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I am having a problem trying to operate a bi-directional solenoid from a RaspberryPi by using an L298N H-Bridge. I am sure it’s a misunderstanding on my part but I hope someone can help me!

The solenoid I am using is this Intertec ITS-LX-2218-12VDC-10mm.

The H Bridge is the same type as this L298N Motor Drive Board Module.

My supply voltage is 15V.

My plan is to use the Pi’s GPIO to control if the solenoid is open or closed.

However when testing (using 3.3V and 5V control voltage and a common ground) I have noticed a problem.

If I don’t have the solenoid attached then when using a multi-meter I when 15V on the output when the control pins are set to 1 and 0 respectively and as expected I get -15V if 0 and 1 are present. This is all fine.

However when I perform the same check with the solenoid connected I’m only reading 7.5V across the H-bridge outputs and the solenoid fails to fire.

To summarise the H-bridge seems to work as I had hoped, apart from when the solenoid is attached. When the solenoid is connected the h-bridge would appear to not provide enough voltage/current to power the solenoid.

The solenoid works fine when using the source voltage directly for a fraction of a second.

I would be the first to confess my electronics knowledge is rubbish and even endless reading on Google has failed to provide any help so I am turning myself over to the good people of stackexchange.

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    \$\begingroup\$ Have you measured how much current the solenoid needs to operate? That bridge is only capable of supplying 2A, so if your solenoid needs more than 2A, you will see a drop in voltage on the output. \$\endgroup\$ – HandyHowie Jul 27 '16 at 11:12
  • \$\begingroup\$ Thank you for your help. My Power source is only 2A and (while being a little naughty because the solenoid is 12V) If I connect the solenoid directly to the power source it works. It is only when the h-bridge is in-between there is an issue. Does this answer your question? \$\endgroup\$ – Tim H Jul 27 '16 at 11:20
  • \$\begingroup\$ You state: "However when I perform the same check with the solenoid connected I’m only reading 7.5v across the H-bridge outputs and the solenoid fails to fire." Please measure the voltage across each active transistor of the H-bridge under this same condition. \$\endgroup\$ – FiddyOhm Jul 27 '16 at 11:30
  • \$\begingroup\$ Tim, you're going to have to provide a schematic. There's a button on the editor toolbar and it's easy to use. Double-click components to edit their properties. \$\endgroup\$ – Transistor Jul 27 '16 at 11:34
  • \$\begingroup\$ @TimH "directly to the power source it works". This doesn't answer the question. How much current does the solenoid consume at 12 V? \$\endgroup\$ – winny Jul 27 '16 at 11:38
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I had a look at the link for the solenoid and the datasheet for the IC used in the H-bridge board from ST found here: https://www.sparkfun.com/datasheets/Robotics/L298_H_Bridge.pdf

HandyHowie made a good point. The details of the solenoid specify 12V DC, 53 Watt nominal power (not peak power as it starts to move). This gives you a current of 53W/12V=4.417A (approximately). Remember that the switch on (rush-in) current for an inductive load (solenoid, motor...) might be many times this figure. The fact that it works when you connect it MOMENTARELY to the power supply, which is only 2A, might be because every power supply has (or should have) a big capacitor at its output to help with these switch-on currents for inductive loads (like your solenoid) or for any circuit that needs to charge its capacitors before it starts to operate.

Therefore, your power supply might NOT be good enough for the task. Also remember that applying Ohm's law, the load (assuming the resistance does not change) will require more current for a supply voltage of 15V instead of 12V. I calculated a resistance of approximately 2.717ohms, so the actual current at 15V should be 5.521A. However, your power supply might be good enough, depending on the load you want the solenoid to move, which means how many Newtons of force it must produce to move it, hence how much current it requires to operate.

In addition, if you look on the first page of the datasheet for the H-bridge IC, you will notice the RSa and RSb sensing resistors which are used by the control circuit to obtain feedback regarding how much current goes through the load. I admit that I didn't read the full datasheet and I have not made any attempt to obtain a schematic for the H-bridge board you are using; however, the L298 has two bridges and is designed for a total current of 4A (2A per bridge) or a non-repetitive peak (100 microseconds) of 3A. You can however connect it in a different way to have higher currents (Figure 7). The previous mean that maybe the control circuitry senses the current overload and 'intentionally' reduces the voltage to reduce the current through the load. On page 7 of the datasheet it says that the control circuit can start chopping the output to reduce the current, so the 7.5V you measure might be the RMS, the True-RMS, or the average (depending on the instrument) value, after this chopping action (if it happens). An oscilloscope could help clarify this.

Alternatively, the reduced voltage might occur because the saturation voltage (figure 1 page 4 of the datasheet) goes up. The datasheet gives a value of 2.4V saturation voltage for 2.4A of current. It is unclear to me if this voltage is for just one or for both transistors of the H-bridge arm that complete the circuit, however, you can observe that the relationship is not linear, so for 4.417A at 12V I expect at least 4.417V of saturation voltage. The situation is even worst for 15V as the current should be 5.521A, so the saturation voltage must be at least 5.521V. So, the 7.5V you "loose" might be because of the non-linear relationship shown in Figure 1, or because Figure 1 refers to each transistor (and remember you have two in series with the load, which is located in between the two transistors in an H-bridge configuration). Keep also in mind that you might burn your IC H-bridge at these high currents.

A further location where you might be 'wasting' voltage is across a component outside your H-bridge IC due to the high currents. The board I can see following your link seems to have more external components.... it also looks as if it has a small surface mount control logic IC made by ST (perhaps the L297 shown on Figure 8 of the datasheet? You can see it here http://www.st.com/content/ccc/resource/technical/document/datasheet/f9/35/6e/3f/48/18/48/51/CD00000063.pdf/files/CD00000063.pdf/jcr:content/translations/en.CD00000063.pdf).

In summary, as a novice, I would have chosen to go with a different H-bridge and a power supply at 12V and much higher current (if needed to move heavy loads with the solenoid). Then again, as an electronics' engineer I would have built a circuit with discreet components and a resistor to absorb the extra 3 volts so as not to overheat my solenoid (if needed). The cost and footprint would have been roughly the same, maybe even better....

I hope this helps,

Best of Luck, Konstantinos

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  • \$\begingroup\$ Thank you for your quick and very detailed response. There is a lot to think about there so I will update this thread once I have tested. Once again Thank you! \$\endgroup\$ – Tim H Jul 28 '16 at 15:05

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