Problem changing from power supply to battery

[Kyle Winters]

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.

 

[The Real Andy]

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.

Without seeing the actual circuit, its hard to say, but is there any
chance that perhaps your batteries are isolating a ground pin???

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.

The problem with most of the common variety of voltage regulators (i
am assuming you are using a 7805) is that require some overhead to
work. This is called drop-out voltage. Most need between 1-2 volts
before they will start regulating. What you need is a low-dropout
regulator.

 

[Kyle Winters]

Andy,
I don't know quite what you mean by "isolating a ground pin", but I don't
think so. Something I didn't mention in my previous post is that I can
still measure the correct voltage across the appropriate pins(6.4V without
the regulator, 4.4V with the regulator) - this is what is confusing me -
everything _seems_ OK.

Thanks for the info on the regulator, would a Zener didoe work better for
this situation? From my (limited) knowledge, it would perform the same
function at a lower overhead?

Thanks,

Kyle.

 

[ehsjr]

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

 

[The Real Andy]

Andy,
I don't know quite what you mean by "isolating a ground pin", but I don't
think so. Something I didn't mention in my previous post is that I can
still measure the correct voltage across the appropriate pins(6.4V without
the regulator, 4.4V with the regulator) - this is what is confusing me -
everything _seems_ OK.

When I say isolating a ground pin, i mean is there a chance that when
connected to the pc power supply, you may be grounding the logic chips
with that rather than a signal ground. Make sure that the ground from
the PC port is connected to ground of the battery.

Thanks for the info on the regulator, would a Zener didoe work better for
this situation? From my (limited) knowledge, it would perform the same
function at a lower overhead?

This would work fine, or as another poster suggested, try a couple of
1n4004's or similar in series with the battery.

Thanks,

Kyle.

Everytime I reply to this thread my floppy drive goes nuts...Bizarre..

 

[Kyle Winters]

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.

 

[Kyle Winters]

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.

 

[Andrew Leech]

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.

I'm inclined to think it will be due to the grounds not being linked.
The separate computer power supplies (one inside the computer and the
one used to power your circuit) often have their grounds linked through
the mains power, whereas the battery is completely isolated.
Try connecting the negative of your battery to the shield around the
cable running to the computer.

Andrew

 

[Kyle Winters]

Andrew,
Thankyou. Spot on. I have connected the two grounds and everything is
working perfectly.

Many, many thanks.

PS - I don't know if you and "The Real Andy" are the same person, but he
also tried to tell me the same thing I think, so thanks as well.

Kyle.