High current 18V Power Supply

[rahdirs]

Hi all,

I think this is one of the most frequent question in this forum,designing a 18 V power supply. I did design a power supply circuit but it gives out low current.
The problem is IC 7818 in my circuit.It only gives a maximum of 1.5-2 A, but i need a power supply capable of giving 20 A.
So,i think i should remove voltage regulator from my circuit???

 

[FoxyRick]

I doubt it is the most frequent question in this forum, and the 7818 is definitely not the problem (or at least is a lesser significant part of it).

First, you need to source a transformer that can give 20A at more than 18V. That is quite a big transformer.

Why more than 18V? Because when you start pulling 20A, there will be some loss in the rectifiers, regulator and wiring.

Then, you need to regulate to 18V. The often-seen method is to use an integrated regulator, like the 7818, with a pass transistor or in this case several paralleled pass transistors.

Unless you either do not need accurate regulation to 18V, or you want to make a SMPS. A SMPS would be much smaller and lighter, but much harder to design unless you really know what you are doing, which is clearly not the case (no offence meant by that).

The principles are simple, and no different to, say, a 12V high current supply than the choice of components for 18V instead of 12V. Search for high current power supplies in general for some idea of the circuits.

 

[rahdirs]

the 7818 is definitely not the problem (or at least is a lesser significant part of it).

But i found in the datasheet of the 78xx series that max. output current that they can provide is only 2.2 A

 

[FoxyRick]

That is true, and I don't think there exists a single-chip regulator for 20A.

However, as I said, it is common to use such a regulator with additional 'pass' transistors. The additional transistors carry the extra current through themselves, with the output being regulated by the 7818.

Find the datasheet for a 7818 (or 7812, etc, they all work the same way). Such datasheets usually have example circuits and nearly always include an example of using an external pass transistor for higher current than the regulator alone can provide. Search here too, try 'pass transistor'.

You still need to find a bigger transformer though, and high rated diodes (or bridge) for the rectifier.

 

[rahdirs]

@FoxyRick:Why aren't TIP2955 working properly as pass transistors ?? They are not generating high current they were used to generate & instead why is 7818 giving out 20 A??? I put six pass transistors in parallel so that each TIP 2955 which can give 5 A max.current,can give out 30 A combined.But somehow 20 A is coming out of 7818.

Why isn't the voltage being kept at 18 V.I understand from ohm's law that, 20.6 A * 0.4 ohm = 8.26 V,but shouldn't IC 7818 keep voltage at 18 V.

I initially thought there must something wrong with pass transistor & replaced them with BD 330 Power transistor and then i get a current of 40 A & 17.8 V.
How do i limit this current to 20 A??Decreasing the number of pass transistors doesn't seem to work & i reduce them to 1 it's making IC 7818 give out 20 A.Why did BD 330 work & TIP 2955 didn't ???

Finally,i kept load very small,if i need high currents for high loads,altering the pass transistors,trying darlington configuration etc... should work right ??