ps3010c.pdf

Bench Top Power Supply, -- Part 3

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Bench Top Powersupply -- Part 3

"Almost done! In this part three we install the final parts, make the last connections, and do the final adjustments with the trimmer pots. Take your time (go-slow)

with the trimmer pots for adjusting current! "

Where applicable, click on the picture for enlargements. Above you see the photo of my finished model. Looks good, performance is excellent, and I am very

happy with it. Notice however that the panel meters are shown non-modified, meaning that I added and modified the stripes for the two settings after the picture

was taken. I don't own a digital camera so am dependent on others to help me out.

I purchased a brand new transformer, model 165S30 manufactured by Hammond. It is of the regular kind. You can get a 'low-profile', horizontal type which

mounts a bit lower. Whatever model you have or buy make sure it fits your case including the large capacitor. The 30V type is only required if you need the full

10A, otherwise a 22V-28V type will do fine. A CT (Center Tap) is NOT used.

The 165S30 transformer has 5 wires, the primary side has two black wires which are connected to your 115 vac. The secondary has 3 wires, in my case two green

and one green/yellow. The green/yellow is not used. Isolated it with some heat shrink and tie it up, see Fig. 10. I never cut the wire off. The two green wires go to

your Bridge Rectifier. It is probably marked 'AC' or '~'. Make sure the transformer makes good ground with the chassis, which in my case meant removing the

paint. I then use a file to take the varnish off one bottom corner of the transformer. When you finish mounting the transformer into the case, take a multimeter or

continuity tester and make sure the chassis of the transformer makes good connection with the chassis of the power supply.

The fuse is a 3.15A slow-blow type to prevent it blows when you switch on the power supply with a bit of load. This transformer is a real heavy one and weighs

several pounds. If you're planning to use it with heavy loads, I would suggest to get one which can provide 12-15Amps at a desired voltage, but, the 10-amp

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transformer listed in the parts list will work. The difference is that the 10 amp transformer gets pretty hot if you use it at the maximum current. If you would have

a 12-15 amp type it will only get a little warm. In my case, I do use the 10amp setting often but not for extended periods. Several minutes at best. Usually around

7 amps or so for charging purposes.

It is probably best to mount the Bridge Rectifier, fuse holder, and power cord first before bolting down the transformer. As you can see in Fig. 11, I used a

mounting bracket for the large capacitor, this bracket is also mounted on the back panel so don't forget to drill the holes first. I used a 115vac receptacle, fuse,

and on/off switch combination which I salvaged from an old UPS and the whole thing is mounted on the back panel.

Crimp spades onto the two secondary green wires and mount the transformer. Connect those wires to the Bridge Rectifier as mentioned earlier. Install the large

capacitor and wire up to the '+' and '-' of the Bridge Rectifier. From this point on all other '+' and '-' connections are taken from the large capacitor terminals. Use

thick wire.

As mentioned earlier, instead of a Bridge Rectifier you could use four separate 'stud-mount' diodes and make your own bridge. The two anti-rattle capacitors, C1

and C2, should be mounted directly onto the transformer or the bridge rectifier for best performance. Power diode D3 is a very vital component in this power

supply and so was chosen a bit over-rated to make sure it will perform satisfactory under all circumstances and temperature changes. You probably already know

that a diode is temperature sensitive which is most noticeable in the 0.7Volt range. Since the same D3 also has a job of current limiting it is best to make sure this

diode does not get too hot. So, we really want a solid diode of 20 amps minimum.

In case of a short circuit, there is at least 2.5 amps of current going through each power transistor, and that is a lot at about 60 to 70 watts of dissipating energy.

That is why the 2N3772 power transistors come in which can dissipate 350 watts or so. And so, as a matter of speaking, at 30 volts we could well assured short

out the output jacks and fry an egg on the output power transistors.

Now lets have a look at L1 & L2. L1, C9, L2, and C10 are soldered as close as possible to the output jacks. We even have to cut the solder leads as short as

possible. C10 is soldered directly on to the '+' and '-' output jacks. C9 is soldered parallel over the L1/C10/L2 network. The two thick wires coming from the

printed circuit board are soldered onto each leg of C9. Oh yeah, I mention again that the PCB has to be mounted isolated from the case. The common ground

connection is connected to the '-' jack via L2. This is a little bit of tinkering but can be done easily. We are working here with 10 amps so worth all the efforts.

The new PCB will have screw connections to make the connections easy.

I call the ferrite beads "pig snouts" because that's what it looks like to me, but hey, call it whatever you want. You need to

make two of them. On the circuit diagram they are indicated as L1 and L2. One or two turns of thick magnet wire will do the

trick. L1 and L2 will keep the output jacks clear of high-frequency interference. Can you omit them? Certainly if you don't

care about that feature.

By now you must be anxious to try everything out. Well, be patient, we're almost done.

