[volt-nuts] Fall of SRAM voltage in a 3457A without external power

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Mon Jan 25 06:46:19 EST 2016

I decided to try a little experiment on my 3457A, When on mains the SRAM
gets 4.8 ~ 5 V. This does not seem to be well regulated, so I assume
depends on mains voltage. Once power is removed, the voltage on the SRAM
stays well above the battery voltage for some considerable time, which I
assume is due to a decoupling capacitor. My 10 M Ohm input Z multimeter is
loading the circuit too much to continuously monitor the voltage, but a few
checks indicated the voltage across the SRAM is falling quite slowly.
Starting at 4.8 V from mains power, after 23 minutes of no mains power, the
voltage on the SRAM was at 3.4V, which is above the battery voltage (3.03
V). and well above the 2.0 V needed to hold the SRAM contents. Assuming the
SRAM takes a constant current one would expect the voltage to fall linearly
with time. If so, it would take 46 minutes to fall to 2.0 V even without
battery power.

ESD and leakage of the human body would probably make screw this up, so I'm
not suggesting replacing the battery that way if you want to preserve the
contents of the SRAM, but there's a fairly good chance the contents would
remain in RAM if one was reasonably quick, especially if you topped the
voltage up from the mains just before removing it from the chassis.

Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
Registered in England and Wales, company number 08914892.
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)

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