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!