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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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5th Sep 2014, 7:41 pm | #1 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Quick Q re LM7805 fault condition?
Are completely new LM7805s known to fail not under load?
I have a brand new one in exactly the usual configuration with correct decoupling and the output voltage under no load is weirdly 6.67V ?? Cheers, Al
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Al |
5th Sep 2014, 7:54 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,996
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Re: Quick Q re LM7805 fault condition?
They need *something* drawing a load for them to regulate against. Whenever I've used these regulators I've always provided this by way of a LED and suitable series-resistor. An onboard "+5V present" indicator on each board is always reassuring.
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5th Sep 2014, 7:57 pm | #3 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
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Re: Quick Q re LM7805 fault condition?
Ah, thanks G6Tanuki... I thought it might be something along these lines. Easily remedied, then! Cheers,
Al
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Al |
5th Sep 2014, 8:46 pm | #4 |
Octode
Join Date: Apr 2009
Location: Aylesbury, Buckinghamshire, UK.
Posts: 1,522
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Re: Quick Q re LM7805 fault condition?
Be sure to decouple the input and output with a 0.1 uF capacitor, as these regulators can self oscillate up into the MHz's without.
Rob.
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6th Sep 2014, 12:21 pm | #5 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
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Re: Quick Q re LM7805 fault condition?
Hey Rob, very interesting! I knew about the decoupling and I've decoupled both input ans output --didn't know these things could oscillate at such a high f though!
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Al |
6th Sep 2014, 4:43 pm | #6 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
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Re: Quick Q re LM7805 fault condition?
Quote:
True say! Thanks so much -- adding this tiny load sorted out the problem. Surprised this only affected LM705, not LM715, for some reason! Mods please close thread.
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Al |
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6th Sep 2014, 5:03 pm | #7 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
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Re: Quick Q re LM7805 fault condition?
I've never had a problem here. Some regulators (eg LM317) definitely do need a minimum load, but the ADJ pin on this is designed for minimum current. The 7805, on the other hand, for which the third pin is intended to be connected to 0V, could easily have been designed with an internal current sink to provide a minimum load, progressively turning off as external load current increased. I'm surprised this wasn't done - i will have to check the small print!
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6th Sep 2014, 5:14 pm | #8 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
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Re: Quick Q re LM7805 fault condition?
Quote:
All's good now, both regulators doing exactly what they should. Be interested to hear what you discover..!!
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Al |
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10th Sep 2014, 3:37 am | #9 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
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Re: Quick Q re LM7805 fault condition?
What brand regulators did you use? The problem is new to me, but possibly I have always avoided it by accident.
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