LM7824 Only 52mA but Hot??

Discussion in 'The Projects Forum' started by tigerapna, Apr 20, 2012.

Dec 14, 2011
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Hello,

I measure only 52mA draw from the 7824 Regulator but It gets too hot to even touch for 2 seconds.

I chose Linear versus Switching because of simplicity during PCB layout design.

I read articles where people were drawing 500 - 800 mA and it should get hot but at 52mA?

Any ideas or just slab a huge heat sink on it?

I tested it over 4+ hours and thermal switch didn't kick in. (normal operation is 1.5 hours)

As always, Thank you for your help!

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2. panic mode Senior Member

Oct 10, 2011
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what is the voltage before it?

i see cap is labelled 35V but that's rather low for 24VAC input to bridge (no elbow room for transients, this could be 'ticking bomb' literally).

if the voltage across the capacitor is indeed 35V, then regulator would have to drop 11V (35-24=11V).
current is Iout+Iq=52+8=60mA

Iq is quescient current and for 78xx is usually about 8mA

11V*0.060A=0.66W

given small size of TO220 that will feel hot...
btw. "hot" is anything above 40degC. most people can't bear holding something that is 45degC which is really not much for electronics.

Apr 5, 2008
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Hello,

Do you use a heatsink on the regulator?
The thermal resistance without a heatsink is rather high:

This means that the temperature will rise 50 °C/Watt for a standard TO220.
In your case the dissipation is about 0.6 Watt so a 30 °C temperature rise (above the ambient temperature).

Bertus

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4. wayneh Expert

Sep 9, 2010
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You're close to the maximum input voltage for the regulator. Maybe a problem.
[Oops, too slow]

Dec 14, 2011
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The maximum inpuit voltage for the regulator is around 40V. I measure around 28AC from the transformer. Is that too high for the input?

Dec 14, 2011
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I am not using a heatsink yet because I didn't think I needed one with drawing only 52mA.

Dec 14, 2011
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I will bump up the smoothing cap to 50V.

Is there anything else I can do to improve this design?

8. paulktreg Distinguished Member

Jun 2, 2008
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This will equate to approximately 40VDC (28 x 1.414) into the regulator which is a little high so reduce this if you can?

9. panic mode Senior Member

Oct 10, 2011
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if you are reading 28VAC, this is way too high because (28*1.41-2*0.7)=38VDC into smoothing cap and regulator. this is exceeding rating of capacitor and it is on very edge of what 7824 can handle safely.

rest looks ok....

btw, why are you measuring current in 24V circuit?
since 24V is only used to power relays, and derive 5V for MCU, i see no need to regulate 24V. i'd probably just save both 7824 and amp-meter and pick transformer with 18V secondary (16-20V will do just fine). then 35V cap can stay.

Dec 14, 2011
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I was just showing in the schematic where I was measuring current to see how much my circuit draws.

Don't I need to regulate the 24V for the PN2222A Collector current?

I have to use the 24vac transformer for another application. Are you saying to pick up a secondary transformer to step down for 5v?

11. panic mode Senior Member

Oct 10, 2011
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i didn't know about something else requiring 24VAC.

this is why i posted idea about reduced transformer voltage.
you circuit draws very little current. this means that capacitor after bridge charges to peak DC voltage. this is approximately 40% above measured AC voltage. if your transformer was 18VAC, then you would get about 24VDC across smoothing capacitor and hence there would be no need for 7824 (if PCB is made, you could just solder jumper in place of it).
your BJTs and relays do not require regulated supply but your PIC does. but there is another regulator (7805) that steps down the voltage for PIC so you are good.

If your transformer provides 28VAC instead of 24VAC, then your smoothing capacitor after bridge will charge to 39V. as mentioned this is border line of safe range for 7824 and it exceeds rating of the capacitor.
if you intend to keep transformer, consider replacing that capacitor with 50 or 63V version. You may want to put a small heatsink on 7824.

tigerapna likes this.

Dec 14, 2011
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If I change the cap rating to 50V+ will it still not pose a threat to my 7824?

I have 2200 uF 50V caps in stock. Can use them for smoothing the 28vac?

Should I just add a small 18vac transformer like this http://www.digikey.ca/product-detail/en/3FS-336/MT2226-ND/285613
rectify and smooth for this whole circuit? (input from 110vac)

13. wayneh Expert

Sep 9, 2010
11,916
2,854
The cap is not providing a voltage-limiting protection for the 7824, so these are two different issues.
Yup. You could add a diode or two in line with the DC supply to the 7824. This will reduce voltage input by about 0.7v for each one you add.

14. panic mode Senior Member

Oct 10, 2011
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298
wayneh is spot on. you can use higher voltage capacitor, that is not an issue. voltage rating of capacitor is simply maximum voltage capacitor can work with safely. this in itself does not pose any risk to 7824, issue is that you are powering the circuit with higher voltage than you should. since the current is low, and board is already made (i think i've read something like that), the most convenient way to reduce voltage before 7824 is probably to use series resistor with one of AC lines of the bridge (insert resistor between transformer and bridge on this board). if you use 100-150 Ohm, you will drop the voltage by couple volts. this resistor will also take over some of the heat dissipated by 7824 (about 30-50%). I would go for big power on the resistor (5 or even 10W) simply to increase it's surface so it feels less hot.

something like
http://search.digikey.com/us/en/products/1623788-1/A102451-ND/2365673

you can use it either before the the bridge or in place of ampmeter.

or, you can forget the resistor, remove the 7824 (replace with jumper) and simply use dedicated small transformer just for this board. transformer you have found looks good, you can parallel the outputs. note, when paralleling outputs, connect pins with dots together (5 connects to 7; 6 connects to 8). it is shown in datasheet too. note: if you parallel them the other way around, you will short the transformer.

Last edited: Apr 20, 2012

Dec 14, 2011
73
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Thank you so much for your help on this panic mode!

The board has not yet been made I have just bread-boarded it.

I will most likely go with the second transformer idea. But I will also test with the power resistor on the AC lines of the bridge.

I love this stuff, so interesting!

16. panic mode Senior Member

Oct 10, 2011
1,308
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cool, good luck with the project...
btw if you are using 18V transformer, 35V cap is ok (apparently it survived 40V, so 24-25V will be piece of cake )

17. atferrari AAC Fanatic!

Jan 6, 2004
2,611
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Are you sure it is not oscillating? (If so, add bigger caps) . Happened to me many times.

The same the LM386 (audio amp) does, surprising lot of people.

Dec 14, 2011
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Oscillating off the LM7842?

Bigger smoothing cap? I'm using 1200uF. I have 2200uF in stock. Was going to use those.

Dec 14, 2011
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So I added a 91 ohm 10W resistor (had in stock only) in AC line and made the CAP 2200uF 50V. Nothing heats up anymore and I checked on the scope and everything is dead on "clean" and no oscillations. Input to 7824 is 25V

Question:
When I'm running three relays the voltage on the input of the 7824 drops 2V. Anything I can do to prevent this? ?

20. wayneh Expert

Sep 9, 2010
11,916
2,854
Well, is it falling off on the regulated side also? If not, there's no problem. That resistor is taking some of the heat - ∆V - that the regulator would be otherwise, and that's a good thing.