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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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Old 12th Sep 2018, 1:38 pm   #1
mole42uk
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Default 78xx regulator with long connections?

Hello,

I'm investigating a 1970's synthesiser in which the power supply has been modified to move the four 78xx / 79xx regulators onto a larger heatsink. While I can understand the need for extra cooling, the mod uses wires of about 400mm to connect each of the three legs of the regulator back into the PCB. There does not appear to be any extra components such as damping capacitors.

I'm concerned that the regulators might be singing away to themselves or refusing to regulate within spec.

Any comments?
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Old 12th Sep 2018, 1:52 pm   #2
Andrew2
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Default Re: 78xx regulator with long connections?

I'd be tempted to add 10uF's directly at the pins. Do you have a 'scope you can have a prod with to confirm any honking?
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Old 12th Sep 2018, 1:58 pm   #3
Studio263
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Default Re: 78xx regulator with long connections?

The datasheets for these show how far the regulator can be from the power supply's reservoir capacitor before an extra capacitor is needed across the input. I pretend to be a big shot and fit them routinely anyway, why mess about?

All you need is 100nF between the input and the common pins. You can solder it to the thick parts of the legs, just where they go into the body of the IC. Nothing more, nothing less. Be careful about adding capacitors across the output, they aren't necessary and too much here will cause instability - the output of a 78xx is in some ways 'inductive' in nature. 10uF absolute tops if you must, and even that is of dubious benefit.
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Old 12th Sep 2018, 2:00 pm   #4
buggies
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Default Re: 78xx regulator with long connections?

My ruler wasn't long enough - 16 inches!
I would definitely add a 0.1uF or 1.0uF on both sides of each 78xx - maybe add a bit of veroboard if they are close enough together to keep the caps within an inch or so?
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Old 19th Sep 2018, 2:22 pm   #5
mole42uk
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Default Re: 78xx regulator with long connections?

Thanks guys, I have fitted 100nF capacitors to the output leads of the actual chips, easy enough to slip the sleeving back, solder the wire and re-instate the sleeve.
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