[time-nuts] need power trans. for HP3325

[Dr Bruce Griffiths]

Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>   
>> Didier Juges wrote:
>>   
>>     
>>> Even though you may have to be careful about noise, and assuming the 
>>> operating voltages are not exotic, you can probably find an 
>>> off-the-shelf switching supply that will fit where the transformer and 
>>> linear supplies were, with lots of room to spare, and significantly 
>>> reduced heat dissipation. I am thinking about doing that with the 5370, 
>>> considering how hot the rear heat sink runs.
>>>
>>> Didier KO4BB
>>>
>>>   
>>>     
>>>       
>> Didier
>>
>> Replacing the HP5370 power supply with switching regulators will be a 
>> somewhat challenging exercise in ensuring that switching regulator noise 
>> doesn't degrade the 5370 performance. Even verifying this will be 
>> challenging.  If you don't burn yourself on the external heatsink there 
>> should be no problem with reliability of the linear regulators.
>>
>> Bruce
>>
>>   
>>     
> Bruce,
>
> I am not too concerned about reliability. Power transistors are designed 
> to run hot. I am concerned about burning myself on the heat sink (it 
> measures well over 60 degree C here), and overall power consumption and 
> heat dissipation in the shack, because the instrument is typically 
> running a lot of time. I have installed it on top of the short rack I 
> made for my main instruments to maximize air flow, but as a result, the 
> heat sink is quite exposed. Also, my shack is above the garage, and 
> therefore the air conditioning is not too effective there (3 walls, 
> ceiling and floor face non A/C spaces).
>
> I have not looked in detail, but I suspect the voltage drop across the 
> linear regulators is a little too high and probably could be reduced.
>
> I may also look at simply putting a switching pre-regulator in front of 
> the existing linear regulators (or at least the worst offender, I am 
> sure the 5V supply must be loaded pretty well) to reduce the voltage 
> drop and the heat, while keeping the good regulation and low noise of 
> the linears. I have a box of LT1074 integrated switching regulators 
> which require only a handfull of components to make a complete switching 
> supply.
>
> A small fan would address the high temperature, but would not help with 
> power consumption and dissipation. That would be an easy fix though.
>
> Didier
>
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>   
Didier

Try using a Murata BNX002 filter between the switching supply output and 
the linear regulator input.
These should eliminate the High frequency noise (0.5MHz - 1GHz) from the 
switching regulator output leaving the linear regulator to deal with the 
low frequency noise.
However for these to be effective a low impedance ground plane is required.

Worst case linear regulator design allowing for low mains input and some 
margin for the ripple from the rectifier certainly increases the series 
pass element dissipation considerably over that when the input to the 
regulator can be held within 0.1V or so with low ripple.

The traditional preregulator employs SCRs in a phse controlled rectifier 
arrangement, the resulting low frequency output noise from such 
preregulators is easily dealt with by the linear regulator.

Bruce