Canberra 2000 NIM Bin Power Supply Question

[ChrisSmolinski]

I'm finally back to working on getting my Canberra 8076 ADC card interfaced
to the computer, and have one question regarding the Canberra 2000 NIM
bin. It has functional +/-12 and +/-24V supplies, but appears to only be wired
for 6V, without having a 6V supply. From some poking around the web, this
seems typical. So I am guessing I can just slap a 6V supply onto the back of
the chassis and wire it in, but I thought I'd ask around first, to make sure
there are not any gotchas I am overlooking. I figure I will take a 5V linear
supply and adjust it for 6V output.

I am curious, why the use of 6V? I see the 8076 board has a diode on the 6V
input to drop it down to around 5V.

[Richard Hull]

The traditional way to obtain the +/-6Volts is via 6Volt nim plug-in power supply modules which power the buss that is pre-wired on many bins. 6 volts plus and minus was a very common early transistor logic supply. This was in the days when the noise immunity band included zero. Zero volts was a logic "I don't know what it is" signal. With DTL and TTL a simple silly-con diode would put the voltage in the range to power these modern devices. Some TTL based nim stuff avoids the 6 volt requirement by hanging a 7805 regulator on the +12 line and letting over half the energy needed for the TTL go away in heat within the 7805!

One large nim module I have gets its 5 volts from the 12 volt supply efficiently using a switcher with two ICs and three transitors and a miniature toroidal xfrmr. Modern, efficient 12v to 5v switchers can now be had in the form of single block, plug-and-pray modules that approach 96% efficiency.

For some reason almost every module I have that was made by Le Croy (LRS) used the 6 volt busses to death with 1.5 amp drains on the plus and minus being rather common!!!

Richard Hull

[ChrisSmolinski]

Thanks Richard, I had completely forgotten about the pre-TTL logic! I was
considering a switcher, but I have concerns over EMI generated by them.
Perhaps with adequate shielding... I'm using an external 5V bench supply for
the 8076 ADC NIM now, I'll have to measure the required current and see if I
have anything laying around.

On a somewhat related note, I have a 100G resistor connected to my
Keithley 610 electrometer now, turning it into a very nice high impedance
voltmeter. I still need to find a place to mount it inside and add a jack for the
voltage input. I run the low voltage output from the Keithley into a DVM for
reading. It's not super accurate but it meets my needs. It's nice to have a
voltmeter that I know won't load down the circuit under test

[Richard Hull]

A Keithly 610 series already has a gang of those ultra-high ohm resistors in it on the range switch!! You can select your input impedance on any keithley electrometer by using the current range positions. ( see the owners manual or the Keithley low measurement handbook.) The highest input impedance is seen on the voltage ranges.

Is yours a 610 A, B, or C? I have about (8), 610 models on hand here with many more of their electrometers, 614s, 600s, etc. I have about 30 keithley items in all here.

The normal input impedance is on the order of a 100 petaohms or better in some tube units! A 100 gigohm is a near dead short!

For the Nim issue, if I needed 5 volts desparately in a nim module, I would just drop in a 7805 on the +12 volt buss and be done with it, letting the module aluminum chassis be my heat sink.

Richard Hull

[ChrisSmolinski]

It's a 610R, the rack mount version. My understanding is that you can't feed
more than 100V into the 610 in voltage mode. So I am using the resistor in
series, as a crude voltage to current converter, then using the 610 in current
mode.

My plan was to use a 7805 if the current requirements are OK; you're right,
the chassis will make a dandy heatsink!

[Richard Hull]

That is correct, you just can't push more than 100 volts into any Keithley electrometer product. You are measuring HV the way they recommend, using the current range with an auxiliary input resistor.

I actually have a rather rare keithley differential impedance selector box for use with their electrometers which is loaded with teflon wafered range switches and a plethora of the precision vacuum resistors.

Measurement on the edge is tough.

Richard Hull