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### Topic: Combining the Ampere of two voltage regulator? (Read 17231 times)previous topic - next topic

#### eloso

#30
##### Jan 06, 2013, 02:42 pm

You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

what is the watts of the resistor?

#### eloso

#31
##### Jan 06, 2013, 02:50 pm

You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

Here's a circuit that I use for more regulated power... the small resistor values cause the 78XX part to provide current limit (around 10 amps with these values) and since the regulator and pass transistor are on the same heatsink, the thermal protection of the 78XX is also provided.

Note the way the output is connected... this provides remote current sensing which compensates for drop along long wires if they are used.

Only disadvantage to this circuit is that a minimum load of a few milliamps is required because of the design of the 78XX regulator. If you use an LM-317 instead, then this is not a problem.

(edit to add): If you use a 79XX regulator and an NPN 2N3055 and reverse the plus and minus terminals, you get the same circuit.

I have LM317 here.

#### dhenry

#32
##### Jan 06, 2013, 02:52 pm
Quote
I tried this circuit a while ago in a breadboard and it is not a working.

That circuit is designed for the pnp to kick in at load current > 700mv/1ohm = 700ma. Anything below that, the voltage regulator is doing the heavy lifting.

If you want the pnp to kick in at a lower current, increase that 1ohm resistor.

#### eloso

#33
##### Jan 06, 2013, 03:24 pm

Quote
I tried this circuit a while ago in a breadboard and it is not a working.

That circuit is designed for the pnp to kick in at load current > 700mv/1ohm = 700ma. Anything below that, the voltage regulator is doing the heavy lifting.

If you want the pnp to kick in at a lower current, increase that 1ohm resistor.

I see. What do you suggest?

#### eloso

#34
##### Jan 06, 2013, 03:31 pm
so the regulator can be very hot at 700mA? i almost got skin burned when i touched it. Im using tip42c pnp.

#### be80be

#35
##### Jan 06, 2013, 03:44 pmLast Edit: Jan 06, 2013, 03:47 pm by be80be Reason: 1

http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
As much problems your having find you a old PSU from a computer kind with a switch to turn on and use that
most have up to 30 amp output on 5 volt and about half of that on the 12 volt.

#### dhenry

#36
##### Jan 06, 2013, 05:48 pm
Quote
What do you suggest?

You want to the regulator to run 50 / 100ma at least. So I would use a resistor of 8 - 10ohm, give or take. Higher where the voltage dropout is big.

Quote
so the regulator can be very hot at 700mA?

Yes, particuarly if the Vin-Vout is large. Your heatsink is puny for this type of applications.

#### eloso

#37
##### Jan 06, 2013, 06:27 pm

You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.

I would do this use a PNP to get you more current

Here's a circuit that I use for more regulated power... the small resistor values cause the 78XX part to provide current limit (around 10 amps with these values) and since the regulator and pass transistor are on the same heatsink, the thermal protection of the 78XX is also provided.

Note the way the output is connected... this provides remote current sensing which compensates for drop along long wires if they are used.

Only disadvantage to this circuit is that a minimum load of a few milliamps is required because of the design of the 78XX regulator. If you use an LM-317 instead, then this is not a problem.

(edit to add): If you use a 79XX regulator and an NPN 2N3055 and reverse the plus and minus terminals, you get the same circuit.

what is the use of 0.1 ohms resistor?

#### Docedison

#38
##### Jan 06, 2013, 10:03 pm
It is used for stability and t provides some short circuit protection for the pass transistor with a little negative feedback.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### krupski

#39
##### Jan 07, 2013, 01:22 am

Quote
Here's a circuit

The 0.15ohm ressitor needs to be 100x of its current value.

You can buffer that pnp with a power npn / n-ch mosfet so you can use a smaller pnp device instead.

No it doesn't. It's chosen so that when the load is around 10 amps, the load that the 78xx sees is 1 amp. The idea is to carry over the current limit and thermal protection built into the 78xx without needing extra components.

I've been using this circuit for literally decades... believe me, it works.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### krupski

#40
##### Jan 07, 2013, 01:25 am

so the regulator can be very hot at 700mA? i almost got skin burned when i touched it. Im using tip42c pnp.

Note that my drawing shows both the regulator and pass transistor on the same heatsink. The idea is for both devices to share the same temperature... that way if the pass transistor gets too hot, it heats the 78xx regulator and trips the thermal protection. The circuit is designed to pass along the current limiting and thermal protection of the 78xx regulator to the pass transistor.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### dhenry

#41
##### Jan 07, 2013, 01:34 am
Quote
It's chosen so that when the load is around 10 amps, the load that the 78xx sees is 1 amp.

With the 78xx flowing just 1amp, the 0.15ohm resistor is providing a voltage of 150mv: how is it going to open up the pnp?

Quote
I've been using this circuit for literally decades... believe me, it works.

I am sure it works. You may want to write the manufactures a note so they can rewrite the datasheet.

#### be80be

#42
##### Jan 07, 2013, 01:57 am
Here one for the OP LM317

#### eloso

#43
##### Jan 07, 2013, 02:52 am
any idea the required watts of the resistor?

#### dhenry

#44
##### Jan 07, 2013, 02:56 am
Tiny: the voltage across it is 700mv at most.

Quote
Here one for the OP LM317

Sounds like high time for them to update their datasheet.

BTW, you can use a n-ch mosfet in place of the npn.

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