Power Supply Modification

[TinMan]

Well,on friday i had another great score from work.
As the boss closed the retail side of the business,a lot of good stuff was getting thrown in the bin.

Anyway,he told me to throw the car audio display cabinet in the bin,once all the sterio's were stripped out of it.
This housed various car audio systems,and you could switch from car stereo to car stereo--speakers to speakers,and amp to amp--so as you could listen to all sorts of combinations,and choose the combo you wanted
Some exceeded 1500 watts of output,and so i knew there was a pretty mean power supply in that cabinet.
So in the back of the 4by it went 

I got to pulling it down this afternoon,and was very happy to find that good sized power supply and regulator.

To cut a long story short,i am hoping to be able to make this a variable benchtop power supply with grunt ,while im awaiting for some parts to turn up for my other project.

Pic one shows what i have retrieved so far from the cabinet.
Needles to say,i will not have to buy any 12 volt relays for a while 
There is also (what i guess to be) over 50 meters of insulated wire of various sizes in there,as well as a push button switch board of about 30 push buttons switches.

Pic two shows the front of the PCB and components.
Note the white arrow i have drawn on the board.
This arrow lines up with the arrow on the reverse side when i flip the board horizontally --so as you can follow the circuit.

Pic 3 shows the reverse side of the PCB.

Pic 4 shows what the ICs are.

I am hoping that one of the VRs controls the output voltage,and that i will be able to change it out for a pot,so as i can adjust the output voltage 
It would seem to me that one of the pots may control the LM723 chip,which is a voltage regulator.
This in turn may control the transistors ?--or perhaps it controls the two voltage comparators,which in turn control the transistors?.

Anyway,i need some help from the EE guys here,as im more of a mechanical hands on guy.
So any help would be much appreciated 


Brad

[ion]

Brad

good catch!

FWIW, I modified a fixed 12V power supply that used an LM723 regulator. I put a ten turn pot on the front panel and wired it into the voltage fixing circuit. Min and max outputs may differ from design to design, but at least now it has some variability.

There are lots of images on the web for LM723 regulator circuits:

https://images.search.yahoo.com/yhs/search;_ylt=A0geKVh5uBtbsGIAwiMPxQt.;_ylu=X3oDMTByMjB0aG5zBGNvbG8DYmYxBHBvcwMxBHZ0aWQDBHNlYwNzYw--?p=LM723&fr=yhs-iry-fullyhosted_003&hspart=iry&hsimp=yhs-fullyhosted_003#id=9&iurl=http%3A%2F%2Fwww.youspice.com%2Fwp-content%2Fuploads%2F2015%2F05%2FpowersupplyLM723.jpg&action=click

Some just vary the ref voltage, some sense actual output voltage for adjustment. Some have  a current limit adjustment which you may also wish to include.

Click through the images to see what suits you.

regards

[TinMan]

Brad

good catch!

FWIW, I modified a fixed 12V power supply that used an LM723 regulator. I put a ten turn pot on the front panel and wired it into the voltage fixing circuit. Min and max outputs may differ from design to design, but at least now it has some variability.

There are lots of images on the web for LM723 regulator circuits:

https://images.search.yahoo.com/yhs/search;_ylt=A0geKVh5uBtbsGIAwiMPxQt.;_ylu=X3oDMTByMjB0aG5zBGNvbG8DYmYxBHBvcwMxBHZ0aWQDBHNlYwNzYw--?p=LM723&fr=yhs-iry-fullyhosted_003&hspart=iry&hsimp=yhs-fullyhosted_003#id=9&iurl=http%3A%2F%2Fwww.youspice.com%2Fwp-content%2Fuploads%2F2015%2F05%2FpowersupplyLM723.jpg&action=click

Some just vary the ref voltage, some sense actual output voltage for adjustment. Some have  a current limit adjustment which you may also wish to include.

Click through the images to see what suits you.

regards

Thanks for the info ION.


Brad

[muDped]

It is absolutely amazing what one is able to acquire by
being in the right place at the right time.  Thank goodness
there are people such as yourself TinMan who recognize
the value of items destined for "the bin" and are able to
rescue them with hope for modifying their capabilities
and thereby extending their lifetime of usefulness.  The
joy and satisfaction of "fixing" things to make them better
is what keeps many of us going.

Linear Supplies were good in their day but are quite
wasteful of power.  Today there are Switching Supply
boards which are inexpensively available from China
which offer Buck/Boost capability with up to 40 Amperes
or more of current at variable output voltages.

Do you know what the DC output voltage and current
characteristics of the Rectifier/Filter portion of the circuitry
which feeds the Linear Regulator?

[ion]

Hi muDped

If Brad is anything like me, he finds it hard to throw stuff out. When you are constantly inventing and trying things out, you need a supply of stuff on hand because you never know when you might need it.

In my case scavenging can be a problem. I stopped it when I filled my house with items I either restored or they are waiting to be restored that otherwise would have been discarded.

The problem is once you become a magnet for the stuff, it just keeps coming, like opening a floodgate, so in my case I need to start building other storage areas or figure out how to shut the flow off.

The universe seems to deliver, be careful what you wish for. 

