Quick Q re LM7805 fault condition? - UK Vintage Radio Repair and Restoration Discussion Forum

Al (astral highway) · 5th Sep 2014, 7:41 pm

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

G6Tanuki · 5th Sep 2014, 7:54 pm

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.

Al (astral highway) · 5th Sep 2014, 7:57 pm

Ah, thanks G6Tanuki... I thought it might be something along these lines. Easily remedied, then! Cheers,

Al

glowinganode · 5th Sep 2014, 8:46 pm

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.

Al (astral highway) · 6th Sep 2014, 12:21 pm

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!

Al (astral highway) · 6th Sep 2014, 4:43 pm

Quote:

Originally Posted by G6Tanuki

An onboard "+5V present" indicator on each board is always reassuring.

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.

kalee20 · 6th Sep 2014, 5:03 pm

Quote:

Originally Posted by G6Tanuki

They need *something* drawing a load for them to regulate against.

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!

Al (astral highway) · 6th Sep 2014, 5:14 pm

Quote:

Originally Posted by kalee20

..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!

Yes, that would have been handy! I was really foxed by the 6.68V to start with and thought I had a manufacturing dud on my hands for my logic power supply.

All's good now, both regulators doing exactly what they should.

Be interested to hear what you discover..!!

Maarten · 10th Sep 2014, 3:37 am

What brand regulators did you use? The problem is new to me, but possibly I have always avoided it by accident.

5th Sep 2014, 7:41 pm	#1
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	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 __________________ Al

5th Sep 2014, 7:54 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	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.

5th Sep 2014, 7:57 pm	#3
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Quick Q re LM7805 fault condition? Ah, thanks G6Tanuki... I thought it might be something along these lines. Easily remedied, then! Cheers, Al __________________ Al

5th Sep 2014, 8:46 pm	#4
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	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. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

6th Sep 2014, 12:21 pm	#5
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	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! __________________ Al

10th Sep 2014, 3:37 am	#9
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	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.