The Adventures ofSparky LeCroy
TK Hemmick TK Hemmick for the Stony Brook Crew and Maximfor the Stony Brook Crew and Maxim
LeCroy Trip Circuit Questions
• PHENIX has long suffered from “myths” about the true functionality of the trip circuit in the LeCroy HV PS.
• We used the schematic to put the current signal on a digital scope trace.
• Triggering on single shot of the scope, we could look at the precise waveform of the current under many conditions.
Spying on the Signals
• Pins 1 & 10 of a particular Op Amp carry the “Fast” and “Slow” current signals for a channel (8 OpAmps 8 channels)
• Unfortunately these carry a lot of common mode noise as compared to our scope and a DC offset for small current surges.
Dual Sum & Invert
• Fast & Slow wires from 2 channels.
• Dual Sum & Invert makes clean signal /w adjustable DC offset.
Calibration
• Current reported by LeCroy via “VT100” interface.
• DC voltage measured by O-scope.
• Good Linearity: we know what V meansC alibration of L eC roy C irc uits
y = 72.202x - 1.8621
y = 72.835x - 1.9639
0
20
40
60
80
100
120
140
160
0 0.5 1 1.5 2 2.5
Volta g e on S c ope (V)
I (m
kA)
s low
fas t
L inear (s low)
L inear (fas t)
Fast vs Slow
• “Sparking” resistor chain with NO CAPACITORS (other than cables etc…)
• NOTE: Slow circuit = Fast output through 10 msec RC and voltage follower.
• Ring Amp a little smaller in slow circuit
Fast Slow
What do you trip on?
• By “brushing” the shorting wire, we can make MANY transients atop the signal.
• The Transients DO NOT(!!) make a trip
Trips Here
No Trip Here
It is a discharge?
• Yup…created visible, aubible spark with a wire shorting a big HV cap.
• Trips on 1st ring.
• NEVER trips with thresh above 1st ring.
NOTE: Change in scale by 10X
Best simulation circuit
• Big RC (GEM) in parallel w/ small RC (strip)
• Spark the strip with a wire
• Smaller overshoot
• Trip point basically at new DC level.
Summary
• Very little difference between “fast” and “slow” trip circuits (“Peak” is misnomer).
• Transients never cause trip.
• Real circuit has little overshoot (rest of GEM supplies most of charge for sparked strip).
• Tripping on extra current due to one shorted strip IS CORRECT!