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Power supply for a Canon camera


almcl

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A question for any electronics gurus, please?

I made up a circuit, readily available on the web, using an LM7808 to convert 13.8 volts from either my stabilised mains power
supply or from a 12 volt accumulator to 8 volts to use with a Canon dummy battery in my Canon 700d.

It worked nicely, delivering 7.9 volts and the camera is quite happy with that.

Went to try it out last week and after about 20 minutes or so everything stopped as the 15 amp power supply (which also powers
the mount and the camera cooler) cut out.  Further investigation showed that the small plastic case in which the LM7808 is
mounted with a couple of capacitors on a strip of Veroboard was too hot to touch. So was the supply cable. Investigating with a
remote infra red thermometer suggests that the LM7808 reaches 50°C within a minute or so of switching the camera on. I haven't measured the current draw, lacking a suitable spot to break into the supply cable, but guess this might be the problem?

If so, is this to be expected when dropping 13.8 v to 7.9 with about (I estimate) a maximum 2 amp demand?  I haven't used a heat sink
as my hope was to keep everything small and light such that the whole unit could simply hang on the cable.

I do have another LM7808 if it is possible that the original is defective, but would appreciate more knowledgeable input before
putting my pretty limited desoldering ability to the test.

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Linear regulators like the LM7808 do need to be used with a heatsink that ideally has free air movement around it so putting it into a small box is not ideal, there are a few options that would work.

a) Mount the IC inside an aluminium case with the heatsink tab connected to the metal (think the tab is connected to ground but check or use an electrical insulating washer etc).

b) Use a switching regulator (normally called buck converters) as these have greater efficiency and dont  get hot and are available with built in volt meters if needed. 

Alan  

Edited by Alien 13
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I bought a mains power supply with dummy battery, chucked all the innards out and replaced with a buck converter, the unfinished bit is a crowbar circuit just in case :grin:

Dave

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13 minutes ago, Davey-T said:

I bought a mains power supply with dummy battery, chucked all the innards out and replaced with a buck converter, the unfinished bit is a crowbar circuit just in case :grin:

Dave

Canon-power-supply.thumb.png.517d6c8f90939551756271cd59b4e204.png

I do like those self contained converters that have screw terminals and a voltmeter.

Alan

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Thanks all!

Looks like the buck converter is the way to go and I do have an almost scrap case of exactly the sort Davey-T shows.  The screw terminals and voltmeter are good, too.

Can you remember where it came from? I can see similar but not-quite-the-same items on ebay, but so far the terminals are proving elusive.

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17 minutes ago, almcl said:

Thanks all!

Looks like the buck converter is the way to go and I do have an almost scrap case of exactly the sort Davey-T shows.  The screw terminals and voltmeter are good, too.

Can you remember where it came from? I can see similar but not-quite-the-same items on ebay, but so far the terminals are proving elusive.

Try Amazon....

https://www.amazon.co.uk/Converter-Adjustable-Step-Down-1-25V-36V-Raspberry/dp/B01DF3JUPY/ref=sr_1_9?crid=WRKENHLSWXOJ&keywords=buck+converter&qid=1558877274&s=gateway&sprefix=buck+con%2Caps%2C744&sr=8-9

Alan

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Looks like a great idea, adding it to my ever growing shopping list

Cheaper on Ebay , search for XL4015 ( and LED if you want that option ).

Edited by Kev M
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