To my surprise I was actually able to get this working pretty quickly!
The problem I’m having is that I’m noticing the solenoid part of the circuit is drawing a lot of current, up to around 5amps max, which is draining my power supply very quickly. The solenoid itself only requires .6 amps and 24 volts. I’m powering it with 3 9V batteries in series. It seems that the batteries will supply quite a bit of current unless a resistor is placed in the circuit.
Unfortunately, when I put a resistor in series to bring down the current to about .6 amps the resistor burns up after about 2 seconds. Some quick math 24V x .6A shows that the circuit is generating about 14 watts of power. The stock resistors I bought on Sparkfun http://www.sparkfun.com/products/9258 say they only support 1/4 watt. So obviously that’s a problem.
My question is what are the best ways to reduce the current in a higher voltage circuit?
Is it just as simple as a buying a resistor that supports a higher wattage? Should I lay a bunch of resistors in parallel to divide the current (that would be a lot of resistors!!)? Or am I thinking about this the wrong way?
9V batteries are not an ideal power supply for your application. As you have noticed, they do not last very long under high loads and even when you solve the current issue they still may not last as long as you would like them to.
If your circuit is drawing 5A, then something is wrong with either a component or how you hooked up the circuit. I would first check to make sure that D1 is installed properly. Flipping D1 around by mistake could cause the excessive current draw you are seeing. I would also verify R1 is the correct value since it will set the maximum current of your circuit. If everything appears to be hooked up properly, I would check the coil of your solenoid. With 24V, the solenoid should draw the specified 0.6A with a little wiggle room for tolerances and your increased operating voltage of 27V. It is possible the solenoid could be partially shorted.
Check these few things out and report back on your findings.
I checked the circuit again. D1 was hooked up correctly but this time I noticed the solenoid was drawing (the expected amount) ~.6 amps. This got me thinking and I realized what I must have done. The only plausible explanation I can think of is that I had hooked up the multi-meter in such a way as to create a connection between the positive and negative terminals of the battery thus causing it to draw the ~5 amps.
As an aside you mentioned that 9V batteries are not an ideal power supply. Is there another battery powered supply you would recommend for this type of application?
gnsortino:
As an aside you mentioned that 9V batteries are not an ideal power supply. Is there another battery powered supply you would recommend for this type of application?
Lithium battery packs are becoming popular, but are still expensive as well as their chargers:
I’m glad you sorted things out. I’ve made my share of mistakes hooking up test equipment that have gone unnoticed for hours while I pull hair out of my head trying to figure out what’s wrong.
In terms of a different battery power supply, what exactly is your application? Does it need to be ultra portable or can it tolerate a larger battery? Dattaway suggested lithium batteries which are better able to sustain high currents. For 24V you could use 2 sealed lead acid batteries in series. They are easy to charge and will provide much more sustained current than a normal 9V battery.
Would it be possible to change the solenoid to some other lower voltage? That could expand your options in terms of finding a suitable battery without having to series connect multiple units.
There is a way to reduce the solenoid current. Most solenoids (and relays) require a lower voltage or current to hold then to pull-in. Therefore a circuit can be built that applies the full voltage when switching on the solenoid then decreases the voltage to reduce the current. The simplest is a parallel resistor and cap in series with the coil. More complicated is to use two transistors.
Thanks again all. Those 11v batteries look great. Not too expensive on their own if you factor in that they are rechargeable but the charger itself is kinda pricy. I’ll have to save up.
The application I’m building is a pressurized feed system (~750psi). Now that I have this circuit working I’m going to try hooking it up to 2 Xbees.
Waltr - do you have an example/tutorial you can point me to of a circuit like that decreases voltage after the solenoid is turned on? If I were to use the resistor presumably I’d need one that could handle a higher wattage?
The circuit I use is a current source to the coil (solenoid) that is switched on/off by another transistor from a digital control line. To get the current to change after switching on I use an cap that turns the current source on max then when the cap charges the bias to the current source decreases to a lower set point.
Google ‘current source’ for starters. Wikipedia has a good over view.
Here is one circuit I designed. C201 is the cap that turns the current source full on and then R313 sets the current source for hold.