Combining the Ampere of two voltage regulator?

eloso: I'm using linear regulator because it is easy to make and it is only a school project. So i will only use it once. I already have a big heatsink and thermal paste so generating heat is not a problem.

I think you under-estimate how hot a regulator will get passing 1.5A of current. I'm too lazy to do the math, but if it's dropping any voltage whatsoever, it may be impractical to keep the die temperature cool enough to survive. I built a 28v-to-15v regulator to supply 100mA or so of current, and the little 1"x2"x0.5" heatsinks got hot enough that I couldn't touch them comfortably. You're asking for a lot more current than that. Trust me -- that "1.5A" spec on the regulator is nothing short of wishful thinking. There's no way you'll keep the case cool enough that the die won't damage itself at that kind of load.

That is unless the 1.5A spec is for a very brief, worst-case, transient load... and average current is really significantly less.

Do you really think you'll use that much power. The link the OP posted said bust up to 2 amps The Lm78 can handle 2.2 Amps in short burst.

These burst of the SM5100B cellular module only last 4.6 mS

be80be:
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.

dhenry: thank god we don't live in a simulator.

You don't really know that. (Sorry, off topic, but couldn't resist. ;))

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.

pekkaa:

dhenry:
thank god we don’t live in a simulator.

You don’t really know that. (Sorry, off topic, but couldn’t resist. ;))

The latest on that is that they have proposed tests to see if we are real or not.

As long as there is a ground connection between the power supplies you don't have to connect the Positive outputs together. Therefore you can have 1,2,3,4,5... regulators connected together (as long as there is a common ground)!

SirNickity:

eloso: I'm using linear regulator because it is easy to make and it is only a school project. So i will only use it once. I already have a big heatsink and thermal paste so generating heat is not a problem.

I think you under-estimate how hot a regulator will get passing 1.5A of current. I'm too lazy to do the math, but if it's dropping any voltage whatsoever, it may be impractical to keep the die temperature cool enough to survive. I built a 28v-to-15v regulator to supply 100mA or so of current, and the little 1"x2"x0.5" heatsinks got hot enough that I couldn't touch them comfortably. You're asking for a lot more current than that. Trust me -- that "1.5A" spec on the regulator is nothing short of wishful thinking. There's no way you'll keep the case cool enough that the die won't damage itself at that kind of load.

That is unless the 1.5A spec is for a very brief, worst-case, transient load... and average current is really significantly less.

I see, but based on the replies in my topic i can draw a lot of current from pnp transistor not the regulator. My heatsink is 80mmx41mmx15mm. Thank you for the warning. Now i will focus on adding a pnp transistor.

I tried this circuit a while ago in a breadboard and it is not a working. I tried to draw 1 ampere by using a load of 10 ohm 5 watt resistor and the regulator gets very hot the pnp transistor is very cold and it seems it is not the one supplying it. I'm sure my connections are correct.

http://www.zen22142.zen.co.uk/Circuits/Power/boosti.htm

be80be: 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?

Krupski:

be80be: 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.

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.

dhenry:

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?

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

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.

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.

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.

Krupski:

be80be: 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?

It is used for stability and t provides some short circuit protection for the pass transistor with a little negative feedback.

Bob

dhenry:

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.

eloso: 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.