content/LPD422FM.rst

=============================
 Lambda LPD 422 FM Bench PSU
=============================

:Author: David Douard
:Category: Electronics
:Tags: test equipment, PSU

This post is a quick presentation and teardown of a small bench power
supply I bought on ebay a few weeks ago for something like 40€.

.. image:: {filename}/images/lambda/lpd422fm_overall.jpg
   :alt: The Lambda LPD 422 FM dual regulated power supply.

It's an old unit, not sure it's exact age, but the components inside
the unit seem to have date codes around 1971/1972. So my unit is
probably from the late 1972 (older than me!). This series of power
supplies seem to have been built for a very long period, since one of
the available user manual on the internet seems to be dated from 1987
(but it's for the "A" version).

Overview
========

It's a dual regulated linear power supply which specifications are:

- voltage range: 0V to 40V
- curent regulation: 0 to 1A current limitation
- voltage regulation: 0.01% + 1mV
- ripple and noise: 500 µVrms, 1.5mVpp


It seems to be "remote programmable", and have remote sensor entries
on the back of the unit.

.. image:: {filename}/images/lambda/lpd422fm_back.jpg
   :alt: Rear view of the LPD 422 FM dual regulated power supply.

Inside the enclosure:

.. image:: {filename}/images/lambda/lpd422fm_top.jpg
   :alt: Top view of the LPD 422 FM dual regulated power supply.

Each of the 2 power supplies have a regulator board on each side of
the enclosure. The power transformer is sitting at the center of the
enclosure and shows a nice "Component Location Diagram":

.. image:: {filename}/images/lambda/lpd422fm_xformer.jpg
   :alt: The power transformer of the LPD 422 FM dual regulated power supply.

A regulator board looks like:

.. image:: {filename}/images/lambda/lpd422fm_board2.jpg
   :alt: Top view of one regulator of the LPD 422 FM dual regulated
         power supply.

.. image:: {filename}/images/lambda/lpd422fm_board1.jpg
   :alt: Bottom view of one regulator of the LPD 422 FM dual regulated
         power supply.
	 
The component, especially the big caps, looks like original parts and
the PCB is a bit burnt by the small power transistor, but nothing too
scary.

The power supply works nice. No smoky or smelly behaviour on load, and
the regulation looks still reasonnable.


Available documentations
========================

- `Lambda LP, LPD & LPT series catalog
  <{filename}/pdfs/lambda/LAMBDA_LPD_LPD_LPT_series.pdf>`_

- `Instruction and Service Manual for Lambda LPD suffix A Series Power
  Supplies <{filename}/pdfs/lambda/LambdaLPD.pdf>`_: this is not
  exactly the same model, but it really very close.


Quick mesurements
=================

In these quick measurements, I've not used the voltage remote sensing
of the power supply.

First, a view of the noise at the output of the PSU; 0V, no load:

.. image:: {filename}/images/lambda/DS1Z_QuickPrint2.png
   :alt: Noise level of the LPD422FM - 0V/0A
      
Same at 20V, no load:

.. image:: {filename}/images/lambda/DS1Z_QuickPrint3.png
   :alt: Noise level of the LPD422FM - 20V/0A

And at maximum voltage (40V, no load):

.. image:: {filename}/images/lambda/DS1Z_QuickPrint4.png
   :alt: Noise level of the LPD422FM - 40V/0A

Then, with the `ZPB30A1 electronic load <{filename}/ZPB30A1.rst>`
connected but not consuming any current (20V, 0A):

.. image:: {filename}/images/lambda/DS1Z_QuickPrint5.png
   :alt: Noise level of the LPD422FM connected to the ZPB30A1 - 20V/0A

As you can see, there is a much higher injected by the electronic
load. When measuring the visible pattern:

.. image:: {filename}/images/lambda/DS1Z_QuickPrint6.png
   :alt: Noise level of the LPD422FM connected to the ZPB30A1 - 20V/0A

it's a 20ms pattern, ie. it's the mains 50Hz being injected... Let's
turn the load on (20V, 0.5A):

.. image:: {filename}/images/lambda/DS1Z_QuickPrint7.png
   :alt: Noise level of the LPD422FM connected to the ZPB30A1 - 20V/0.5A

The injected noise from the mains has vanished, and the RMS noise
level is back to the same value as it ws with no load (around 40mV).

At 1 amp, it's roughly the same:

.. image:: {filename}/images/lambda/DS1Z_QuickPrint8.png
   :alt: Noise level of the LPD422FM connected to the ZPB30A1 - 20V/1A

Now, trying to capture the transient curve when activating and
deactivating the load. As I cannot remote control the load, I cannot
create a nice square load pattern which make this measurement much
easier. So I've just adjusted the trigger level to single capture the
moment I press the 'on/off' button of the load:

.. image:: {filename}/images/lambda/DS1Z_QuickPrint10.png
   :alt: Transient response of the LPD422FM connected to the ZPB30A1 - 20V/1A -> 0A

.. image:: {filename}/images/lambda/DS1Z_QuickPrint11.png
   :alt: Transient response of the LPD422FM connected to the ZPB30A1 - 20V/0A -> 1A

The overshoot is around 75mV and takes around 200ms to vanish, and the
undershoot is a bit less (50mV) and lasts 150ms.

At one amp, the ripple voltage is around 8mVpp and 3mVrms:

.. image:: {filename}/images/lambda/DS1Z_QuickPrint13.png
   :alt: Ripple voltage of the LPD422FM connected to the ZPB30A1 - 20V/1A

It's quite out of spec: ripple should be less than 500µVrms and
1.5mVpp, but it's not really a surprise considering the age (over 40
years old!) of the PSU. A future recap may improve its performances
back to its original specifictions.

I also plan to replace the banana jacks on the front panel (their
barrels are very short, less than 1cm: I cannot plug a standard banana
plug more than half its lenght). I may also try to bring the remote
sensor plugs on the front panel (with a pair of easy to set or remove
jumps): I like to have the sense terminals ready to use.