0-30 Vdc Stabilized Power Supply

[Blacque Jacque]

Aha, this is a theory test isn't it ?

Ok here goes: High value resistors are needed to prevent current leakage through the existing LED, probably 100x Q3's bias pair values, hence a darlington to get enough current gain to turn on the 2nd LED. Correct ? ( My transistor theory was never very good :-\

 

[audioguru2]

Hi Blacque,
You are correct, the high-value resistors and darlington transistor are needed to keep the existing LED dimmed. Our vision's response to brightness is logarithmic so even a tiny current in an LED is obvious.

What's wrong with PNP circuits? Can't you read upside down?  ;D ;D

 

[Blacque Jacque]

LOL,

Ok final question, would I need a proper darlington or could I use 2 x BC557 for a discrete one ?
Thanks for your help so far Audioguru.

 

[audioguru2]

Hi  Blacque,
Sure, make your own darlington with a couple of BC557's. Make certain the 2nd one turns-off with a 100K resistor from its base (and the emitter of the 1st one) to its emitter (the positive supply). Use a 1M resistor from the base of the 1st one to the positive supply, and a 470K resistor to the collector of Q3. Use another 3.9K current-limiting resistor connected to the collectors of your darlington for the new LED. Use 1W resistors for the current-limiting resistors (R22 and the new one).

 

[aergotic]

The TIP31A (Q2) disipate 5.1W, are this properly? Whether have heatsink 21

 

[audioguru2]

Hi Aergotic,
I always design circuits for "worst case" conditions because you don't know what the current gain of your transistors will be, just a range of current gain. If your transistors have the lowest minimum guaranteed gain, you still want the design to work, and reliably.

The project's max output current is 3A. The modified circuit uses two paralleled 2N3055 transistors so each one will pass a max of 1.5A. The minimum current gain of a 2N3055 is 20 at 4A, but the datasheet has a graph that shows its gain doubling at 1.5A, which would be 40. Therefore the base current for each 2N3055 is 37.5mA and a total of 75mA max.

The 30VAC transformer will produce a rectified and filtered 40VDC at full load. If the project is set for 3A output current and the output is shorted, Q2 will have about 37.6V across it. Therefore the max dissipation of Q2 is 75mA X 37.6V = 2.82W.

The TIP31A can dissipate 2.0W at room temperature without a heatsink but will be extremely hot. If you want its max internal temp to be only 100 degrees C, then it will need a 26 degrees C/W heatsink and good thermal grease. 

 

[Blacque Jacque]

Hi All,

I have an idea which may solve a difficulty raised earlier in this thread ie: you cannot see the current limit set point easily, particularly if you want to use a multi turn wirewound pot.

Expanding my theory for current metering, you could use another opamp buffer to read the voltage on U3 pin3 ( current limit set point ) & feed it through a momentary switch to the ammeter. This would need to be done with no current flowing to the load, otherwise the

 

[audioguru2]

Hi Blacque,
Good idea, but I would just use an ordinary marked pot and a range switch.

 

[Blacque Jacque]

Hi Audioguru,

I hadn't thought of a range switch, how would you implement this ???

Do you think current & voltage setting would be easier with fine & coarse pots instead of multiturn ones ? If the current setting range is from 2mA to 3A that's a big adjustment range, it would be difficult to provide accurate control at each end of the range. On the other hand, how much use is adjustment down to single milliamps ?

I think calibrating the meter scaling is surely easier than calibrating front panel markings & higher resolution too.

 

[audioguru2]

Hi Blacque,
To use a range switch, you use a fixed main resistor and in parallel with it a high-value pot and a bunch of series connected scaled resistors in series. The switch shorts the resistors one at a time to change the range. Use a make-before-break switch to keep the current from jumping. Since the pot and resistors are high-value, the opamp's input that they feed must have a very high impedance, like a FET-input opamp.

I don't like having fine and course pots because it is difficult to see what their setting is, and you keep hitting the stop on the fine one.
I also don't like to use multi-turn pots. I just use a big knob and hold my breath when adjusing it. Sometimes I wish I could hold my heart too! Avoid coffee before adjusting.  ;D

I have never used a current regulated supply, but maybe I could use a few mA to measure the current gain of small transistors or something.

