" bravely wrote to "All" (15 Dec 04 21:53:56)
--- on the heady topic of "Philips PM3055/Fluke PM3055 Scope, cold start =
problems."

ro From:


ro When the Unite is cold the overvoltage SCR will fire, replaced the
ro SCR, you can also blow freeze on the SCR and it will latch.

ro The over voltage circuity is on the other end of the board, and all
ro voltages are OK.

ro Could be that the DV/DT specs of the SCR are to low or the SCR specs
ro change when its cold, or the inverter is running to fast

ro Has a look at some specs and this is funny as this problem could only
ro happen when the SCR get hot..


ro This Scope is not mine and has been sent to Philips many times, but
ro they gave up on it in the end.


ro There are 4 Diodes been added around the SCR that are not shown on the
ro circuit, may be that was a Philips attempt to fix it.


ro SCR BT151 is a low gate sensitivity & DV/DT 200v/us

ro Also sold as a Fluke PM3055


ro Any one know of any mods for this, or the frequency of the PSU
ro Inverter.

ro Thanks

You might try modifying the circuit by adding a snubber network across
the SCR's anode to cathode, (physically close to the device to
minimize inductive effects). This consists of a low value resistor
(with diode) and capacitor in series but the proper values depend on
the amount of current involved. As long as the product Rload x C is
greater than your 200v/us. Where Rload is approximately the applied
voltage divided by current, the resistor R is to discharge C safely,
etc...

: o +V
: |
: Rload
: |
: +----+--D|--.
: a | `--R---|
: SCR |
: g o-/| k C
: _|_ _|_
: /// ///

A*s*i*m*o*v

.... The current limits placed are based on resistance