We still have to adjust the eight trim potentiometers on the pcb. And you really need to

sit down for it and carefully take your time. Look for a time and place when you can do

this quietly and undisturbed. The last thing you want is to open this heavy power supply

up again and re-adjust it because of initial sloppy adjustments. So, take your time, go slow, and verify each adjustment until

you're satisfied.

The adjustments are done in a special sequence and if you keep yourself to this procedure then I doubt you would encounter

any problems. Okay then, here goes it.

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Adjustment procedures:

FIRST check for correct wiring from and to pcb, jacks, meters, and coolrib. Very important.

Before starting the adjustments, familiarize yourself with the trim pots on the printed circuit board and the potentiometers on the front panel. The two on the

front panel are R3 and R12, the others are on the pcb. I mention this to avoid confusion while doing the adjustments. If you wish, mark all the pots ahead of time

by writing the 'R' numbers on a piece of scotch tape or something. It will help a lot!

Important: make sure to 'zero' the panel meters with the little plastic screw attached to the needle movement unit.

Open up the connection of the thick wire between the pcb and the positive of C3 and insert a small fuse of a couple hundred milliAmps. If you don't have a

small fuse handy then you can also use a 1/2 watt 10-ohm resistor or something similar. Do NOT plug in the power supply yet! Turn all trim pots to the left

(counter-clock-wise) all the way. Set the two potentiometers on the front panel about halfway. Set the two switches on the front panel (1/10A, 6/30V) to the low

settings, meaning the current switch on 1 amp, and the volt switch on 6 volt.

Take your digital multimeter and secure its minus (black) lead on the minus output jack. Plug in the power supply and switch on the power. If all is well and

there is no smoke, the main fuse and small temporary fuse on C3 remain okay, we can continue.

Put the plus (red) multimeter probe on topside of potentiometer R3. This is the position closest to the minus of C4 on the pcb. You have to measure there a

voltage of precisely 6 volts.

If needed, this voltage can be adjusted by turning the R2 pot. Turning the potentiometer (R3)

on the front panel will move the panelmeter but NOT the multimeter. If you move the red

probe to the '+' output jack and you should find a variable voltage (via R3) between 0.7 and 6

Volt. Don't worry about the current meter at this time, it probably will not move at all

because there is no current. All you do at this time is adjusting the low-voltage scale.

Leave the multimeter probes connected to the '+' and '-' output jacks and switch the Volt-

switch (S2a) on the front panel to the 30 Volt position. You will see that the voltage makes a

big jump upwards. We adjust R3 all the way to the right (clockwise) and adjust trimmer

potentiometer R23 until your multimeter shows 30 volts. We now adjust R26 until the panel

meter shows the same, 30 volts. Switch back to the 6-volt position and adjust the panel meter

to 6-volt full-scale with R24. If you're done with this and you are satisfied then have beer on

me for a job well done. You are half way finished!

Switch off the power, unplug the powercord. Remove the temporary fuse between the positive of C3 and the pcb and re-connect the wire to start adjustments on

the current settings.

Switch the panel meters to the 6 volt and 1 amp positions and turn current-limiter R12 on the front panel all the way to the left (counter-clock-wise). Set the volt

meter on the frontpanel to 4 volt with R3. Select a setting on your multimeter of 100 or 300 milli-amps dc. Take the red probe and insert a resistor of 39 ohm

between the red probe and the '+' of the output jack. You will notice that current flows through that resistor. The panel meter also shows a bit of current and at the

same time the needle of the panel-voltmeter falls back a little to about 2 volts or even lower. If that is the case you know your current limiter is working properly

and you can continue with the adjustment procedures.

Remove the 39 ohm resistor. Switch your multimeter to the highest current setting (preferably 10A) and connect it directly to the '+' and '-' output jacks. The

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meter should show no more current than with the 39-ohm resistor, even less this time. Carefully open up R12 (front panel) clockwise until you see increased

current on both multimeter and panel meter. A good multimeter will go to at least 10 amps, but I guess the job can be done with 2 or 3 amps also. On the other

hand It would be actually better to borrow a good multimeter from a friend or rental shop if you don't have one yourself.

Okay, on with it. Open R12, slowly, as far as possible and note the current reading. REMEMBER you are still at the 1A/6V setting! If there are no problems the

current reading probably shows 1/2 amp or something in that area. Let it sit in that condition for awhile and observe the temperature of the large cool rib. It

should warm up a little bit. If all is okay and still no smoke you can safely assume that the circuitry works correctly.

Before continuing:

Please be aware that 10 amp is a lot of current. Any carelessness on your part will cause damage, guaranteed! I can't stress enough to do these adjustments

without disruption and with great care. Over-adjusting any of the current-trimpots or adjusting them too fast will surely blow one or more 2N3772 power

transistors, or 2N6388, and possibly other related circuitry.

Okay, on with it--carefully.