Use digital meters then you don't have to mark or calibrate them. With their 4 digits, they have very fine resolution.  ;D

 

[Thomas1]

Hi Blacque Jacque,

Annother way to preset the current is to shorten the output with a suitable switch and measure the voltage drop across R7. This voltage gets to a 100k/27k potential divider. Now we have 0,3V @ 3Amp across the 27k resistor which can be fed to a panelmeter. A 1k pot can be added in series to the 27k for fine tuning.

A second switch can be used to disconnect the output from the psu. That gives us the ability to preset the voltage.

I intend to do it that way. Maybe there could appear a problem when R7 dissipates 4.23 Watts and gets mighty hot which will change its value. Those expensive Resistors in a TO-247 case could be a solution because their temperature coeffizient is less than 50ppm, but I can't find any with 0.47 Ohm.

Hi Audioguru,

1) Can a 0.56R or a 0.39R Resistor be used for R7?

2) I've read both threads about this psu and am a little confused about the emitter leg of Q1. Is the original schematic correct or not? ???

 

[Blacque Jacque]

Hi Audioguru,

Thanks for the description of range switching.
I prefer to use just pots rather than the range switch you describe, I will get both a 10 turn and separate pots for coarse/fine & decide which I like best. This psu might be overkill for gain testing transistors, one slip and BOOM!

 

[audioguru2]

Hi Thomas,
If you can't get a powerful 0.47 ohm resistor, use a 0.39 ohm one and change the other parts around the current-set pot to match.
I bought a 25W aluminum-clad resistor and was amazed at its tiny size. I thought it would smoke for sure since at what extremely high temp does nichrome melt? I bolted it to an aluminum box with some thermal grease and let it dissipate 25W. It reached the same temp as the box, just nicely warm!
The original project spec'd only a 5W resistor for R7. It, Q2 and Q4 must get incandescent!

Yeah, I'm sorry to confuse you, I made a mistake. Q1 should be wired like the original.  :-[

Hi Blacque,
That darn majic smoke smells really nasty doesn't it?  ;D

 

[Thomas1]

Hi Blacque,

We are talking about these, right?

I think you are right, they shouldn't get that hot when they are bolted to the case... I think I'll try one of these. I just now discovered that they have also only 50ppm. That should work.

Annother advantage of "my" current measuring is that you can always see how much current flows to the load. You just need a switch that can stand 3 Amps.

Hi audioguru,

Okay, now I'm informed about Q1. Thank you 

When both values can be presetted, the usage of multi-turn pots should be no problem anymore, right? Saves space, can be well adjusted and a accidential touch of the knobs would not produce that much smoke

 

[audioguru2]

That's what I used, a 25W one is much smaller than that.

 

[Blacque Jacque]

Hi Thomas,

Yes that's exactly what I had in mind, take note though, they are only rated for full power when bolted to another larger heatsink or chassis, eg: the 25w types are only good for 9w on their own and remember to derate if you run them hotter than 25 degrees.

 

[costa]

hello guys!!!
this is my first post here, since i registred few days ago...
i built this power supply with org.specs and it works fine...2.5 A with almost 34V....
i am going to modify it so it can hold more than 3A output current as suggested by you all...
I read all the posts about this PSU and only thing not so clear to me is the emiter of Q1...
does it go on junction of C1- and R7(negative input)  or on the junction of R7 and C7(negative output)...
Also should C7 be of a greater value???
Please help...

 

[audioguru2]

Hi Costa,
Welcome to our forum.
It is wonderful to hear that your original project works fine and produces 2.5A at almost 34V. ;D

It is difficult to believe that its 24V transformer has such a high rectified DC output voltage. Without losses, 24VAC has a peak of 34V and the bridge rectifier has a 2V drop, giving an output with ripple of only 32V. The original circuit has voltage drops of at least 5V so its max output voltage at 2.5A should be only 27V. Other people's original circuits produced less.

The emitter of Q1 is correct on the original schematic at R7 and C1, sorry I made a mistake by changing it. :-[
With the original value of only 3300uF, C1 will have a lot of ripple voltage at full output current. At least 10,000uF is recommended for 3A, much more for 5A.

 

[costa]

thanks audioguru...
sorry i didnt mention that i use stronger transformer as specified... ;D
but i ask again for the cap C7 (not C1) if it could or should be of a bit larger capacity???
thanks...
btw. audioguru, i am searchin for a timer circuit that would turn off my aquarium lamp in the evening 22.00h and turn it on in the morning 07.00h,every day...have you heard of something like it???

 

[gangadhara]

[move][/fffff