15-12-2016, 09:42 PM
From what I remember the -5V rail was quite lightly loaded. Unfortuantely we'd already done the PCB before I realised that a pair of 7905 would have been better. Glad I remembered the site for the resistor. I think we were already using the wallwart for another design so it made sense to use the same one again.
The problem with a 2 diode 2 cap circuit for -5V is that I think I'd have to have used a half wave rectifier for +5V, something I would rather not do.
As is often the case, there's more than one way to do a design. Some ways are better than others. I'd be a liar if I said all my professional designs were as good as they could be - I've made my share of mistakes and some of them have gone into production. Nobody died.
Once there was very nearly a lethal problem on one of my deiigns. Not my fault at all but still scary. I'd specified a mains transformer (something like a 12+12 30VA toroid) and the client found they could get a good deal from a smaller compnay whose main business was custom wound parts. One day their technician got a tingle from the metal case. It SHOULD have been earthed but for odd reasons it wasn't. A whole batch of these transformers had faulty insulation. So much for the transformer manufacturer having proper test procedures in place. Fortuantely none of the kit had been shipped to customers yet. While my client would have been first in line for the public liability claim if there had been a nasty accident, some flak could easliy have come in my direction too.
The problem with a 2 diode 2 cap circuit for -5V is that I think I'd have to have used a half wave rectifier for +5V, something I would rather not do.
As is often the case, there's more than one way to do a design. Some ways are better than others. I'd be a liar if I said all my professional designs were as good as they could be - I've made my share of mistakes and some of them have gone into production. Nobody died.
Once there was very nearly a lethal problem on one of my deiigns. Not my fault at all but still scary. I'd specified a mains transformer (something like a 12+12 30VA toroid) and the client found they could get a good deal from a smaller compnay whose main business was custom wound parts. One day their technician got a tingle from the metal case. It SHOULD have been earthed but for odd reasons it wasn't. A whole batch of these transformers had faulty insulation. So much for the transformer manufacturer having proper test procedures in place. Fortuantely none of the kit had been shipped to customers yet. While my client would have been first in line for the public liability claim if there had been a nasty accident, some flak could easliy have come in my direction too.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