Regarding bench power supplies, often they have such low usage, wasted power is not that big a problem. When it comes to repairing one, I'll take the linear over the switchmode type, which can be a real bear to fix. For high use items, I agree, switchmode is the way to go to save energy.

Each type has advantages and disadvantages, as always, engineering trade offs.

Regards

[Room3327]

Are you sure it's not a switching regulator? 723's can be used to build switching regulators as well as linear, a very versatile chip, designed many circuits with it.

[TinMan]

It is absolutely amazing what one is able to acquire by
being in the right place at the right time.  Thank goodness
there are people such as yourself TinMan who recognize
the value of items destined for "the bin" and are able to
rescue them with hope for modifying their capabilities
and thereby extending their lifetime of usefulness.  The
joy and satisfaction of "fixing" things to make them better
is what keeps many of us going.

Linear Supplies were good in their day but are quite
wasteful of power.  Today there are Switching Supply
boards which are inexpensively available from China
which offer Buck/Boost capability with up to 40 Amperes
or more of current at variable output voltages.

Do you know what the DC output voltage and current
characteristics of the Rectifier/Filter portion of the circuitry
which feeds the Linear Regulator?

Being that it was a power supply for car audio equipment,i would say the output voltage is between 12 and 13 volts.
I have no idea as to what the current value would be,but judging by some of the equipment it powered,and the size of the output cables,i would say it is pretty high--in the ball park of 40 to 50 amps i'd say


Brad

[TinMan]

Are you sure it's not a switching regulator? 723's can be used to build switching regulators as well as linear, a very versatile chip, designed many circuits with it.

Well.im not sure.
That is why i posted pictures of the circuit board,in the hope some one would be able to tell me.

Guess i could just power it up,place a small load on the output-like an incandescent bulb,and start turning each pot one by one,and see what happens to the output ?.

The large caps you see are 10,000uF each,and rated at 25 volt's,so that would be the limit of our output voltage.
The transformer output says 15 volts,but the primary has many taps,so could be increased if needed.

There is 3 wires on the output of the regulator/rectifier.
1 is a common ground
1 is the main 12v+
And 1(the smaller yellow one)is also 12v+,but would have been for the constant/permanent power supply to the stereo units,so as they retain all the settings,such as time and pre programed stations.


Brad

[TinMan]

OK,i placed an incandescent bulb across the output as a load.
By adjusting VR1,i can lower the voltage across the output to 10.8 volt's,and raise it to 15.5 volts.
VR3 and VR4 seems to have no effect on voltage output,so i put them back to the position they were in.

As the output on  the transformer says 15 volts,i believe that we are reaching the maximum output voltage of the power supply. But how to get the voltage down to near 0 on the lower end of the voltage range?

If we look at the pic below,we can see a few resistors near to VR1.
Could one of these be some sort of limit resistor placed across the trimmer pot?
Can the trimmer pots them self have a lower limit of resistance each way,so as the resistance is never 0 across two of the three pins?


Brad

[ion]

Brad asked:

Could one of these be some sort of limit resistor placed across the trimmer pot?

More than likely it is in series with one leg of the pot.

If you look  at the image page I referenced earlier, and observe the schematics, there are two methods, one is to vary the reference voltage, the other is to sense the output and use the output to adjust the voltage as part of a voltage divider. In this scenario, there usually are two resistors on either side of the pot for voltage range limiting.

[TinMan]

Brad asked:
More than likely it is in series with one leg of the pot.

If you look  at the image page I referenced earlier, and observe the schematics, there are two methods, one is to vary the reference voltage, the other is to sense the output and use the output to adjust the voltage as part of a voltage divider. In this scenario, there usually are two resistors on either side of the pot for voltage range limiting.

Hi ION

I pulled the PCB back off of the heat sink,and there is a 1.2k resistor across the two outer pins of the VR--see pic below.
I pulled the resistor out,and measured it with a DMM,and it reads 1.2k.
The colors are hard to make out with such small resistor's.

Then from the middle pin of VR1,it go's through a small cap,and then to pin 4 on the LM723 chip--the inverting input.

It's hard to trace some of the components,as this seems to be some sort of double sandwiched PCB,where half of the tracks cannot be seen from either side of the board.


Brad

[ion]

Brad



Attached is a schematic of typical voltage adjustment for LM723. On this schematic there are two resistors that set the range of adjustment.
The designators may be different on your board, but they will be on either end of VR1. If there is only one resistor, just replace that one with a jumper and see what you get.


R1 sets the lower limit

R2 sets the high limit

You can jumper out these resistors if you want full range adjustment.

To go to the very minimum voltage output, you may need to divide down the reference voltage pin 6 to pin 5. We can get to that later, try the first step of removing R1 and R2 and replace with jumpers. With the typical LM723 implementation, you cannot go down to 0 volts without some major circuit modifications.
Only first cup of coffee, so hope I got that right. 

edit: in looking at other arrangements of LM723 it might be a bad idea to eliminate the  resistor in series with VR1 if it is wired differently than the first schematic as it would burn up the VR1 and short the supply...see second attachment. In this case, you should wire your external adjustment pot as in the first schematic.

Note that in the second schematic the reference is divided down to 2V so that the min Vout is 2 volts.

Typically the reference voltage is 7.15V so that is the min output if you don't divide the ref down.