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Topic: Why choose LM7805 over mini DC-DC Converter? (Read 1 time) previous topic - next topic

Bjerknez

Apr 01, 2020, 05:56 am Last Edit: Apr 01, 2020, 05:57 am by Bjerknez
I'm just wondering. What is the basis for choosing, for example, an LM7805 over a mini DC-DC converter?

There are probably some good reasons for economics and commercial use, but I don't quite see why someone like me should choose an LM7805.

Yesterday I tried to run a so-called load test of the LM7805 and a mini DC-DC converter (Buck converter). The mini converter could withstand at least three times the load and did not get as hot as the LM7805.

It may well be that the way I ran the load test is not entirely after the book, but my point is just that the Mini converter is much more usable than I first thought and it is not much bigger than the LM7805.

This is the Buck converter I have:

https://www.aliexpress.com/item/32261885063.html?spm=a2g0o.productlist.0.0.6ff2f15bQh2UdS&algo_pvid=5a4691dc-a593-4310-8844-826565171121&algo_expid=5a4691dc-a593-4310-8844-826565171121-6&btsid=0ab6f8ad15857132490614801e932b&ws_ab_test= searchweb0_0, searchweb201602_, searchweb201603_


Smajdalf

DC-DC converter adds (electrical) noise that may be undesirable for the application.
Sometimes it can even produce audible noise.

LM7805 (or some other linear regulator) may be considered more reliable. DC-DC converter are quite complicated. You need to trust the Chinese seller that  it will work as intended in any combination of conditions that may happen. The are little specifications of such modules from eBay/AliExpress and even those cannot be trusted. They often claim many amps output current which is far from reality.

For some protects with low power requirements a linear regulator may be more efficient because of the quiescent current.


Bjerknez

I see.

My home project draws around 3-400mA as total and my LM7805 with a little heatsink goes wery hot. I cant even touch it. Since i'm going to plase it inside a box i'm afraid heat is an issue.

My little DC-DC converter is almost the same size and with 2 amp load and 9V in i cant feel any high temperature and i can touch it without problem.

When i tested the 7805 and tried a load test, i never gets over 450Ma. 400mA is good, but it gets VERY hot.

But as jo said, the LM7805 is maybe more suitable for lover current circuits.

Thanks for answer btw :)

wolframore

#3
Apr 01, 2020, 06:36 am Last Edit: Apr 01, 2020, 06:36 am by wolframore
Linear convertors are favored in high end audio where switch mode noise is of concern.  Same in many circuit where you just can't have the noise whether it's audible or on sensitive circuitry.  I don't like the way the switching converters whine sometimes in some units,

Switch mode if designed for the right situation can be beneficial for battery life and can pass a lot of current as you've noticed.  Linear circuits can be designed to do same.

Think of it as tools. You don't throw away one tool because just got a different one... nothing is free and sometimes you trade off one benefit for another.
Bad Boys Race Our Young Girls But Violet Generally Wins - Get Some Now :) - ELI ICE man

Bjerknez

I see. So for my home projects thas has nothing to do with audio, the mini converter is the best choice?

wvmarle

I'm just wondering. What is the basis for choosing, for example, an LM7805 over a mini DC-DC converter?
1. most important: current requirement. Over 500 mA a buck converter; under 50 mA linear regulator.
2. second most important: required voltage drop. 5V to 3.3V drop or 12V to 5V drop makes a world of difference. In the first case I'm happy to use a linear regulator up to a couple hundred mA, in the second case no more than 80-100 mA.
3. breadboard space. Linear regulators are much smaller.
4. cost. Linear regulators are much cheaper (not that a buck converter is expensive, I normally pay something around 1 USD for a 2-3A model, but linear regulators are cheaper).
5. battery life. Here it gets a bit tricky: a linear regulator is far less efficient, but a buck converter has much higher quiescent current - by a few orders of magnitude, typically. So for an application that is most of the time idle and requires the lowest possible idle current, a linear regulator will win hands down - unless you pick a 7805, get an LDO. For an application that's most of the time active and drawing significant current, the better efficiency of the buck converter will win. But when doing battery very often you can get away without regulators, for even better efficiency.

Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MarkT

I see. So for my home projects thas has nothing to do with audio, the mini converter is the best choice?
Yes unless its dealing with a fast low level analog signals from a sensor and precision is important, switching
noise/hash from a switch-mode converter might add interference to the readings.  For low bandwidth you'd
be low-pass filtering anyway.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

Paul__B

3. breadboard space. Linear regulators are much smaller.
Only for less than one watt.  :smiley-roll:

MarkT

I see. So for my home projects thas has nothing to do with audio, the mini converter is the best choice?
If you are doing RF stuff too, supply noise is very bad news and you'd use linear regulators exclusively.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

wvmarle

Only for less than one watt.  :smiley-roll:
True. Though when running into that kind of dissipation the final decision would most likely have been made in steps 1 or 2 already :-)
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

westfw

You didn't used to able to buy (or build) a mini switching dc/dc converter for a reasonable price...
Even now, you can pretty much only get poorly-specified modules from somewhat questionable vendors smuggled past customs, which will be a problem for any "real manufacturer "...

MarkT

You didn't used to able to buy (or build) a mini switching dc/dc converter for a reasonable price...
Even now, you can pretty much only get poorly-specified modules from somewhat questionable vendors smuggled past customs, which will be a problem for any "real manufacturer "...

https://uk.farnell.com/xp-power/tr10s05/dc-dc-converter-5v-1a/dp/2708277
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

CrossRoads

Can also look at 3.3V, 5V output DC/DC converters from Murata Oki at Digikey, $4.30 USD and 7805 size & pinout.

Or little DC/DC converters from Pololu, size about comparable to a 7805.

Both offer cool running switching regulators where 7805 heat & inefficiency are a concern.
The Murata parts switch at 500 KHz, any noise on the DC output should be inaudible as audio.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Smajdalf

How much 7805 costs? I guess much less than $4. The user is the one who pays the electricity bill, you don't care. More heat = failure sooner. For any real manufacturer this is another advantage unless it fails too soon.

wolframore

Bad Boys Race Our Young Girls But Violet Generally Wins - Get Some Now :) - ELI ICE man

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