HP 456A AC Current Probe

I wanted a simple project to take a break from working on my RCA WV-98C VTVMs. I’ve had a Hewlett Packard 456A AC Current Probe sitting around for a while. 

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The 456A is a clip-on AC current probe. It is comprised of a clip-on probe and a current-to-voltage amplifier and converter. It has a ±2% accuracy bandwidth of 100 Hz to 3 MHz and a 3 dB bandwidth of 25 Hz to 20 MHz. Its dynamic range is from “below 1/2 mA" up to 1 A RMS. The current-to-voltage conversion ratio is 1 mV per mA. It can be used with a scope or an AC voltmeter such as the -hp- 400D.

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The 456A was introduced in the July/August 1960 issue of the HP Journal and first appeared in the 1960 HP catalog.

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A scan of the 456A manual is available at the HP Archive.

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The probe itself is similar if not identical to the current probe in the HP 428A DC Clip-on Milliammeter, which was introduced in the June/July 1958 issue of the HP Journal. HP later produced an AC version as a plug-in for the HP 150A oscilloscope. This was introduced in the May/June 1959 issue of the HP Journal.

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The 456A was powered by two Mallory TR 233R and one TR 234 mercury batteries. Mercury batteries are no longer available due to environmental concerns. They are a bit of a problem to replace, as they are an odd size and an odd voltage. The TR 234 battery supplies +5.33 volts to the circuit and the two TR 233R batteries supply -8.0 volts.

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An AC power supply was available as an option, but mine came with the battery supply. The AC supply raises the noise floor from less than 50 µA on batteries to 100 µA on AC power.

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The circuit is pretty interesting. It uses two OC170 germanium transistors. These apparently suffer from the internal growth of tin whiskers that short out the transistor.

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I checked out the passive components, and all were fine except for C2, which was leaky, and C6, which had a high ESR, presumably because it had dried out. I replaced those two electrolytic capacitors and powered the circuit from my HP 6227B dual power supply. It functioned fine, so the transistors are still good. It’s just a matter of time, though, before they get tin-whisker disease. 

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To accommodate the optional AC supply, the battery holders are mounted on a removable plate, as is the AC power supply. 

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I removed the battery holders and designed a power supply using two modern 9 Volt batteries

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I used a single Linear Technology LT1120A micropower regulator in the supply. I built the circuit dead-bug style on a piece of PC board cut to the same size as the battery mounting plate. I soldered a couple of nickel-plated nuts to the board in lieu of real captive nuts.

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I measured the power draw at about 4.5 mA on both the +5.33 and -8 Volt rails. I didn’t measure the draw on the 0 Volt rail, but it can’t be more than a 100 µA or so. A post on the internet purporting to relay information from David DiGiacomo says it’s 20 µA. Rather than use two LT1120As, I took advantage of the data-sheet application information to use the comparator as an op-amp to generate the zero Volt rail.

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One trick I used was to rewire the power switch to switch the connection between the negative battery voltage and the -8 Volt circuit common for the regulator.

© Steve Byan 2011-2019