Battery Top Power Suppy

© Brooke Clarke 2005

Fixed Output Voltage	Adjustable Output Voltage	BTPS on 10 AA holder

In the last few weeks I've had the need for a small battery type power supply to power equipment either at 5.0 Volts or a nearby voltage.  Regulation is needed so just using raw batteries is not an option.

By using the 9 Volt battery snaps, like are part of the 68BA battery adapter, and a Printed Circuit Board (PCB) that the same size as the top of a 9 Volt battery you get a small low cost power supply.  For this application where the current draw is way below 100 ma, using the TO-92 packaged voltage regulators is the way to go, both for cost and for board area reasons.  The TO-92 takes up less board space than a SOIC surface mount package.

When working with prototype boards the use of the BTPS allows saving the holes for your circuit rather than using the proto board for a power supply.

It turns out that there are a number of TO-92 packaged fixed output voltage regulators and so far they all have the same pinout so that a single PCB will work for all of them.  The table below is based on the offering by National Semiconductor, but I'm sure that are many companies that offer TO-92 packaged voltage regulators.  Some of these may have a price - performance point different from the NSC parts.

Single 9 Volt Battery

Model	Voltage	Imax	Iq typ1	Vin min	Vnom -1% Measured 100 Ohms	1k load	100 load	47 load
	Volts	mA	uA	Volts
LM2936Z-5.0	5.0	50	9	5.20	5.14	5.025	5.002	0
LM2936Z-3.0	3.0	50	15	3.20
LM2936Z-3.3	3.3	50	15	3.50
LP2950CZ-3.0	3.0	100	75	3.45	3.27	3.000	3.000	3.001
LP2950CZ-3.3	3.3	100	75	3.75
LP2950CZ-5.0	5.0	100	75	5.45
LP2950ACZ-3.0	3.0	100	75	3.45
LP2950ACZ-3.3	3.3	100	75	3.75
LP2950ACZ-5.0	5.0	100	75	5.45	5.31	5.002	4.999	5.004
LM2931Z-5.0	5.0	100	400	5.3	4.93	4.929	4.929	4.957
LM2931AZ-5.0	5.0	100	400	5.3
LM78L62ACZ	6.2	100	2000	7.9
LM78L05ACZ	5.0	100	3000	6.7	6.56	4.975	4.974	4.969
LM140LA-5	5.0	100	3000	7.0
LM340LA-5	5.0	100	3000	7.0

Note ¹ - Table arranged in order of Iq. which turns out to in 1/$.

Summary of Key Specs

LM2936 - lowest Iq  Good choice for battery powered device that pulls less than 50 ma and is on all the time.  Also good for use as current source where the minimum current (Iq) needs to be considerably higher than the minimum regulated current.  Regulation at low currents not as good as others.  Low drop out voltage.  Highest price ($1.85).  Only choose this one if you need the Iq.

LM2950 - Slightly more Iq but you get 100 ma drive capability and low drop out voltage.  The output voltage seems to be more precise than the other regulators.  Very good regulation at all currents tested. (2950A: $0.69, 2050: $0.80)  The "A" version has tighter specs on output voltage and a lower price.

LM2931 - much more Iq than the 2936 and 100 ma drive with lowest drop out voltage. (2931A: $0.49)

LM78L05 - very high Iq not recommended for battery power apps.  100 ma drive.  Low price. ($0.27)

Multiple 9 Volt Batteries

Model	Voltage	Imax	Iq	Vin min
	6.2	100
78L09ACZ	9.0	100
78L12CZ	12.0	100
	15.0	100

Adjustable Output Voltage

By using the LM317L regulator an adjustable output power supply can be made.  With a single 9 Volt battery and stock value resistors you can get 1.25 to 7.7 volts, settable to about 1 millivolt when a 25 turn pot is used.

Note ¹ - can not set to 1 mv like for higher voltages without some effort.  Best to use fixed type regulator for these low voltages or use different pot and fixed resistor.
Note ² - The spec sheet says 50 to 100 uA so the 4 ma measured may be a problem with the HP E3617A power supply current metering.

Note² - In a couple of cases there is a few tenths of a volt difference in the regulated output and the spent battery voltage, but this just means you give up a little battery capacity when using these combinations of Vreg and # of batteries.

    AA Battery Holders w/9 Volt snaps

    Wall Wart w/9 Volt snaps

ALS-208

Converting the AccuLab ALS-208 into a stand alone Hall Effect magnetic field probe.
The fixed voltage BTPS has an LM2936-5 and a Radio Shack 275-624 SPST micromini switch connected to the BTPS.
Brown wire = +5 volts in. Green wire is ground.  White wire is output.  The DIN plug was removed and the brown wire in the cable disconnected at both ends.

Range	DMM
+/- 2 G	2.3 to 2.9
+/- 20 G	2.06 to 2.10
+/- 200 G	2.005

There's some offset and slope factors that need to be determined.  When used in the Accu-Lab system the software applies the offset and slope factors and reads out directly in gauss.  There are 3 pots inside the ALS-208.  One seems related to some external input and the other two seem to be coarse and fine adjustments of the Hall sensor drive voltage based on an LM385-1.2 Zener type voltage standard.

Sanity check on +/-2 range.  If zero field output was 2.5 volts and the range is -2 to 0 to +2 gauss and the supply is 5 volts then a rail to rail output would be 0 to 2.5 to 5.0 volts.  The theoretical slope would be 1.15 volts per gauss.  The Earth's field would be + and - 0.6 volts from 2.5 or a range of 1.9 to 4.1 volts.  The actual range is more like 0.6 volts total for the Earth's field so the actual slope is closer to 0.3 v / 0.5 g = 600 mv / g.

Then the probe is placed into the center of a demagnatizer and the DMM is in the AC volts mode, the reading goes up to about 1.7 volts rms.  It seems that this probe can be used for either AC or DC fields.

If you have technical info on the ALS-208 please let me know.
The Hall Sensor is an Allegro A3503 which is now obsolete.  The A1302 is the recomended replacement replacement and has better specs.  The A1301 (2.5 mv/G) has twice the sensivity of the A1302 (1.3 mv/G).

FLC-100

Powering the FLC-100 Fluxgate magnetometer.  A couple of pair of CAT5 LAN cable has been pulled out of the jacket and the Orange pair used for 5 volts and the blue pair used for the DC output.  Zero volts out for no field and a scale factor of 1 v / 50 uT.  Current use is for testing zero magnetic shielding cans.  This sensor has an advantage over others in that there is a defined output for no field.  The FGM-3 type sensor has a continuos change of frequency as the field changes, but the frequency for no field needs to be determined by the user (not a simple task).

Battery Top Signal Generator -

Should this be made into a product?  If so, sold as:

• assembled and tested
• kit of parts including PCB with snaps installed
• just PCB with snaps installed?
• There are a number of different PCBs possible:

Fixed Output	Adjustable Output
1 Batt	1 Batt
2 Batt	2 Batt
3 Batt	3 Batt
4 Batt	4 Batt

It might make sense to only have the 2 Single Battery PCBs and the 2 Quad Battery PCBs made and have provision for a jumper on the Quad board to allow 2, 3 or 4 batteries to be installed?  But a jumpered 4 battery PCB takes up space that a smaller board would not.

Let me know your thoughts.