Undesired voltage drop using a solenoid

[pirator]

Hello, I use two maxim ICM7250 programmable timer/counter triggering one another to recycle a solenoid valve on and off. This 7250 is neat and serve perfect for programmable timers as they have digital inputs for the time count rather than fighting RC networks. Check them out

http://pdfserv.maxim-ic.com/arpdf/ICM7240-ICM7260.pdf

Anyhow, I am powering this circuit using battery power. I have four 3V 1/2AA batteris in series(12V). I tap of at 2 batteries so that 6V is for my logic and use a 2N2222 transistor to apply 12V to a solenoid valve. The 12V is obtained from all 4 of the batteries, so a common ground between the 6V and 12V. The problem I am having is everytime the solenoid turns on, the voltage of the whole circuit would drop significantly.
I originally had the circuit being operated at 6V and used a 5V coil solenoid valve but the drop in voltage caused the voltage to drop below the pull-in of the solenoid valve. So my thinking was to go to a higher voltage coil and thus reduce the load induced on the circuit when the solenoid valve turns on. This approach allows the solenoid to operate and is the lowest current consumption (with 12V solenoid=25mA, 5V solenoid=110mA), but I am curious how could I get rid of this problem if I decided to go with 6V supply and 5V coil or even 3V coil? I've tried a regulator but the same problem existed. Is there anyway to avoid this? To avoid some unknows I do have a fly-back diode. The 12V solenoid is 250ohm. The 5V solenoid is 47ohm. The batteries are 3V lithium's. The only thing I can guess to be the problem is the relationship between the internal resistance of the batteries and the circuit when the solenoid valve comes on. I also tried a 3V solenoid, but the problem seemed worse(?), maybe cause it is 15ohm.(?)

Any help would be most appreciated!!!
TIA
Nathan

 

[automatic2]

your absolutely correct. The internal resistance of your batteries results in no go. Increase your battery capacity or, as is common, is to have a seperate supply for your solenoid(s). Then you can optically connect with the freedom to choice any coil /supply configuration.

 

[ve7brz]

I agree that the Lithium batteries are suspect because of the notoriously high internal resistance of Lithiums. On the other hand I'm suprised that your batteries, considering the size, are having problems at 110 mA unless they are a low discharge type. Newer high discharge types are available.

I also agree fully with the previous tip, however I do have one more suggestion: switch over to a ni-cad pack. You have to recharge them, of course, and the capacity is lower, but the internal resistance can be an order of magnitude less than the Lithium.

Bruce

 

[peebee]

Interposing relay.

 

[pirator]

Thanks automatic2 and ve7brz for your replies. I will try different batteries, hopefully the problem isnt as bad. I'm more proficient in automation (PLC's,etc.) than electronics, so I ask is a voltage drop unavoidable beyond that of the diode, etc?
This device is to last at least one year in industrial temperature range before battery replacement. Charging is not an option. This thing goes in a manhole and possibly could be submerged in sewage. We already sell a device for our systems that uses this recycle functionality but it utilizes an off-the-shelf component. The SSAC Inc. timer is 40 or 50 dollars alone, in the quanity we get!

Thanks for the help.
Good day,
Nathan

 

[buzzp]

You need seperate power supplies for your solenoids. That or get a larger source that is capable of sourcing more current. Solenoids can pull 20 times the continuous current at energizing. This is browning out your power supply which is giving you all your problems. So get a battery that can source more current or seperate the power supplies for the solenoids and the other electronics. You might try a large cap to help source some more current at pull-in. Good luck.

 

[pirator]

Thanks buzzp for the reply. Will try suggestion.

Thanks,
Nathan

 

[fxguthri]

Hello

I'm new here, so I don't know my way around at all. However, I came across your thread while searching for some info on the ICM7xxx devices. I have ~40 years of electronic experience. If you are still having problems with your solenoid circuit, then please contact me or e-m me a schematic (pdf file maybe?) and I will be happy to review the circuit. I am currently using the ICM devices in a design and therefore familiar with the devices. You can e-m me directly at fxguthri@excite.com, if you wish. Please put some reference to the ICM in the subject line or you might get 'trashed'! If you solved your problems - GREAT!

Have a good one!

Frank

 

[Chuckles159]

On a solenoid the pull in current is a lot more than the hold current. A 6V solenoid may need 5V to pull in but can hold at 3V. Therefore, a solution that often works is to put a capacitor of several thousand µFd across the power source feeding the solenoid and it's driver. Perhaps a 10,000µFd 16V capacitor would do the job. Even a smaller value may do. The capacitor supplies the extra current needed to pull in the solenoid and as long as the batteries can supply the needed hold current, you will be fine.