PDP-11/70 Boards

Tony Duell ard.p850ug1 at gmail.com
Wed Dec 8 22:25:55 CST 2021

On Wed, Dec 8, 2021 at 9:58 PM Rob Jarratt via cctalk
<cctalk at classiccmp.org> wrote:

> > I have a variac rated at 2.5A. I am not quite sure whether that would be
> > sufficient to test the bricks individually. Otherwise, I guess one of these
> > would do the trick https://cpc.farnell.com/block/steu250-48/transformer-
> > 250va-230-400v-2-x/dp/TF01418?st=24v%20transformer ?
> So, to supply the bricks on the bench, would a variac rated at 2.5A be OK? I am not sure I know how much current the bricks will draw at 20VAC, and at what voltage the 2.5A rating is given. Otherwise, would this do the trick? https://cpc.farnell.com/block/steu250-48/transformer-250va-230-400v-2-x/dp/TF01418?st=24v%20transformer

Most Variacs do not provide isolation between the input and output,
that is they are variable _autotransformers_. So if you use one to
test a power brick, the latter is directly connected to the mains
supply. Connecting an earthed 'scope to any point in said power brick
will then blow fuses at least. Touching any point in the power brick
could give you a nasty shock.

The power bricks draw very little current with no load on the output
and 2.5A would be fine to power one up.

Most, if not all, of the  power bricks start by rectifying and
smoothing the AC input. As a result they will run off a DC input too.
You can test them using a bench supply,  normally do.

One final thing. Do not, (as I did when younger and even more foolish)
disable the over-current trip in a power brick. If the crowbar
operates, you will end up with transistors blown off the PCB.


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