1. Home
  2. Electronics Design
  3. Problem changing from power supply to battery

Problem changing from power supply to battery

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.

Reply to
Kyle Winters
Loading thread data ...

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.

Reply to
Kyle Winters

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

Reply to
ehsjr

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.

Reply to
Kyle Winters

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

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.

Reply to
The Real Andy

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.

This would work fine, or as another poster suggested, try a couple of

1n4004's or similar in series with the battery.

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

Reply to
The Real Andy

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.

Reply to
Kyle Winters

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

Reply to
Andrew Leech

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.

Reply to
Kyle Winters

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.