Current-sensing resistors in the diode current driver
-----------------------------------------------------

The low-noise Jan Hall current driver relies on a resistor in series
with the diode.  The voltage across this resistor provides the
feedback for the current control circuit.  Changes in this resistance
will change the diode current, for example due to temperature
fluctuations.

Our lasers run at:

   SDL	   50mA
   Sanyo   100mA (new batch)
   Sanyo   130mA (old batch)
   Max	   200mA

Max current, 25 ohms = 1 Watt.

Nominally we use:
  Vishay VPR221, either 2x50ohm or 1x25ohm.  
  Power 8W, TC 5ppm/deg C.
  Price is approx $30 each but there's a minimum 
  order quantity of about $500.
  Supplied by Mayer-Krieg, (08) 8291 3240 fax (08) 8291 3200


   Current	Delta-T	    Tempco      Delta I
     mA		 deg C    (ppm/deg C)     uA
   ---------------------------------------------
   100mA	50 C	      3           15
   100mA	50 C	      5           25
   100mA	50 C	      50          250

   100mA	10 C	      3           3
   100mA	10 C	      5           5
   100mA	10 C	      50          50
   
Alternative resistor suppliers
------------------------------

Farnell
-------
Notes: 
a) TC = temp coeff in ppm/degC
b) Page numbers are from year 2000 catalogues

Part#	Page	Description	TC 	Power	Value	Price ea
---------------------------------------------------------------------
341-903	1726	Rhopont 8E16D	3	0.33W	20R	11.91
341-915						50R

Other choices:
309-860	1718	Vishay S102J	0.6	0.6W	100R	30.65

544-206	1719	Multicomp	50	0.5W	24R	0.126
544-280						51R

335-540	1720	BC		50	0.6W	24R	0.137
335-629						51R

643-038	1721	Meggit MCP	100	5W	20R	2.09
643-040						50R

342-294	1726	Welwyn W21	75	2.5W	24R	1.59
342-350						50R

344-450	1731	Welwyn WH	50	10W	27R	3.65
344-497						50R

306-3616 1732	Vishay SH25	50	25W	22R	5.67
306-3628					47R

RS
--
217-3166 681    Neohm UPW50     3	0.5W	50R	7.34
	 682    Welwyn W21      75	2.5W    27R	1.14

We will go for two of the Neohm from RS, cost $14.68 a pair.