# Solenoids pulling more than rated current (6VDC, 300mA)

I'm building a musical instrument using thirty JF-0530B solenoids, rated 6VDC and 300mA. But I've discovered when I power them directly with 6VDC they draw ~1.5A, because they only have a resistance of ~4 Ω (they all do, except for three that appear to be shorted/bad).

(The values above are as measured with my multimeter, like so – just to make sure I didn't mess something up here.)

My concern is that I'm going to damage the MIDI decoder board, which switches the solenoids via ULN2803 chips (Darlington arrays with embedded flyback diodes) that are only rated up to 500mA per output.

The odd thing is, before discovering all this I tested the solenoids with the decoder – and not only does it work at 6VDC, it works at 12VDC when the solenoid pulls nearly 3A! (I just used the wrong supply.) Maybe I just got lucky here because the solenoids are only powered for a fraction of a second.

I'm not familiar with solenoids, but isn't 4 Ω a low resistance for a 300mA-rated solenoid? Did I get a bad batch or do I misunderstand how to use them? I'm trying to ask vendors for the resistance of the 12VDC version of this solenoid (which I'm thinking of using instead) but no success yet. I've seen this thread (among others) but that seems to have boiled down to misuse of equipment. Another forum suggested I was seeing inrush current, but this seems to apply only to AC solenoids.

Thanks in advance for any thoughts!

Midi decoder board

• If it's relevant, here is the MIDI decoder board in question (PDF). I don't have enough reputation to have linked it in the main question. Jun 11, 2017 at 19:03
• Well, to start with, the solenoid you linked to is rated for 300 mA at 24 volts, not 6, so the coil resistance should be about 80 ohms. It's very likely that the cheap Chinese manufacturer mislabelled some solenoids. It's likely that you have some beefy solenoids which are intended to provide a good deal more force than the ones you think you have. Jun 11, 2017 at 19:14
• Why didn't you read the solenoid data sheet before buying? Jun 11, 2017 at 19:14
• Can't find JF-0530B manf. or data beyond picture on Amazon with Chinese markings. Can see why they tempted you, at quick glance they're much less than half price of those from typical distribs (some of which are 4 ohm). You're in dodgy territory, though. You could try working with it and PWMing them at 24 V with big PSU caps but may be more complicated than you wanted. Get yourself a refund. Jun 11, 2017 at 19:33
• @WhatRoughBeast Thank you. The same model number (JF-0530B) is used for 6, 12, and 24VDC solenoids, and I do have these 6VDC ones (my reputation prevented me linking it in the original question). I can certainly see how they could have been mislabeled. Jun 11, 2017 at 19:50

I can only find data for the 12 V version of the JF-0530B solenoids. These appear to be the type used in vending machines, etc., where they typically have short "on" times. I suspect that the voltage and current ratings are not simultaneous: you can run at 6 V (or 12 V in the case of the datasheet version) for a while or at 300 mA continuously. For the latter condition the voltage will be reduced.

Assuming the frame size and power rating is the same then we can use the temperature rise versus time curves from that data sheet. If we use the rated voltage and your measured resistance then we can calculate the power dissipated in the coil: $P = \frac {v^2}{R} = \frac {6^2}{4} = 9 \; W$. That would give you a temperature rise as shown by the curve 1. After 30 s continuous on you would have a coil temperature rise of 50°C.

Figure 1. Graphs from solenoid datasheet.

Some of these applications require a good pulse of current to overcome inertia, etc., but not so much to hold the solenoid in position for the time required. It is possible to charge a capacitor to 6 V through a resistor to give a good initial "kick" to the solenoid but limit the continuous current to a safe value.

simulate this circuit – Schematic created using CircuitLab

Figure 2. C1 charges up to 6 V via R1 and provides a short pulse current when Q1 turns on. The current will rapidly fall off to the value limited by R1 and the solenoid's internal resistance.

You need to read and understand the stroke details on the graph on the right too.

ULN2803

Yes, they're not adequate for this task. Even if you parallel the outputs you'll likely exceed the chip total current limit. (Remember that all the current has to pass though the internal gold wire to the common pin.) Look for a suitable Darlington transistor.

Ideally you would be able to share the one RC pair between all the solenoids but this depends on the RC time values selected as it would have to recharge adequately between firings.

You might also need to prevent the actuator damping the ringing. The piano mechanism does this by propelling the hammer to the string but it's allowed to fall back to a rest position if the note is held. This allows the string to vibrate undamped. You may succeed by just getting the pulse length right or you may need a separate hammer mechanism.

• Thank you for taking the time to write this out! You may be right – it takes 5-6V to actuate the solenoid – and at a lower voltage that brings the current closer to 300mA, it won't actuate but it will hold if actuated manually. The capacitor approach is neat, but wouldn't the 6V discharge send the same high current through the trigger (the current-sensitive component here)? This application calls for the solenoids to strike bells directly, so they are only powered for as long as it takes to actuate, no continuous sustain necessary; and mechanical load is also negligible. Jun 12, 2017 at 0:08
• Sorry, I forgot to address the ULN2803 question. See the update. Jun 12, 2017 at 5:38

Probably they are AC not DC. Last time when I installed a hydraulic valve had the same problem and the solenoid had some new chinese standard RAC. I couldn't understand what this RAC means, then like lightning hit me : RAC = rectified AC, half wave rectifier (diode). Possibly your solenoid is 6 VRAC.

• Rectified is largely DC, with a pretty good dose of 50/60 Hz and harmonics superimposed. It will behave pretty much the same way DC will. Jun 11, 2017 at 19:35
• They're DC. I had the same thought as you though. Jun 12, 2017 at 5:39