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


Relevant Pages

  • Re: PWM a truck alternator
    ... In the circuit I've designed, the frequency varies with duty cycle -- it ... holding the system voltage at a measly 14.2 volts. ... 15V might damage your battery. ...
  • Re: Faraday "magnetic" induction without a magnetic field.
    ... 1mA through the circuit. ... result of an induced voltage around the circuit. ... ends located at different point modeling an EMF where the ends are at ... source i.e a battery in this case. ...
  • Re: rewiring an emergency light with tubes for LED strips
    ... 220v AC and when power goes out, the batt kicks in and lights the tubes, simple huh? ... My main goal would be keeping the case, the battery, and as much of the ... I have disassembled the unit and found that the circuit board is just ... I've also measured the voltage of the wires leading to the batteries, ...
  • Re: battery charger topology
    ... My battery charging knowledge is limited, but I know a bit about SMPSU ... to give unity PFC and constant voltage ... normally have to build into the design. ... Should I parallel up boost circuit or go to duel-phase for higher ...
  • Re: Faraday "magnetic" induction without a magnetic field.
    ... I already pointed out that instead of 2 resistors with two taps as ... If you consider the circuit as having a string of n ... resistors- all but two being 0, then the total voltage across these two ... would be distributed with the battery in circuit- even if non-linear. ...