[time-nuts] Low noise power supply recommendations

Wed Jun 15 04:14:52 UTC 2011

John,

I will check it out, and may do some experiments myself on these. Also, I
will take a look at the photo-FET's, as I had forgotten about those.

What has me wondering is how neon bulbs act in the circuit, their low
brightness, and their drop out times, as I think the on voltage is around
90 volts or so, but the square wave going to them is around 100-115 volts
if I recall. I thought about using a simple 10:1 resistive divider, then
using a series resistor from that junction going to the LED, the same as
for a 10-15 volt supply. The neon bulbs light goes through two Lucite tubes
to the CDS cells, and it couldn't be too bright by the time it reached
them.

I also thought about using a new form of chopper, as Paul mentions, but
making it fit and work could get complicated. ESI quit using the HP 419 in
the last models of their 801 DC supply and detector-null meter, and started
using a Keithly 155. I either figured it was over this very thing, or HP
dropped the 419 from its line. An engineer at Vishay told me that they quit
using the Fluke over this neon problem, and went to the Keithly in the last
versions of this bridge.

I'm going to be using the bridge not only as it was intended, but to do
other null measurements, as I added a circuit to use the meter circuit
seperatly from the bridge.

Thanks,

Will

*********** REPLY SEPARATOR  ***********

On 6/14/2011 at 8:13 PM J. Forster wrote:

>Hi Will,
>
>I don't know. Check the HP Group archives. I only vaguely followed the
>thread. I'm not so sure a phototransistor will work without circuit mods.
>A photoFET might.
>
>Best,
>
>-John
>
>===============
>
>
>> John,
>>
>> I wondered about that, as using a LED is generally in conjunction with a
>> photo-transistor, and not a CDS cell. The reason I thought it might work
>> is
>> that a company years back used them together to form a safety light
>> curtain.
>>
>> The Fluke, and the HP, had a bad rep for those neon bulbs going out and
>> having to be replaced. I am going to replace the ones in the 844 before
I
>> button it up, but was wondering if something else could be done. I
looked
>> up the life cycle for the NE-3 and it's kind of low, especially compared
>> to
>> a LED. I imagine the problem has to do with the brightness of the LED,
as
>> compared to a neon bulb, when using a CDS cell.
>>
>> As far as the power supply is concerned, I think I am going to go with
>> Ni-Cad batteries, and regulate the voltages. I think what they had was
>> nothing more than four step voltages from the battery supply, going from
>> 3,
>> 6 (7), 12, and 24 Vdc, or X2 of the other. From what I saw in the
article
>> earlier, an easy zener with emotter follower regulator should do the
trick
>> by the comparison with batteries. They used some resistance in series
with
>> the zener to reduce noise, but it did decrease the stability somewhat. I
>> have seen this used in some old bias regulation circuits for tubes years
>> ago.
>>
>> As far as the noise, I also wondered about this, as ESI used a current
>> limited DC power source to do the same thing, and it was ran off the AC
>> line.
>>
>> Thanks,
>>
>> Will
>>
>> *********** REPLY SEPARATOR  ***********
>>
>> On 6/14/2011 at 7:14 PM J. Forster wrote:
>>
>>>The reason I asked is that most microvolt bridges use choppers and have
>>> BP
>>>filters at the chopper frequency, so noise is largely uncorrelated.
>>>
>>>On the neon photochoppers, it has been discussed at length several times
>>>on the HP-Agilent Yahyoo Group. A similar thing is used in the 410C.
>>> There
>>>are apparently issues with the CdS cells.
>>>
>>>Best,
>>>
>>>-John
>>>
>>>===============
>>>
>>>
>>>> John,
>>>>
>>>> No, it's DC. I'm trying to meet the original specs of the batteries in
>> not
>>>> only voltage/current, but cleanliness of the current.
>>>>
>>>> Now inside the Fluke 844, it has a chopper, and it uses an AC power
>> supply
>>>> internally to feed its circuitry. The Vishays bridge uses the
batteries
>>>> voltage, and one small AC power supply to run the digital portion of
>>>> the
>>>> bridge for the PPM readout. The bridge itself is half analog and half
>>>> digital. The analog for the measurement, and the digital for the PPM
>>>> difference.
>>>>
>>>> By the way, has anyone ever converted a neon NE-3 driven chopper to
>> using
>>>> LED's? The bulbs have a sqaure wave coming into them of around 100-200
>> Hz
>>>> if I recall.
>>>>
>>>> Thanks,
>>>>
>>>> Will
>>>>
>>>> *********** REPLY SEPARATOR  ***********
>>>>
>>>> On 6/14/2011 at 6:50 PM J. Forster wrote:
>>>>
>>>>>Is the bridge excitation AC or DC. If it's AC, you may not even need
>>>>>low-noise power supplies.
>>>>>
>>>>>-John
>>>>>
>>>>>==============
>>>>>
>>>>>
>>>>>
>>>>>> Hi Will,
>>>>>>
>>>>>> You could consider building your own power supply system for the
>>>>>> replacement of
>>>>>> the batteries.  Use a separate power transformer which, I suspect,
>>>>>> would
>>>>>> be small
>>>>>> as the current requirements would be low for driving a bridge
>>>>>> circuit.
>>>>>>
>>>>>> The main company to look toward for high quality, low noise
>>>>>> regulators
>>>> is
>>>>>> Linear
>>>>>> Technology.  Here is a candidate part to look at :
>>>>>> http://www.linear.com/product/LT3082
>>>>>>
>>>>>> This is the lowest noise (33uv/10Hz to 100KHz) low dropout regulator
>>>> that
>>>>>> can
>>>>>> handle regulating 24 volts and is adjustable, meaning use can use
>>>>>> this
>>>>>> part for
>>>>>> all three voltages (i.e., three regulators).  You will have to
decide
>>>>>> if
>>>>>> it noise
>>>>>> specs are suitable for your needs.  Keep in mind that battery noise
>>>>>> is
>>>>>> typically
>>>>>> random whereas the noise out of a regulated system tends to be
>>>>>> constant.
>>>>>>
>>>>>> If you really think you need lower noise then you could consider an
>>>> LT1000
>>>>>> shunt
>>>>>> reference.  Typically the circuit designs for LT1000 type devices
are
>>>>>> as
>>>>>> references and lack current capability above about 10 milliamps.
>>>>>> Adding
>>>>>> additional current circuitry would add more noise.  See:
>>>>>>
>>>>>> http://www.linear.com/product/LTZ1000
>>>>>>
>>>>>>
>>>>>> Bill....WB6BNQ
>>>>>>
>>>>>>
>>>>>> Will Matney wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> I am in need of a very clean 24 Vdc power source, to replace some
>>>>>>> old
>>>>>>> mercury cell batteries with. What would a good low noise, clean,
>> power
>>>>>>> supply be in your recommendations? I thought of using two 12 Vdc
>>>>>>> lead-acid
>>>>>>> batteries in series, and making a charging circuit with regulators,
>>>>>>> but
>>>>>>> I
>>>>>>> am hoping to purchase a good used supply off ebay, etc. The old
>>>>>>> circuit
>>>>>>> used two 12 Vdc snap terminal mercury batteries in series, for 24
>> Vdc,
>>>>>>> along with a 7 Vdc mercury cell, and two plain AA carbon 1.5 Vdc
>>>>>>> batteries
>>>>>>> in series for 3 volts. Any help and or ideas would be appreciated.
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>> Will
>>>>>>>
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>>>>>
>>>>>
>>>>>
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