Convert 0-30V 3A PSU to 5A or more

[DerFly]

So, I am also building this PSU and have box, panel meters, transformer, etc.  I have also finished the circuit board.

I am thinking to use a thermistor circuit and 12 volt fan to keep the heatsink cool.  Have not decoded to mount the heatsink inside or outside the box yet.

I think of two ways to power the fan circuit;

1. Use the rectified 30 volts and a voltage regulator to get 12 volts, or,
2. Use a dedicated 12v 1a transformer with rectifier and filter cap to provide unregulated 12 volt for the fan circuit.

What do you think?

DerFly.

[MP]

Why not a 12V zener and a resistor? Keep the parts cheap and the count down this way.

MP

[Staigen]

Just use the 30-35 Volts from the rectifier, a fan dont load much. Why not use 2 fans in series to cool both sides of the heatsink. But then you must do somthing to hold the middle point between the fans at a proper voltage. Can be done with a op-amp, i belive. But costs goes up!

//Staigen

[Staigen]

Hey Mixos, have you tested your PSU for low voltage/ high current yet? When you are testing, also check the voltage over the reservoir cap (C1). Also check the voltage over the resistor R7, and also the voltage over D7 and the output voltage from U1 (at point 5 ). Also the temperature on the case of Q4, you dont have to measure the exact temperature, just put your finger on it, it must not get to hot!

//Staigen

[Staigen]

Hi all, i belive we should take the most of the discussions about the PSU and its coversion to higher currents in this forum. Mixos and me has have a small discussion about how to do the tests in ICQ and via email, and he have performed some tests now. The tests did not fall out as good as the promises in the projects text. Trying to get more then about 25 V at 2.1 Amps show increasing ripple in the output, and trying to get higher voltages than that was not possible. At lower output currents it was of course possible to get higher voltage out. This is so because the reservoir condenser C1 is to small, maybe his transformer is not enough good to, i am not shure about this, he have to test that to. Can you test the output AC voltage from the transformer when the PSU is loaded at 1 V / 3 Amps? Also the pass transistor Q4 get very hot at low output voltage / high current (1V/3A)!

//Staigen

[mixos]

I measured the AC voltage across the transformer at

1V/3A the output was a distorted sine with 32V amplitude

and it goes till:

1,4V/4,4A where the output was also a distorted sine with 31V amplitude

Is the transformer goes to current saturation?

At these outputs except the Q4 also the rectifier diodes becomes hot.

[Staigen]

Yes, the transformer is rated at about 72 VA, and you try to take out about 90-100 VA! Is there any place on the internet that describe rectifiers, somewhere? I have difficulties explaining exactly how a rectifier works, i know how it work, but explaining it in a few words is harder. And i mean rectifiers with its asociated transformer and reservoir condenser, also under load.

Shortly, the transformer have to be rated about 1,5 times the actual load, in this case 24 Volts times 3 Amps times 1.5 = 108 VA! Thats a 4.5 Amps transformer.

Then we have the ripple. This is because the current you draw from the reservoir cap when the voltage out from the transformer is lower than the actual voltage over the cap. The current is unloading the cap during that time and the voltage is dropping. The remedy for this is to heavily upgrade the cap to about 10-20 times the normal value. Also the rectifier diodes have to be at somewhat higher rating.

Now to something else. How much shall we upgrade the PSU to? I have a little suggestion here, how about 0 to 30 Volts and up to 5 Amps with good regulating and ripple and 7 Amps at somewhat lower voltage, also with good regulation ripple and 10 Amps intermittent but not moore than about 25 volts output with good regulation and ripple. I belive this is possible. At least i am going to build the PSU that way, if we success!

//Staigen

[bogdan]

ok. i have a suggestion. what if the  2N3055 transistor could be replaced by 2? this will lower the dissipation ok each transformer.
but the drop on the transistors means that the output will not be as high as 30V....
or, another suggestion. put a resistor in series with the transistor.  and a relay with a control circuit. when the output voltage is more than 15V(let's say) then the resistor is shortcircuited by the relay and so you can get the maximum voltage. when at the output is lover than 15V then the relay is OFF and some voltage gets dropped across the resistor. this way you minimise the power dissipation ok the transistor.
and for the heatsink...i think that mounting a small fan, maybe one similar to a computer cpu fan coud be used. having an artificial air flow cools the transistor better than a large heatsink. and for getting 12V for the fan, i would go for a 7812 regulator. it is cheap, and for the current that a fan will draw, less than 200mA it is very good.
well, hope that my suggestions will do some good.

[Staigen]

Hi there Bogdan, shure they do! But we come to that later. Of course the pass transistor must have headroom, and i am also going to suggest multiple pass tansistors. Also fan cooling for the heatsink, in order to use a cheaper one. But the relay and its control circuit can easily be moore costly than a few transistors.

//Staigen

[bogdan]

well, yes, the realy and resistor and control circuit(a low power transistor, a zenner and some resistors) can be more expensive than the transistor itrself. but what about adding the heatsik? you will not need a heatsink for the resistors. yet, i think that the power resistors can be more expensive.
what about using something else in place of the resistors ? like car lamps or other types of lamps....?

[Staigen]

Adding a heatsink? The heatsink is already there. Do you mean that we shall use the pass transistors without heatsink? I dont think that will work. Of course a bigger heatsink costs moore, but you only buy it once, but the PSU you use year out and year in, so if it cost i little moore in the beginning, but lasts longer i belive its worth it.

//Staigen

[bogdan]

nope, you did not understand me right.  
i was suggestiong to use the heatsink and the transistor that are already there and another resistor in series with the transistor, wich will be short-circuited(the resistor) when the voltage required at the output of the power suply is highter. what i was trying to say is that for the resistor, a heatsink is not required.

[Staigen]

I did understand you from the beginning, but i dont think the idea is so good, it complicate things, and it can make the PSU less reliable. The idea is not so dumb, it is much power dissipated at low output voltages and high currents, and that is transferred to heat, so if you can dissipate that in a resistor there will be much to gain, but it have moore disadvantages to, for example, where to put the trip point etc. And lamps dont have constant resistance, they have heavy positive temperature constant.

//Staigen

[bogdan]

yes, lamps are not more reliable.
i think that the best option here is to either put tu 2n3055 transistors, but it think that tha will require having a 30V a.c. transformer to obtain 30V at the output, or to use a highter power transistor.
or, maybe what about using 2 2n3055 transistors in paralel?
i have seen this in many schematics. maybe it will work better than having 2 transistors in series?