Using a Capacitor to Provide Energy to Solenoids

Discussion in 'The Projects Forum' started by boingaon, Mar 16, 2013.

May 15, 2012
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Hello,

I'm creating an array of 25 solenoids that are each actuated by an Arduino using a simple transistor setup. The solenoids draw 1.25A at 9V. I plan on having a maximum of 5 solenoids active at a time for no longer than 10ms, with about 25ms between actuation, minimum. Within that 10ms timeframe, the solenoids will theorietically draw 6.25A. The 9V wall adapters that I am looking for do not go up to thus current range, and the ones that do are extremely expensive and bulky.

I'm wanting to add a capacitor to the circuit that will act as a buffer, providing energy so that the sudden amperage increase is not 'felt' by the power adapter. To do this, I believe that I need to calculate how much energy the solenoids will consume in that timeframe, and find a capacitor that can store that amount of energy when placed across the voltage rails. The problem is, I can only find equations that show the energy across a resistor, not something like a set of solenoids.

The second part of the challenge is to determine if the capacitor can fully recharge in the time between actuation (an estimate would be 25ms).

This equation may be of use:

Vc = 1/C ∫ Ic*dt

I could solve for 'dt', but I do not know what time frame this would actually be specifying.

Thank you

2. ErnieM AAC Fanatic!

Apr 24, 2011
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1,627
Five units at that duty cycle will have an average current of:

1.25A * 5 units * 10ms/25ms = 2.5A

So you need to provide that much continuous current from your wart.

One trick you might try is to see what the min pickup voltage of the relays is. Typically that is somewhat lower and may drop the current to some range you can support.

3. KMoffett AAC Fanatic!

Dec 19, 2007
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What are the spec's on the solenoid operating times? 10/25mS seems pretty fast. Got a mfg/model or a link?

Ken

4. John P AAC Fanatic!

Oct 14, 2008
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I think you might be best off doing an experiment. Set up a 1-shot (LM555 or the like) to deliver a 10msec pulse. Charge up a capacitor, then use the 1-shot to turn on a transistor and operate a solenoid. You'll be able to see how much energy was used if you measure the voltage left on the capacitor at the end, assuming the leakage rate is low. From that you should be able to calculate how many solenoids can fire, how often, with a given voltage on a given size of capacitor, and what the average energy input needs to be.

Note that if the voltage on the capacitor has dropped from the peak, solenoids that fire late won't get as much energy as earlier ones.

May 15, 2012
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They didn't come with a spec sheet, but I know that they have 12ohm impedance and operate from 6-9V. The solenoids will always be activated no more that 10ms at one time, but the period of time between actuation can vary a lot because each of the solenoids will be playing a note in a song. The 25ms spacing is an arbitrary minimum time between activation that I have set. This is intended to prevent notes from being played so quickly that the solenoids are running continuously.

ErnieM:
Modeling the actuation as a PWM signal would give me an average current, but wouldn't the actual current still be 6.25 amps during actuation?

6. tubeguy Well-Known Member

Nov 3, 2012
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Please verify the current draw. 12ohm impedance would draw 9/12 = 0.75amps at 9V.

7. KMoffett AAC Fanatic!

Dec 19, 2007
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Impedance is frequency dependent. Are you referring to the coils DC "resistance"?

Ken

May 15, 2012
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The 12 ohm impedance was given by the vendor. Actually, the 9V power supply I was testing these with was operating at about 14 volts, so I apologize for the inaccurate measure of the amperage. Indeed, tubeguy's calculation of 0.75A is correct at 9V.

I did try to determine the resistance of the coils with a multimeter and found a resistance of about 10 ohms. However, I am not sure how accurate this value actually is since the coils have such little resistance.

To throw a new number out there, it looks like the maximum amperage will be 0.75A x 5 = 3.75.

If my duty cycle is 10ms/35ms = 28%, then the average amperage would be about 1A. However, if I am thinking correctly, the 3.75A will still be drawn from the wall wart if only for 10ms. Is this alright, assuming that the wart is rated at something like 2A?

