right, the output should Not be connected over to the AC input,

They are not connected to AC input... they are connected to chasis... Terminal 4 has the GND symbol.

he S terminals are for remote sense, if used, If unused, they should each be connected to their respective + & - output terminals.

hmm im not sure why they are like this.

 

[Minder]

hmm im not sure why they are like this.

Certainly not the typical or Manuf. suggested way of using them.
If left unconnected, the output usually does not work.
M.

 

[HellasTechn]

and this FBT000158 IC. its on ebay for 100usd...
On aliexpress for 6 but sold out...

 

[Minder]

and this FBT000158 IC. its on ebay for 100usd...
...

One on ebay for $100 , but you could buy a new supply for less.
Why do you need to replace?
M.

 

[HellasTechn]

i guess i could just buy two 28V units (or maybe two 24V units and adjust them to 28V) for like 30-40.

 

[Minder]

Did you try hooking them up in the way I said -S to -V etc.?
It might pay to spend a little time on them as they are superior to any modern SMPS out of China.
Electrolytics dry out etc.
M.

 

[HellasTechn]

Did you try hooking them up in the way I said -S to -V etc.?

No. i think there is no point since i know it worked before connected like that.

Electrolytics dry out etc.

I measured the capacitors and found them good as new after 27 Years.

 

[HellasTechn]

It might pay to spend a little time on them as they are superior to any modern SMPS out of China.

Check this out... I have measured all diodes, transistors, capacitors, transformers and found them all good.
The only components i cant confirm are the FBT00158, the TL431IJG (though i did the test with a 5V psu and the 1kresistor and reads good (about 2.3V)) and the TLP635 (the led side reads good) and that component top left side next to the transformet that has an arrow on it...
What else could be damaged ? Also i can not find a schematic.

 

[Minder]

No. i think there is no point since i know it worked before connected like that.
.

I think I would at least start from the way the manufacturer suggests at least, It would be the correct start point for trouble shooting.
M.

 

[hevans1944]

as you may see in the picture on each smps the lead used as "negative" is connected to the chasis.

No it isn't. In one image, +S and +V (bridged) are connected to the chassis, meaning this power supply produces negative voltage on -S and -V (bridged) with respect to the chassis. In another image, -S and -V (bridged) are connected to the chassis, meaning that power supply produces positive voltage on +S and +V (bridged) with respect to the chassis.

The bridge conductor installed between +S and +V and between -S and -V is used in lieu of a separate pair of sense wires (usually small gauge) installed between the power supply output sense terminals, +S and -S, and the load terminals to provide voltage regulation at the load. With those two jumpers in place, voltage regulation occurs only at the output supply terminals, +V and -V. The load conductors connecting to +V and -V may be considerably larger than the pair of sense wires to reduce the voltage drop in the load conductors, but without a separate pair of sense wires a voltage drop will occur between the power supply output on +V and -V and the load connected to those two terminals. This may or may not be important, depending on circuit design, load current, and other factors.

Many power supplies have internal resistors of about 100 ohms permanently wired between the output terminals and the corresponding sense terminals. This allows the power supply to operate more or less normally if someone happens to remove the bridge jumpers, but still allows normal remote sensing if sense wires are used.

The outputs of SMPS units are isolated from mains power, so there is no reason not to use one as a positive output supply and the other as a negative output supply with respect to chassis common, as shown in your excellently photographed images..

I would attempt to repair the Lamda power supplies before replacing them with cheap Asian imports of unknown design quality. Back in the day, Lambda power supplies were virtually bullet-proof, but it seems like today everything is disposable rather than repairable. <sigh>

 

[HellasTechn]

No it isn't. In one image, +S and +V (bridged) are connected to the chassis, meaning this power supply produces negative voltage on -S and -V (bridged) with respect to the chassis. In another image, -S and -V (bridged) are connected to the chassis, meaning that power supply produces positive voltage on +S and +V (bridged) with respect to the chassis

I get the point.

I would attempt to repair the Lamda power supplies before replacing them with cheap Asian imports of unknown design quality. Back in the day, Lambda power supplies were virtually bullet-proof, but it seems like today everything is disposable rather than repairable.

I want to repair these power supplies indeed. In here i am the most inexperienced and ignorant when it comes to electronics but i can tell when i see quality electronics such as these PSU's.
My problem is that i can not pay 100USD for the FBT00158 IC... And to make matters worse i have been unable to locate other damaged parts.