[volt-nuts] [offlist] Re: Cropico DO4A Digital Ohmmeter
gandalfg8 at aol.com
gandalfg8 at aol.com
Mon Apr 30 15:41:05 EDT 2018
I'm quite sure you're right, and certainly if properly designed the pass transistor should be inside the feedback loop, but I've also learned the hard way over many years never to take anything for granted, especially when it's not my design and I'm trying to avoid stripping out the board to check the circuit.
I would have wanted to cut back the pins of any replacement anyway, in order to add flexible tails and hopefully avoid similar mechanical mishaps in the future, so a bit of time spent with some swiss files has achieved a very similar result, although perhaps given the choice I might not have cut the pins quite that short:-)
From: Charles Steinmetz <csteinmetz at yandex.com>
To: gandalfg8 <gandalfg8 at aol.com>; Discussion of precise voltage measurement <volt-nuts at febo.com>
Sent: Mon, 30 Apr 2018 14:56
Subject: [offlist] Re: Cropico DO4A Digital Ohmmeter
It is vanishingly unlikely that the parameters of any individual
transistor (in particular, Vbe and hFE) would change the calibration of
the instrument or the operation of the PSU protection circuits.
A heatsunk TO-220 is clearly the pass element, not a voltage-determining
component, of the PSU. It will be inside the error amplifier's feedback
loop, so any difference between individual transistors will have no
effect on the regulated voltage. Also, consider that the LM723 has a
fair amount of temperature and aging drift -- orders of magnitude
greater than any possible effect of changing the pass element.
To reiterate, replacing the transistor with a different sample (or a
different TO-220 part of the same gender and configuration, for that
matter) will have no effect on the calibration of the meter.
Of course, after repair it will be only as accurate as it was before the
damage, so you are still left with the problem of calibration. But you
can be secure in the knowledge that replacing the transistor will not
affect the meter calibration or the PSU protection circuits.
On 4/29/2018 4:57 PM, Nigel Clarke via volt-nuts wrote:
> Sorry, I realise now I could have explained better, I do know what transistor it is but that's not the problem, this looks to be quite a complex power suply/charger circuit, with at least three unmarked adjustment pots so if I change the device, even for the same part number, it's quite possible it will need readjustment and that's what I don't have any information on.
> An additional concern is that the whole instrument is built onto a single PCB without any obvious way of isolating the supply, so there's not too much room for error.
> I think for now, assuming the existing transistor checks out ok, I'll try my plan B option and see if I can remove enough plastic from around the broken legs to attach some flexible leads and hope the original settings hold up.
> Nigel, GM8PZR
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