9. tubeguy Well-Known Member

Nov 3, 2012
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KMoffett pointed out the spec was 12 ohm impedance which implies these can operate on AC. You might want to experiment with different voltages both AC and DC as John P suggested to determine 'how low you can go' and still get reliable operation. Then you might be able to use a lower voltage higher current wall-wart.

10. KMoffett AAC Fanatic!

Dec 19, 2007
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How about a link to the vendor and specific solenoid? The only solenoid valves I can think of that operate at those short intervals are fuel injectors.

Ken

Last edited: Mar 17, 2013
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May 15, 2012
31
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They did not come with a data sheet. I'm likely running them at a faster actuation time than they are built for, but that's because if I go any longer, the music notes don't sound good.

May 15, 2012
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Sorry to bring this thread back to life but my question is immediately relevant to this topic.

When a capacitor is used for excess energy reserve/storage, how and why does the charge from the capacitor get used before the charge from the main power supply (wall adapter/battery, etc.) In the instant that a sudden load in the system turns on, will that load drain the capacitors BEFORE turning to the main rail for "sustenance"?

For example, my project uses 5 solenoids that each draw 700mA at 9 volts (3.5A total). Since power supplies at this voltage range are expensive and huge in size, I want to put some capacitors in my system to provide the extra current needed to power the solenoids while they are on (about 5 millisecond "on" time with 100ms in between actuation), worst case scenario.

I'm going to add the capacitor(s) into the system in way shown in the following diagram. However, I don't understand why the solenoids would draw charge from the capacitor and not the 9V power supply. That's what I want it to do.

Thanks

Last edited: Jun 24, 2013
13. wayneh Expert

Sep 9, 2010
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The power will be drawn from all the sources that are in parallel. The amount delivered will depend on the internal resistance of each source. The capacitors have relatively very little internal resistance, so they can deliver a lot quickly.

If it helps, imagine a bank of water valves on the same actuator so they all turn on together and deliver water from the same pressure source. The amount of water that pours out each one will depend on how large each valve is. The capacitor has a big pipe but not much reserve. The wall wart has an infinite reserve but a small pipeline.

14. crutschow Expert

Mar 14, 2008
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As wayneh noted, both sources will deliver current to the load based upon their relative impedance. To limit the contribution from the power supply you can put an appropriate value resistor in series with the supply (or just allow the supply to go into its "current limit" mode if it has one).

May 15, 2012
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Thank you both. If I did not add the resistor, would the capacitors still take precedence over the power supply, but perhaps not as much as if I did add the resistor? With the amperages that I'm dealing with, a huge resistor may be needed.

16. wayneh Expert

Sep 9, 2010
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When you close the switch, current will start to flow from the two sources (supply and capacitor) in inverse proportion to their respective impedances. If the current is larger than the supply can keep up with, the capacitor voltage will start to drop. How far it drops depends on the size of the capacitor and the time it it has to discharge. The time constant is RC, and the capacitor will discharge nearly completely in 3 x RC.

The load demand seen by the power supply - the voltage on the capacitor that is less than the supply voltage - depends on how low that capacitor voltage drops. If the capacitor is large enough, the supply will hardly be affected. If the capacitor is too small or not present, the supply will see something resembling a short.

You can calculate reasonable estimates of all the voltages, but I think the bigger question is that you need to know more detail about your supply and how it might behave with large transients.

May 15, 2012
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Very well explained, thank you.

18. LDC3 Active Member

Apr 27, 2013
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I think you didn't look hard enough. Here is a 9V 3.5A wall-art for less than \$20.

19. Duane P Wetick Active Member

Apr 23, 2009
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You say the solenoid coils have 12 ohms impedance...at what frequency? At 60 Hz...this translates to 31mH. and the current is mostly reactive. A free wheeling diode (reverse biased) must be added to the coil to protect your Arduino transistor driver. A scope view of the coil would be most helpful in calculating actuate-release times and subsequent cycling times.

Cheers, DPW [ Everything has limitations...and I hate limitations.]