The following adjustments have to be done in the correct sequence. Switch the power supply OFF. Set the panel switch to the 10-Amp setting and also the

multimeter to as high a current setting as possible. Turn R12 all the way to the left (0 amp), the multimeter is still connected to the '+' and '-' output jacks on the

front panel. Turn the power supply ON . You should notice almost no current at all. The setting of the 'Volt' potentiometer (R3) does not matter much at this time

so don't worry about it. Carefully and slow adjust R12 to a high as possible value and stop when it shows about 5 amps on the multimeter. Adjust the panel meter

with R20 until it shows the same value as the multimeter. When you're done the panel meter should show half way the 10-A scale. Just make sure that your

multimeter can handle 10 amps. If not, then don't exceed that value with R12 or you blow up your multimeter. Adjusting the PS3010 at 5 or 8A is acceptable too.

Turn R12 again all the way to the left and flick the switch on the front panel to the 1-A setting. Adjust R12 all the way to the right and with R18 adjust the value

of the multimeter with the value of the panel meter until they're equal.

In the mean time the coolrib is getting quite hot during all the adjustments in the 10-A settings. But that is done now. You have now adjusted six of the eight

trimmer pots and so still two to go.

Remove the multimeter. Turn both potentiometers on the frontpanel (R3/R12) all the way to the left (0 position). Return the switches to the 1-A and 6-V settings.

Short out the output jacks on the frontpanel with a piece of wire. Turn R12 all the way to the right and adjust R14 until the 'current' panel meter indicates

precisely 1-amp (full scale).

That done, turn R12 back all the way to the left and place the current switch in the 10-A setting. Adjust the full scale of the panel meter with R16 until it shows

exactly 10 amps. At this setting the cool rib heats up quickly so keep an eye on the temperature. You are done. Finished. I'll bet you are smiling now. After all,

you now have an analog piece of equipment equal or better then the commercial unit(easily can cost $800+) and for a fraction of the cost!

AGAIN: Be careful when adjusting the 10A settings! Adjustments with the trim pots to adjust the current should be done very SLOW or you're guaranteed to

blow one of the 2N3772 power transistors and/or other related components.

*If* it so happens that the PS3010 blows a power transistor, it will go to maximum current and may do damage to your multimeter, blow it's fuse, etc. Quickly

switch the powersupply off, and unplug. Let everything sit for 10 minutes or so to drain the voltage off C3. If you have a diode-setting on your multimeter, use

that position otherwise put it in the ohms setting. Put one probe on the base (base of Q2 to Q5 are all connected together) and the other probe on the emitter of

any of the 2N3772's. If any of the four are shorted it shows for all of them. To check which transistor it is, disconnect the four wires going to R6 to R9. Put your

multimeter on the 'Diode' setting if possible and check each transistor between the 'base' and 'emitter'. You will find hopefully only one shorted transistor.

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Replace it, and re-do the current adjustment. To prevent repetition of the same, adjust the PS3010 at 5A is good enough.

I'm looking into protecting each of the transistors and maybe a warning indicator of sorts.

Inside the enclosure I keep a little plastic container which contains some spare parts just in case I need it in the future. The parts I use are the 723 IC, the zener

diodes, 2N6388, and one 2N3772 power transistor. A medicine container will work real well. Why? Well, just because everything is so-called short-circuit-

protected it does not mean it can't happen, for example by a power surge, abuse when in the 10A setting, or lightning. Murphy's applies here too. But on a

positive note, this power supply is almost indestructible when used within its boundaries.

Now, what can you do with this power supply? Anything you want. Charge regular NiCad or Lead-Acid batteries, power your projects, check the current draw of

a particular project, run all kinds of motors, styro-foam cutters, etc. It is limited only by your imagination.

Parts List:

Resistors:

1/4 Watt, Carbon, 5% (or better), unless otherwise indicated

R1 = 470 ohm, 1/2 watt

R2 = 2 K, trimmer pot

R3 = 5 K, potmeter (lin)

R4 = 560 ohm

R5 = 47 ohm

R6,R7,R8,R9 = 0.1 ohm, ww, 5%, 1-watt

R10,R11 = 0.33 ohm, ww, 5%, 10-watt

R12 = 470 ohm, potmeter (lin)

R13,R22 = 820 ohm

R14,R23 = 500 ohm, trimmer pot

R15 = 150 ohm

R16 = 100 ohm, trim pot

R17,R18,R19,R20 = see text

R21 = 5K, metalfilm, 1% (2 10K parallel 1%)

R24,R25,R26,R27 = See Text (non-variable: 25K trim pot)

R28,R29,R30,R31 = 3.3K

Capacitors:

C1,C2 = 3.3 nF, ceramic C7 = 470pF, ceramic (see text)

C3 = *8,200 uF/50V+, electrolytic C8 = 10 uF/63V, electrolytic

C4 = 1000 uF/63V, electrolytic C9 = 1 uF, foil type, see text

C5,C6 = 4.7 uF/63V, electrolytic C10 = 22 nF, ceramic

Semiconductors:

D1 = 1N4004

D2 = 1N4148

D3 = 1N3209 (or NTE5942) power diode, 15A+ (see text)

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