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LM1085 VReg Overheating

12/06/2016 12:30 AM

I have a programmable (using a Arduino MEGA) 32x32 RGB LED matrix:

that I want to play with. Power for it is 5VDC and up to 4A, so I am using a 5V LM1085 LDO vreg which is supplied by a 19.5V, 6.7A laptop power supply.

The program I am running on it now is only taking 0.5A, but the vreg is getting very hot.

My question is; why is it overheating at 0.5A when it can easily supply 3A? It is attached to a heat sink with heat transfer paste. I have checked all my connections and they are correct and solid.

Here is a schematic of what I have:

It isn't visibly apparent, but there is a 4-way (electrical) junction with: the load, the capacitor, the IC Ground pin, and the negative output of the laptop PS.

Thanks to anyone that can help

Mike

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#1

Re: LM1085 VReg Overheating

12/06/2016 6:28 AM

The power that the regulator has to get rid of is equal to the voltage drop x the current.

In this case that's (19.5-5) x 0.5 = 7.25W.

Multiplying that by the heatsink rating (degrees C/Watt) will give the (approximate) temperature rise above ambient.

Don't know what heatsink you're using, so can't comment further, but one of the little PCB mounting jobs is about 7ºC/W, which would give a running temperature of about 72ºC, which is pretty dam' warm!

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#8
In reply to #1

Re: LM1085 VReg Overheating

12/07/2016 10:01 PM

Nailed it.

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#2

Re: LM1085 VReg Overheating

12/06/2016 6:49 AM

That is right, with such a high voltage drop the regulator should be used with very low output currents.

A practical solution would be to use a power supply with lower voltage. If you do not have a lab power supply, you can use the PSU of any PC, sold at 10$ online.

If you still want to use your linear regulator, put 5x1.5V Ni batteries in series (total of 7.5V) at the input of the regulator and you will see a big difference in temperature.

In this particular case, since you need 5V, I would use a typical USB charger, from a USB charger connected to the AC mains or one of the ports of your computer. Old equipment chargers can also be handy.

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#3

Re: LM1085 VReg Overheating

12/06/2016 9:31 AM

Why are you using a linear regulator for running a digital circuit? A linear regulator will have to dissipate about three times as much heat as your load with these voltages.

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#4

Re: LM1085 VReg Overheating

12/06/2016 3:15 PM

As others have pointed out: you're using a linear regulator, that means that you are wasting all the power from 19½V down to 5V, and of course it's the regulator which has to get rid of that power.

You'd be better off just buying an AC/DC 5V 4A supply. You should be able to get one for about $15.

http://cpc.farnell.com/ideal-power/25hk-ab-050a400-d5-1/psu-desktop-5v-4a-3-pin-iec/dp/PW03335

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#5

Re: LM1085 VReg Overheating

12/06/2016 4:22 PM

Thanks everyone for the replies! I suspected the large input vs output voltage difference was the reason, but needed some qualified EEs to tell me. Linear LDO applications have dwindled with the appearance of more efficient switching supply designs. But it was what I had at hand.

I had already selected a 5V 5A switcher from DigiKey for ~$20 and so will now order it.

Thanks everyone!

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#6

Re: LM1085 VReg Overheating

12/07/2016 2:42 AM

I prefer to let you work this out for yourself. Between the o/p of the psu and the o/p of the regulator is 14V that you need getting rid of. The only way the regulator can get rid of voltage drop is with heat/Watts and it does that by increasing current to 0V volt reference leg and that cause internal heat that will dammage and destroy the regulator. What can one use that you can choose yourself in size and wattage that is specific made to drop voltage and where you can use the Very Important Rule to get the size and then the Weer(again in my first language) Very Important to get the wattage to do the calculation. Get one of those as only cents and drop the voltage over it to the prefered Voltage of the regulator. Do not drop the total 14,5V, but read the Data sheet for the regulator for the preferred supply.

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#7

Re: LM1085 VReg Overheating

12/07/2016 3:39 AM

Using a linear PS chip and such a high input voltage is the problem as a few have pointed out.

I simply do not use Linear chips for anything, I use only SMPS power supplies of which there are plenty on tebay for example.....

If building a power supply, I use chips which although technically cannot be called SMPS chips, they work internally in a similar way, but use a lower DC input voltage, so they don't "bite" when you least expect it!!

The one I use a digital mainly will handle up to 5 amps, 10 with 2 in parallel, can handle input voltages up to 60VDC and give a clean output up to 42VDC. All you need as a front end is a cheap transformer, a full wave rectifier and a cap for smoothing.....

But really, its probably cheaper to buy a 5 amp 5 volt SMPS off ebay.....though I enjoy the building as well.

I looked on ebay USA and you could buy many different "things", that would get you going, but see what you think!:-

Adjustable-10A-DC-DC-CC-CV-Step-down-Power-Supply-12v-5v-24v-LCD-volt-amp-meter

5-volt-30-amp-power-supply-LED

DC-Voltage-Regulator-Converter-Constant-Volt-Amp-Step-Down-1-25-32V-12V-5A-75W

Some of them have variable voltage and variable over current protection, a useful thing to have! They do need a cheap transformer, bridge and cap as I mentioned before, but they might give you a few good ideas.....but all are cheap!

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#9

Re: LM1085 VReg Overheating

12/07/2016 10:47 PM

It is dissipating 7 watts and this is a lot of heat.
You need a 10cm x 5cm heat sink for this amount of watts, TO START WITH.

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#10

Re: LM1085 VReg Overheating

12/15/2016 1:23 PM

A nice little trick I have used is to put a Zener diode in series between your power supply and the regulator. You still have to be careful in selecting that component because its voltage times the current equals the Watts that it has to be able to burn off, but it removes the heat problem from the regulator. A bunch of normal diodes in series will do the same thing. Each one drops the voltage to the regulator by about 0.6 to 0.7 volts so it may require 12 to 14 in series (as long as they can stand the 0.5 Amps).

Still, it is better to step the AC power down a bit more so that you don't have to waste energy as heat.

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#11
In reply to #10

Re: LM1085 VReg Overheating

12/15/2016 2:32 PM

A good old and useful trick!!

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