Bench 6V - high current power supply

[dan3460]

I have been working on old car radio for a while and I'm tired of caring a battery upstairs. So I want to build a power supply that could supply 6 and 12V at between 6 to 10A. I found a transformer that can deliver up to 16 Amps for around $20. Now looking for a voltage regulator that I can use, any ideas of something that will not break the bank.

Thanks

[tautech]

Have a look at LM338, good for at least 5A and IIRC 7A peak. Data sheets have typical applications examples that will meet your requirements.

[dan3460]

I saw the LM338 and LM138, but this old radios will pull a little more than 6A and on testing you probably can run this thing for a long while. If I understand correctly, those can only sustain more than 5A for 5ms. Now reading a different spec sheet it says "Typical 8A" if Vi-Vo <10V. Is that means that can handle 8A continually?

[Seekonk]

If the transformer has two 6V ish windings, you can go with two full wave bridges with associated capacitors.  Put these in serial for 12V and parallel for 6V by means of a switch.  That will also give double the current at low voltage.  Remember vehicles are not that well regulated. I would do something like a low drop out 7805 driving several PNP power transistors in parallel.  Check out the data sheets of fixed regulators for schematic.

[dan3460]

Yes the transformer that I saw has 2 windings, and yes I was thinking on use some kind of a switch to change from 12 to 6. Because I want to put this on the bench I would like it to be fairly good regulated, just in case I wanted to use it on other things. The 7805 only handles up to 1.5A or I'm incorrect?
 

[Kleinstein]

Getting 6 V DC from a 6 V AC transformer can get tricky - this would need to be a very low drop design, possibly even using the 12 V to fill in ripple. So I would more think of something like a LT1575 and external MOSFET. So this will not be a simple circuit. It might work with something like LT1083 from the 12 V.

Also 6 V lead batteries are usually more like 6.5 - 7 V when full.

[Seekonk]

"The 7805 only handles up to 1.5A or I'm incorrect?"

Correct, but 1A driving the base of two or more PNP power transistors gets you a lot of current easily more than 10A.

[dan3460]

That is an idea. Would you suggest the transistors?

[oldway]

Very simple power supply: variac, transformer, bridge rectifier, big electrolytic capacitor and a fuse.
No need of regulation.
Ajust output voltage using your multimeter.
Battery voltage is not regulated. (from 11 to 14.5V)

[dan3460]

Yes, but i'm looking for something to put on my bench permanently. It would be nice that when you turn it on produces 6 or 12 volts.

[mariush]

LM1084 , LT1084, *1084 ... linear regulators capable of up to 5A and with a low dropout voltage (1v, up to 1.4v depending on temperature, extreme case) and they can be paralleled with a very small resistor to balance them.

See http://www.ti.com/lit/ds/symlink/lm1084.pdf or http://cds.linear.com/docs/en/datasheet/108345fh.pdf (see page 15 for paralleling regulators)

LM1084 is cheap, about 2-3$ for one piece, the LT1084 is more expensive, as Linear parts often are (but if you're super cheap you can request samples from them - up to 2 parts per product, up to 3 products per request if I remember correctly - and you'll get them by mail in a couple of weeks)

If you want 6v out using a linear regulator like the one above, you'd need a minimum voltage always above 6v + dropout voltage , or about 7v. It's in your best interest to keep the minimum and maximum voltage these linear regulators see as close as possible to that 7v figure, because the difference between input and output voltage is dissipated as heat and no matter what heatsinks you use,  the maximum you'd dissipate on the regulators would be about 15-20w (see note 4 on page 6 of Linear's datasheet for maximum dissipated power depending on regulator and the case)

So for example, if you'll use two regulators in parallel and each of them will output 4 amps (out of maximum 5 amps), that 20w figure means you have a budget of 20w / 4 amps = 5v , which means you should not exceed 6v + 5v = 11v at the input voltage.

So keep these in mind when you pick a transformer.

For 6v and 12v, I would pick a transformer that has two independent secondary windings of about  9v AC and if you want  up to 10A, then you'd need one rated for at least 18v x 10A x 2 = about 300-350VA

If you want to output 6v , you connect only one winding to the bridge rectifier and capacitors , if you want 12v you connect both secondary windings. 

So let's say we have 2 x 9v AC and 300VA transformer and you want 12v and 10A.

The 18v AC is RMS, which means when you convert it to DC using a bridge rectifier, you get a peak DC voltage of   Vdc peak = sqrt(2) x Vac  - 2 x Vdiode = 1.414 x 18 - 2 x 0.8  = ~24v DC
The maximum AC current is Iac = 300VA / 18v = 16.66 A  and the max. DC current would be around 0.62 x 16.6 = 10.3A which does the job.

Now you need to make sure the minimum voltage always stays above around 13.5v so that regulators like LM1084 will output 12v even at 4-5A per regulator ... so now you figure out how much capacitance to use :

C = Current / [ 2 x mains frequency x ( Vpeak - Vmin) ]   =  10A / 2 x 60Hz x (24 - 13.5)  = 10 / 120 x 10.5  = 0.00793650 Farads so you'd need a minimum of 7936 uF  capacitance to keep this minimum voltage above 13.5v  .. so two 4700uF 35v rated capacitors in parallel should do the job. You can go with more capacitance but you'd only get the minimum voltage higher at lower currents which means the linear regulators will get hotter.

For 6v, you could connect the 2 secondary windings in parallel, which means you'll have 9v AC and up to about 20-25A of DC current available.

Still, 300va transformers are quite expensive, so you can see why people would prefer to use a switching regulators, because they're more efficient. But in your application (radios) they may not be a good idea.

[oldway]

Yes, but i'm looking for something to put on my bench permanently. It would be nice that when you turn it on produces 6 or 12 volts.

Buy on ebay 2 low cost fixed voltage SMPS: one 6V 10A and another one 12V 10A.

[Seekonk]

Use this  http://www.eleccircuit.com/boosting-regulator-current-for-ic-78xx-by-mj2955/

Increase R1 to about 2 ohms and add two more PNP transistors of at least 10A.

[donmr]

Regulators like the LM723 can work with an external power transistor so you can handle as much current as you wish.
You will need some head room from your rectified DC to the output.

[Paul Price]

You can have the basic for a 6V power supply that will give you all the current and most of all the parts you need by using a discarded desktop PC ATX power supply's 12V output and a 7805 with PNP power transistor boost circuits.

You can raise the 12v output to 18V or so to charge 12-V batteries with a boost converter.

https://www.eevblog.com/forum/projects/do-laptops-typically-have-battery-over-voltage-protection-strange-project-ideas/msg850843/#msg850843

See the boost converter PCB I posted in reply.