Author Topic: Let me make sure I got this. Right with 7805  (Read 1387 times)

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Offline unicoderTopic starter

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Let me make sure I got this. Right with 7805
« on: May 10, 2009, 09:55:51 PM »
Im im pushing the 7805s current limit. I should branch off and make another 7805 source circuit yeah? It there anything else liek a 7805 with a higher current rating?

Im not sure and Id like suggestions. If i need to push a heavy load ill probly just push the current across a relay or contactor. I just need to find some higher amp'd regulators.
« Last Edit: May 10, 2009, 09:58:50 PM by unicoder »
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Offline SmAsH

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Re: Let me make sure I got this. Right with 7805
« Reply #1 on: May 11, 2009, 12:32:33 AM »
having them in parallel does work, and a heatsink can help too.
Howdy

Offline Soeren

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Re: Let me make sure I got this. Right with 7805
« Reply #2 on: May 12, 2009, 04:37:58 PM »
Hi,

Im im pushing the 7805s current limit. I should branch off and make another 7805 source circuit yeah?
7805 comes in various versions up to 2A.


It there anything else liek a 7805 with a higher current rating?
Plenty, and most of them better!
Did you try a bit of googling for some?

A personal favorite of mine is MIC29752BWT, which is a variable voltage LDO regulator from Micrel, which is good for 7.5A at a drop of around 425mV.
That means that you can make do with a bit less than 5.5V in for 5V out and at a drain of 7.5A, you'd still dissipate less than 3.2W, less current equals less drop and hence less dissipation.
For comparison, an 78S05 needs to drop 3V at only 1.5A, which means that it will dissipate at least 4.5W at only 1/5 of the current.

The above is only valid if you are able to use 5.5V and 8V respective, if you use eg. 8V on both, they will dissipate the same for the same output current (Ohms and Watts laws).


Im not sure and Id like suggestions. If i need to push a heavy load ill probly just push the current across a relay or contactor. I just need to find some higher amp'd regulators.
Alternatively, you could boost your 7805 with a PNP power transistor - MJ4502 (PNP) is a golden oldie, handling almost twice of an 2N3055 (NPN, which you cannot use, but it's probably the best known "work horse" to compare with). MJ4502 used to be extremely extensive back in the seventies, but can be picked up for next to nothing these days (think I got them at 50 cents a pop last time I found some on the .net).

If you want to make a more complicated circuit with less dissipation (less wasted power), you might consider a switch mode circuit, but it's not a beginners project.
If you choose to take that route, I'd recommend the switchers from LT, as they are fairly simple to work with (compared to similar chips from other manufacturers) and their Switcher CAD (SPICE) is free and can be used for other stuff (if you believe in sims).

Finally, you could make a linear regulator yourself, based around an op-amp and a couple of transistors.

Paralleling single voltage linear regulators is a bad thing to do, as they have a nominal tolerance of around +/-0.25V, so you might end up with one giving 4.75V in parallel with one giving 5.25V, which means they will fight each other for half a volt, instead of supplying the power to your circuit(s).

Options sorted from easy to hard(ish).
 - More powerfull linear regulator.
 - Boosted linear regulator.
 - Regulator made from discrete components.
 - Switched mode regulator.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline protoadv

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Re: Let me make sure I got this. Right with 7805
« Reply #3 on: May 12, 2009, 07:18:23 PM »
What are you trying to control?  Should like you might be going a bit overkill.  Is it motor?  Here's a like to a 7.5A 5V reg on digikey:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=576-2254-ND

Hope this helps.

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