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Lm 7805 !!!

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watzmann

New Member
Hi All ,

i'm running a stepper motor with a supply from LM7805 , my problem is the stepper draws more than 1.3 A , so the regulator goes so hot in a few seconds ,

i need to use another regulator LM too , but can stand this high current ,
i know the 138, 338 ..... i don't like them they are variable , i need a constant one like the 7805 and high current

could you suggest one please ,

Thank you
 

Hero999

Banned
Does the regulator have alarge heat sink?

What about using an LM317 which can handle 1.5A.

The chances are that your LM7805 is fine but you haven't got it on a decent heat sink.
 

watzmann

New Member
o.k , i can fix a heatsink , but i think this wrong too ,

the datasheet says it just for 1 Amp , so why i use it with more than that ???
 

Hero999

Banned
The chances are it'll stand up to 1.5A but use the LM317 which requires a couple of resistors if you have any problems.
 

picbits

Well-Known Member
What is your input voltage ? If your input-output voltage is too large then it will heat up a lot more quickly.

Even the LM338 will shut down quickly if you try powering something that takes a couple of amps at 5v if you're using a 30v input.
 

blueroomelectronics

Well-Known Member
watzmann said:
o.k , i can fix a heatsink , but i think this wrong too ,

the datasheet says it just for 1 Amp , so why i use it with more than that ???

Only you know why you're using a part unsuitable for the task. You should try to overdesign. If your input power is much higher than 9v it'll probably go into thermal shutdown anyway.

Why are you regulating stepper motors anyway?
 

Boncuk

New Member
Lm7805

You have three choices to get a higher current than 1A.

The

- LM78S05 delivers 2A,
- LM7805K delivers 3A (TO3 package)
- LM7805 delivers any current if you boost it with a power transistor. (The max. current splits into two currents where the LM7805 will stay at 800mA if the dimensions are correct.)

All of them require a heat sink!

Bosco
 

mvs sarma

Well-Known Member
watzmann said:
Hi All ,

i'm running a stepper motor with a supply from LM7805 , my problem is the stepper draws more than 1.3 A , so the regulator goes so hot in a few seconds ,

i need to use another regulator LM too , but can stand this high current ,
i know the 138, 338 ..... i don't like them they are variable , i need a constant one like the 7805 and high current

could you suggest one please ,

Thank you
Perhaps you need a regulator that can deliver at least a peak of 2.5 times the measured load on stepper motor. Thus it is better to work with a regulator mod (with series pass element) to handle a peak load of 5AMPs to be consistently stable.please study the series pass elements to increase the current load to 5 amps. As mentioned by Bill , you definitely need a Good heat-sink for the entire power section.
 

Hero999

Banned
blueroomelectronics said:
Why are you regulating stepper motors anyway?
Exactly, he shouldn't be powering stepper motors from a linearly regulated power supply.

He should use the unregulated supply for the motor and just use 5V for the microcontroller.
 

Hero999

Banned
It's a switching regulator, therefore it will require a large heatshink.
 

ecerfoglio

New Member
Hero999 said:
It's a switching regulator, therefore it will require a large heatshink.

It's a LINEAR regulator, therefore it will require a large heatshink.
 

Hero999

Banned
Sorry, I meant linear regulator. :D
 

Dean Huster

Well-Known Member
Usually, the biggest contributor of heat to a linear regulator besides the current draw is the supply voltage. If you feed a 7805 from an 8v supply, you'll drop 3v across the part and at 1a of current, that's 3 watts of dissipation, easily handled. On the other hand, if you're supplying it from a 20v supply, that's a 17 volt drop across the regulator and at 1a, 17 watts of power dissipated.

Besides that, a 7805 usually reacts "negatively" to overcurrent spikes like that and tries to shut down unless you've hung a ridiculously-large amount of capacitance on its output.

Dean
 
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