Can L7805CV replace LM2940-5? - UK Vintage Radio Repair and Restoration Discussion Forum

'LIVEWIRE?' · 9th Mar 2013, 11:29 am

One of the latter type is faulty in a Ferguson FRG-R121D, and, noting that Maplin stock the L7805 in a TO-220 package, I thought 'that will do!', then I noticed that, according to Maplin the maximum i/p voltage for the L7805 is 8V. In the FRG-R121D the regulator is fed from 12vDC. The STC Data sheet for the L7805 specifies a max. i/p of 35v, so who is right? The LM2940-5 is a 1.5v TO-220 case device rated @ 1.5A, whilst the L7805 is rated @2A Max, so, AFAIK it should be a suitable replacement. Can someone confirm, this, please?

Nickthedentist · 9th Mar 2013, 12:07 pm

Quote:

Originally Posted by 'LIVEWIRE?'

according to Maplin the maximum i/p voltage for the L7805 is 8V.

Minimum input voltage, more like!

dinkydi · 9th Mar 2013, 12:52 pm

There is unlikely to be any problem interchanging these parts.

The LM2940 has a lower dropout voltage (1V versus 2.5V), but this is not an issue here. The realistic maximum output current of both is about 1A. Most important the pinouts are identical. As Nick notes the Maplin voltage spec is in error.

Peter

'LIVEWIRE?' · 9th Mar 2013, 7:20 pm

A small correction is needed here. The regulator listed by Maplin is actually a L78S05CV, but the data I quoted in my OP is correct according to their website. I will most likely get a L78S05CV and fit it. As a matter of interest the original regulator had developed low resistance between the I/P & O/P pins, and was dragging the +12v unregulated supply down to <8v, though the O/P still measured +5V on a 20k/v Analogue Meter.

dinkydi · 10th Mar 2013, 1:46 am

The L78S05 is the 2A version of the L7805. To work at higher currents the dropout voltage increases (3V for 1.5A).

Peter

9th Mar 2013, 11:29 am	#1
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Can L7805CV replace LM2940-5? One of the latter type is faulty in a Ferguson FRG-R121D, and, noting that Maplin stock the L7805 in a TO-220 package, I thought 'that will do!', then I noticed that, according to Maplin the maximum i/p voltage for the L7805 is 8V. In the FRG-R121D the regulator is fed from 12vDC. The STC Data sheet for the L7805 specifies a max. i/p of 35v, so who is right? The LM2940-5 is a 1.5v TO-220 case device rated @ 1.5A, whilst the L7805 is rated @2A Max, so, AFAIK it should be a suitable replacement. Can someone confirm, this, please?

9th Mar 2013, 12:52 pm	#3
dinkydi Retired Dormant Member Join Date: Feb 2011 Location: Sydney, Australia Posts: 277	Re: Can L7805CV replace LM2940-5? There is unlikely to be any problem interchanging these parts. The LM2940 has a lower dropout voltage (1V versus 2.5V), but this is not an issue here. The realistic maximum output current of both is about 1A. Most important the pinouts are identical. As Nick notes the Maplin voltage spec is in error. Peter

9th Mar 2013, 7:20 pm	#4
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Can L7805CV replace LM2940-5? A small correction is needed here. The regulator listed by Maplin is actually a L78S05CV, but the data I quoted in my OP is correct according to their website. I will most likely get a L78S05CV and fit it. As a matter of interest the original regulator had developed low resistance between the I/P & O/P pins, and was dragging the +12v unregulated supply down to <8v, though the O/P still measured +5V on a 20k/v Analogue Meter.

10th Mar 2013, 1:46 am	#5
dinkydi Retired Dormant Member Join Date: Feb 2011 Location: Sydney, Australia Posts: 277	Re: Can L7805CV replace LM2940-5? The L78S05 is the 2A version of the L7805. To work at higher currents the dropout voltage increases (3V for 1.5A). Peter