L7824 goes in thermal shutdown , at only 350mA

[andre tchmil]

what is going on  ?

I'm making a new PSU for  4 Neve modules.
Total current draw is around  350mA.

AC 24V > after rectifying and smoothing , I have approx. 36VDC feeding a 7824.
Regulator shuts down after 20 seconds or so.
replaced the regulator with a new one ( same batch ), nada
What am I doing wrong ?

 

[peterc]

HI Andre

Sounds like thermal shutdown
Does it get hot? Is it on a heat sink?

The 7824 will only dissipate 3w without a heat sink, your requirement is (36-24) x .35 which is over 4W

Peter

 

[andre tchmil]

yes , proper heatsink (  the entire 19" chassis )
I know  It sounds silly but I have no idea what's going on.
maybe I should try a 1000uf cap in front , now it's a 2200

 

[Rob Flinn]

The more voltage dropped across the reg the less current it can supply.  As peter says you are dropping too much voltage for the current you are drawing.  If you look at the data sheet there is a graph normally to tellyou this info.  You could use a series dropper resistor before the reg to reduce the voltage across the device.

 

[andre tchmil]

The more voltage dropped across the reg the less current it can supply.  As peter says you are dropping too much voltage for the current you are drawing.  If you look at the data sheet there is a graph normally to tellyou this info.  You could use a series dropper resistor before the reg to reduce the voltage across the device.

true , let me try with the resistor.
or a 783 ?

 

[JohnRoberts]

This is PS 101 but worth understanding...

The iconic 3 terminal regulators are hard to kill with all kinds of built in protection. The thermal shut down is one of those protections, back in the day before they got smart they would just melt and release their magic smoke, but back  in the day we didn't have 3 terminal regulators at all.

In an ideal world for semiconductors, cooler is always better for longer life and reliability, so beyond just not thermalling, it would be better to run it even cooler.

In practice 3 terminal regulators want a few volts across them to regulate with so 24V output wants 27+ input voltage. Then add some extra for ripple, since that voltage headroom needs to be from the lowest ripple voltage.

Peter described the math for power dissipation in a pass element (voltage dropped across the device times current).

There are several ways to drop the unregulated voltage....  with a constant current draw of  0.35A every 1 ohm in series will drop .35V, so 20 ohms in series would drop roughly 7V and get the unreg down to a happier 29V unreg..... BUT now that resistor is dissipating power itself, (P=I^2xR) so around 2.5W (not a tiny resistor. )

Some less obvious tricks are using a smaller reservoir cap, so the higher ripple voltage would result in less average unregulated voltage.  Another circuit trick is to change to half-wave rectified (if you aren't already) The unregulated voltage will be lower again. Finally a trick I have used to scrape off some unregulated voltage is to add a series resistor before the diode charging the reservoir cap, this will drop more voltage when the cap is charging and drawing high current, for less wasted power as heat. But I can't give you an easy answer for how much resistor, just a lot less than 20 ohms.

Good luck...  It is not uncommon to use heatsinks for regulators, I've even seen them bolted to metal chassis, but look out for ground loops when you do that.

JR

 

[iampoor1]

Maybe the thermal transfer between regulator package and the chassis is not good? I have had this problem before with regulators. I use heatsink s now and save the hassle.  ;D

 

[andre tchmil]

ok , problem solved witn a series resistor in front.
Thinking about going with a SMPS next time

 

[mjrippe]

Funny stories/lessons learned:

I bought a few power supplies from a well known DIY supplier that were originally designed for Yamaha PM1000 modules (+44vdc for the channels and +48v for phantom).  Before purchasing I asked if they would be useable for some +24vdc modules I wanted to rack.  "Yes", said the seller, "there is a trim pot to adjust the 44v output".  Great!  I bought three of them and power transformers to match.

First project was a four channel Neve rack with 33114 modules.  Each draws about 100-120ma, 317 regulator is 1A, so plenty of reserve, right?  Nope.  When you are dropping nearly 30v from the raw DC it would power ONE module without catching fire, but two caused thermal shutdown after about ten seconds!  Figured out that was the problem and used a Power One +24v and a Meanwell +48v.

Next project was a pair of Urei/UA 1108 discrete preamp modules.  These only draw about 40ma so I knew two would be OK.  Just to be safe I decided to bolt the +24 regulator to the chassis.  Used a silpad for insulation and wired up the first module.  Had a 1/4A fuse on the 120vac mains, it blew immediately.  Turns out I insulated the back of the 317 regulator, then put the bolt through without a plastic washer, duh!  Corrected that and all was well.