Re: Problem changing from power supply to battery

OK - I'm getting close to my wit's end here.
I put 2 x 1N4004's in series with the +6.4V - they dropped the voltage to
4.7VDC - but the circuit still didn't work. I tried just a single 1N4004 -
voltage was 5.4VDC and circuit still didn't work.

It's not a problem with isolating the ground pin as Andy suggested - the
power supply I used for the testing was seperate to the PC which is
connected to the circuit via LPT port.
The two power sources are totally unrelated, I can still get the circuit to
work by connecting to the +5VDC of the benchtop power supply (which is a PC
power supply scavenged from a PC case)

Any help much appreciated.


"Kyle Winters" <kyle@xxxxxxxxxxxxx> wrote in message
> Ed,
> Thanks for the advice. I measured the voltage drop when directly powering
> the relay - it was 0.01V difference, so I'm guessing that's OK. I will
> pick up some 1N4004's tomorrow and let you know how I go.
> Thanks again,
> Kyle.
> "ehsjr" <ehsjr@xxxxxxxxxxxxxxxx> wrote in message
> news:ArpCe.2691$xe3.1845@xxxxxxxxxxx
>> Kyle Winters wrote:
>>> Hello all,
>>> I am having a problem moving my circuit from a power supply to a
>>> battery. The circuit (I can provide more details if necessary) is
>>> basically some 74LSxx IC's which are turned on/off via signal from a PC.
>>> The outputs control the state of relays which allow motors to be driven
>>> via 12V.
>>> To date, I have been doing all my testing using a PC power supply -
>>> 12VDC for the motors and 6VDC for the circuit - and this worked
>>> perfectly. I (wrongly) assumed it would be a simple matter to change
>>> from the power supply to a battery for the power source.
>>> I sourced two SLA batteries (1 x 12VDC, 1 x 6VDC) to replace the power
>>> supply but the circuit no longer works. The 12V for the motors is
>>> working fine. When I run the circuit from the battery, the relays never
>>> switch.
>>> The circuit is drawing 150mA @ 6V.
>>> I can switch from the battery back to the power supply and everything
>>> works OK, so I know the devices in the circuit are still working fine.
>>> But this is about the extent of my troubleshooting knowledge. I get the
>>> feeling I am missing something basic. I thought maybe the 6.4VDC being
>>> output by the 6V SLA battery was too high, so I put a 5V voltage
>>> regulator in which dropped it to 4.4VDC (I still haven't figured out why
>>> so low - but that's another story!) and it still doesn't work. If I
>>> leave the voltage regulator in, I can still run fine from the power
>>> supply.
>>> I thought it may be drawing too much current for the battery, but that
>>> doesn't really make sense to me - it's only 150mA. The details of the
>>> battery are (I am copying this straight off the battery case):
>>> 6V 2.5AH
>>> Cycle Use: 7.25-7.45V
>>> Standby Use: 6.8-6.9V
>>> Initial Current: Less than 1.2A
>>> Before I start blundering around blindly with the "insufficient current"
>>> theory, is there anyone who could have a stab at this for me and tell me
>>> why I can't just switch from power supply to battery?
>>> Many thanks,
>>> Kyle.
>> You want to run the IC at 4.5 to 5.5 volts. Put a couple 1N400x
>> diodes in series with the battery. Each diode will drop about
>> .6 volts. Forget the 7805 - it's a needless expense, and won't
>> work at 6.4 volts in. You need about 3 volts above the reg's
>> nominal output - or in this case, about 8 volts - on the input.
>> Next, if it still doesn't work, charge the battery properly. Then,
>> use it directly to operate one of the relays, and measure the
>> voltage across the battery with the relay energized by it. The
>> voltage should not drop much. If it does, the battery is no good.
>> Ed