LM7805 getting too hot

G

GeorgeC47

Senior Member
My circuit for the dual stepper motor driver for a telescope is complete and works like a treat.
The only problem is, the LM7805 regulator gets viciously hot in less than a minute.
I want to power the circuit from a portable source and had a 12V battery in mind.
The circuit and motors run off 5V.
I knew heat would need to be dissipated so I bolted the LM7805 to a lump of aluminium as in the picture.
Clearly this is insufficient.

Can anyone suggest a better way to get a 5V supply?

George
circ1.jpgcirc2.jpg
 
Thanks. Each stepper motor pulls about 0.5 amps so I suspect that an adequate heatsink will not fit in the box.
Also the box has no ventilation holes.
 
GeorgeC47 said:
Thanks. Each stepper motor pulls about 0.5 amps so I suspect that an adequate heatsink will not fit in the box.
Also the box has no ventilation holes.
Click to expand...

So why not simply put a bigger heatsink on the outside of the box where it can work properly, i.e. where there is airlflow ?
 
GeorgeC47 said:
Thanks. Each stepper motor pulls about 0.5 amps so I suspect that an adequate heatsink will not fit in the box.
Also the box has no ventilation holes.
Click to expand...

You may want to try a Murata 78SR-5/2-C. A bit pricey, but a switching regulator the same size and shape as a 7805.
2 amp output, and needs no heatsink.

Rip
 
Dropping 7V at high current will cause an incredible amount heat with a linear regulator. Such systems would normally use 12V steppers so there is no need to drop their voltages.

I'm assuming the steppers are powered off 5V through the 7805 regulator. If not there may be some other issue. Perhaps post your circuit diagram.
 
That Murata box of tricks looks ideal! About £10 but that is not a problem.

Hippy, my circuit is straight out of the Picaxe manual, except 5V instead of 12V.
steppercircuit.jpg
 
The Murata 78SR-5/2-C seems to be only available from the USA with a huge delivery charge.
Can they be bought in the UK?
 
All links end up with a large shipping charge. Not to worry, I ordered one from Mouser.com
£9.61 for the converter and about £12 shipping.
Thanks for your help, everyone!
George
 
Are they really 5V only steppers? running them at 12V would remove the problem completely.
 
Use a 6V battery direct to your steppers, no regulator. No harm done and you can use the extra power.

To power the Picaxe from a 6V battery, use a low dropout 5V regulator, such as http://www.jameco.com/1/1/7841-lt1086ct-5pbf-1-5a-low-dropout-positive-regulator-fixed-5v.html

or (if you can live with 100 ma) http://www.ebay.com/itm/25-x-LM2950..._Supplies_ET&hash=item3a7f72efc3#ht_672wt_679

Using a 1970's-era 7805 is usually a bad idea unless you are drawing small currents and only need to drop a few volts. Its high overhead voltage (over 2V) is just plain silly.
 
Last edited:
erco said:
Use a 6V battery direct to your steppers, no regulator. To power the Picaxe from a 6V battery, use a low dropout 5V regulator, such as http://www.jameco.com/1/1/7841-lt1086ct-5pbf-1-5a-low-dropout-positive-regulator-fixed-5v.html
Using a 1970's-era 7805 is usually a bad idea unless you are drawing small currents and only need to drop a few volts. Its high overhead voltage (over 2V) is just plain silly.
Click to expand...

That makes sense. If I was starting again I think that is the approach I would take.
However, the Murata device is on order and should do the trick.
 
George

The video shows the basic circuit, the one I linked has an extra stage of filtration which removes more ripple and switching noise, well worth paying the extra coppers for
 
Does the regulator get hot when the steppers aren't running? If it does, I think you have a problem in your circuit somewhere.
 
GeorgeC47 said:
Thanks. Each stepper motor pulls about 0.5 amps so I suspect that an adequate heatsink will not fit in the box.
Also the box has no ventilation holes.
Click to expand...

The LM7805 is only rated for 1 Amp and at that current would need very good heat sinking.
If your two motors draw 1 amp in total plus some allowance for the other parts of the citucit then overload is the situation.

There is the 78H05 which is a 5V 5 Amp regulator in a TO3 package but I have not purchased any of those for years so availability may no longer be good.

As mentioned earlier by others, consider a switching mode voltage reguilator which can be compact and far less losses (= heat generated).
 
Hemi345 said:
Does the regulator get hot when the steppers aren't running? If it does, I think you have a problem in your circuit somewhere.
Click to expand...

My experience with steppers is they draw current all the time since you usually want them to hold a position. I have considered putting a switch input on my lathe project to turn off the enable pins on the motor controller when I'm not using the stepper but I certainly wouldn't want that to happen after each motion because the the carriage would likely move after the power was cut to the stepper. Likewise my guess is that the OP wants the telescope to remain in the position after the stepper moves the instrument.

Most of the LM7805 regulators I'm familiar with handle 1.5A. They DO produce quite a bit of heat at that current and a proper heat sink in free air is needed. The chunk of metal pictured earlier likely won't do the job because it is inside a box with no free air flow and chunks of metal have poor heat transfer characteristics. In any case GeorgeC47 has posted he is going to replace the LM7805 with a small switching regulator.

I actually like erco's notion of powering the steppers directly from a 6V battery the best. 6V won't hurt them and the weight savings will be significant.
 
lewisg said:
I actually like erco's notion of powering the steppers directly from a 6V battery the best. 6V won't hurt them and the weight savings will be significant.
Click to expand...

+1

You could just two diodes in series to drop the voltage to a Picaxe safe level.

And the battery will be easier to charge from solar panels :)
 
boriz said:
+1

You could just two diodes in series to drop the voltage to a Picaxe safe level.

And the battery will be easier to charge from solar panels :)
Click to expand...

Capturing the energy from the moon and stars through the telescope to charge the batteries... I like it.
 
lewisg said:
My experience with steppers is they draw current all the time since you usually want them to hold a position.
Click to expand...

Ahh, I've only dabbled with a small stepper from a floppy drive and never thought to measure the 'idle' draw. Good to know :)
 
lewisg said:
My experience with steppers is they draw current all the time since you usually want them to hold a position.
Click to expand...

Some time ago I dabbled/experimented with the AMIS30622 Stepper motor controller chip rated for 800 mA per coil/winding.
http://www.onsemi.cn/pub_link/Collateral/AMIS-30622-D.PDF
I did post some information on this forum.
i2c control with accelleration and deacceleration and selectable running and holding currents plus microstepping options.
 
How are steppers voltage actually rated, as the steppers i have on a CNC router have a manufacturer voltage given for 2.8 volts, but are run at 30 volts with the CNC and just get warm to touch after extended use of the machine.
I have also pulled steppers out of early printers that have a 5v rating and were operated at 24 volt within the printer.
 
lewisg said:
Likewise my guess is that the OP wants the telescope to remain in the position after the stepper moves the instrument.
Click to expand...

The right Ascension motor runs permanently to track the stars, and the handset will speed up or slow the motor down to centre the target in the field of view. The Declination axis is perpendicular to that, and for most of the time will not be used, but the stepper will be powered. Because the driver uses two pins as NOT inverters I can't switch the power off to all the coils - two will always be on.
 
GeorgeC47 said:
. Because the driver uses two pins as NOT inverters I can't switch the power off to all the coils - two will always be on.
Click to expand...

BINGO. That's your problem. You must switch your coils off immediately after stopping, or you'll overheat your steppers. Change your circuit.
 
The Murata converter arrived this morning - only 48 hours from the USA! It is wired in and works perfectly.
No heat generated as far as I can see.
Thanks for the advice, Rip.
 
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