IC heating problem while using 24 volt smps

[shrihari]

Dear all,

Hi to all,

My query is, I am using 24 volt smps to power my board having few relays, led displays, microcontroller and other passive components. I am using 7805 to power microcontroller and 7812 to operate relays. But while in working condition 7805 and 7812 are getting to hot because out of 24 volt input 12 volt is dissipated in regulator ic. To avoid the heating of ic I have connected resistor in series to load to drop 12v across it but now resistor is getting to hot not regulator ics. I have tried resistors of different values up to 10watt, still resistor is getting to hot. My board load current is 100mA.

Please try to suggest efficient solution for my problem

 

[ramussons]

You are using a 24 volt power source to feed a setup that requires 12 volts and 5 volts.

You ARE going to dissipate a lot of heat - your choice to dissipate it in 78xx regulator or a resistor.

The solution is to get Switching Regulator(s) that gives you 5 and 12 volts output from 24 volt input.

 

[BGB]

You are using a 24 volt power source to feed a setup that requires 12 volts and 5 volts.

You ARE going to dissipate a lot of heat - your choice to dissipate it in 78xx regulator or a resistor.

The solution is to get Switching Regulator(s) that gives you 5 and 12 volts output from 24 volt input.

or, alternatively, use some heat-sinks and a fan (such as an old computer fan).

I am using a 7805 to drive a 700mA load, but this depends on the fan operating. if the fan is not running, the 7805 fairly quickly goes into thermal shutdown.


FWIW: the watt rating on a resistor does not mean that it wont get hot, but rather how much heat it can dissipate in normal conditions and not fry itself.

for example, a 10W resistor would probably get hot, but remain fairly stable, whereas a 1/4W or 1/2W resistor would likely burn up.

 

[shrihari]

thank you ramussons and BGB sir,

I appreciate with your suggestions.

Re: ramussons, As you mentioned i can use switching regulator to generate 5 volt and 12 volt from 24 volt SMPS, instead of using two switching regulator shall i use one switching regulator that can step down the 24 volt to 14 volt so that i can safely feed 14 volt to 7812 and 7805 and this will protect regulator from overheating. please mention your view regarding it.

Re: BGB, As you mentioned 10W resistor would probably get hot, but remain fairly stable. But will it affect the lifespan of my system, please mention your view regarding it.

 

[BGB]

thank you ramussons and BGB sir,

I appreciate with your suggestions.

Re: ramussons, As you mentioned i can use switching regulator to generate 5 volt and 12 volt from 24 volt SMPS, instead of using two switching regulator shall i use one switching regulator that can step down the 24 volt to 14 volt so that i can safely feed 14 volt to 7812 and 7805 and this will protect regulator from overheating. please mention your view regarding it.

Re: BGB, As you mentioned 10W resistor would probably get hot, but remain fairly stable. But will it affect the lifespan of my system, please mention your view regarding it.

usually a switching regulator would be used to produce the target voltages directly.

there are pros and cons, typically the benefit is that they are more efficient (less of the power is turned into heat), but the disadvantage of generally being more expensive, generally most often in the form of a PCB (rather than a through-hole component), and producing a bit more high-frequency noise (sometimes relevant for things like audio electronics and similar, there may often be a slight high-frequency squeal or similar in the output if not sufficiently filtered out).


a 10W resistor should be fine for a long time if you are running less than 10W.
if it is a wire-wound type, internally they are usually made out of nichrome or similar, which can handle being red-hot for extended periods of time (such as in heating elements), and presumably significantly longer when inside a ceramic case.

 

[ramussons]

thank you ramussons and BGB sir,

I appreciate with your suggestions.

Re: ramussons, As you mentioned i can use switching regulator to generate 5 volt and 12 volt from 24 volt SMPS, instead of using two switching regulator shall i use one switching regulator that can step down the 24 volt to 14 volt so that i can safely feed 14 volt to 7812 and 7805 and this will protect regulator from overheating. please mention your view regarding it.

Re: BGB, As you mentioned 10W resistor would probably get hot, but remain fairly stable. But will it affect the lifespan of my system, please mention your view regarding it.

Why a SR for 14 Volts? (doubt if it's available anyway ) Why not 12 volts itself?
Any Linear regulator WILL dissipate heat.

If your regular source is 24 Volts, it is better to get standalone SR's for 12 and 5 volts. It can be used to power other projects too, in future.

 

[Kiwi]

Can you swap the 12v relays for 24v ones?

Can you replace the 24v power supply with a 12v one?

There are fairly cheap 24v-12v reducers available for automotive applications, eg 12v stereo in 24v truck.

 

[shrihari]

Can you please suggest any low cost, widely used 5 volt and 12 volt switching regulator IC.

 

[ramussons]

Give you a lead

http://www.digibay.in/lm2577-dc-dc-adjustable-step-up-power-converter-module

Google for more

 

[shrihari]

Thanks all of you for your valuable suggestions.

I have one idea to stop heating of regulators, that is i am planning to use three regulator ICs that will be connected in series. first is 7818, 7812 and 7805, the concept behind this is maximum input voltage for 7818 is 35 volt and for 7812 is 19 volt and for 7805 it is 10 volt. I hope this logic may work theoretically, please give your suggestion about how this idea is effective.

I don't want to change the relay with 24 voltage because it will not stop the heating problem of 7805.
I don't want to use switching regulator IC as it produces high frequency noise and also not readily available.
I cant change the SMPS because it is already integrated with the system for which i am designing the PLC board.

Please give your suggestions for this concept.

Thanks

 

[ramussons]