Coil Voltage - Page 1
Forum section: electronics / beginners

author: JLynch
author_rank: Contributor
timestamp: October 01, 2022, 03:41:59 pm
content: Built a vacuum former where I’m trying to regulate the temp & pressure using an Arduino coupled with a BMP180 sensor and a solenoid value. Problem is that I have no idea what the working voltage of the solenoid value should be.The solenoid value is a Vapor Canister Vent Valve for a car with a coil resistance of 17.5 ohms. Started off with 12 volts but the coil got way too hot. Tried 6 volts but that wasn’t enough to close the solenoid, settled on 9 volts which seems to be working.Instead of just guessing / trial and error, is there a way to know what the proper working voltage should be based on the coils resistance?

author: Gyro
author_rank: Super Contributor
timestamp: October 01, 2022, 05:34:29 pm
content: I think the vapour canister solenoid valve on a car is meant to operate briefly, ie. just to release the vapours to be burned once the engine has warmed up. It probably is a 12V solenoid, but with a short duty cycle rating.If you are operating the valve on a continous duty cycle, you probably are looking at trial and error to get reliable opening without overheating. There are many factors involved that prevent you being able to calculate from just coil resistance - mechanical factors such as strength of return spring, wire gauge, number of turns etc.There are techniques where you pulse the coil at full voltage to open it and then reduce the voltage / current to hold it there, but it requires a bit more circuit complexity.

author: JLynch
author_rank: Contributor
timestamp: October 01, 2022, 07:01:36 pm
content: Yeah was trying to cheat by using the EVAP Vent Valve, wanted a fail-safe solution (normally-open) pneumatic valve able to operate within a vacuum, not an easy item to find without spending a ton of money.A lot of pneumatic valves won’t release while under a strong vacuum getting stuck in a closed position. The EVAP Vent Value is designed just for that purpose and works really well under the specific conditions needed for the vacuum former.Like the idea of varying the voltage, starting high enough to close the solenoid then dropping lower just to keep it closed. I’m okay with programming an Arduino and hooking up a sensor here and there, but varying the voltage between closing / latching vs continuous operation is a bit beyond me - circuit wise anyway.Guess I could setup the Arduino to toggle between two relays each with there own 'load' voltage. Energizing one at 12volts to close the solenoid then a second to hold it closed, after some delay the Arduino could de-energize the first and the second would take over at the lower voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: October 01, 2022, 07:12:31 pm
content: The simpletons way of doing it is to use a capacitor and resistor in parallel and that combo in series with the solenoid. The capacitor will briefly allow full voltage/current at the solenoid and then as it charges, the current tapers off to whatever the resistor will allow. You'd have to do some measurement and experimentation to get the right component values and the capacitor will probably have to be pretty big. Maybe start with 12V, 15 ohms and 1000µF.

author: JLynch
author_rank: Contributor
timestamp: October 01, 2022, 07:38:21 pm
content: I’m all in for a simpletons approach, have a bunch of linear taper pots, resistors and a bag of capacitors, not sure if any are as big as 1000µF, but should have enough to put something together. Thanks for the suggestion.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 02, 2022, 09:55:12 pm
content: Mail bag/tear idea for Dave:Modern industrial control relays and motor contactors have made the shift from line voltage coils to 24 vdc coils. All but the smallest have some form of voltage boost for solenoid armature pull-in then dropping to a regulated constant holding current. The current requirements are drastically improved over the original dumb coil designs and they also now have build-in snubber circuits, no doubt to protect the coil control circuitry.Would love to see one reverse engineered in Dave-Cad.

author: james_s
author_rank: Super Contributor
timestamp: October 02, 2022, 10:40:06 pm
content: If it's for a car, it's a 12V coil. A common approach used for coils that need to pull in powerfully and then hold for extended periods is to drop the voltage once the coil has pulled in. Pinball machines use a switch or sensor to detect end of stroke. A simple time delay also ought to work.

author: rob77
author_rank: Super Contributor
timestamp: October 02, 2022, 11:42:59 pm
content: or you can use 12V and PWM the solenoid - start with 100% duty to close the solenoid and the lower the duty cycle to keep the solenoid closed but prevent overheating. you can figure out the needed duty cycle to hold by lowering it in 5% increments till it opens (and then add 10% back to be on the safe side).

author: Glotedom
author_rank: Newbie
timestamp: October 03, 2022, 11:38:54 pm
content: ATTN: JLynch - Thinking outside the box...Before I retired, I owned an Electronic Security Company. On Access Control Systems, we had a similar problem. The solenoid coils on electrified door hardware would get warm and prematurely fail if energized and retracted all day (needed if the front door of a business was unlocked all day). The solution was a small inline module that would allow full voltage to pass through to the solenoid and then reduce the voltage a short time later. The solenoids retracted, then remained energized and retracted all day, locking at the end of the day when voltage was cut. Sounds like you have similar requirements...Look at these installation instructions and see if you think this will work. Module works at both 12 and 24 VDC. The limiting factor is only 1A current draw is allowed. I've got a couple; I'll look for them. If you think it will work, I'll send one to you to test out...

author: JLynch
author_rank: Contributor
timestamp: October 04, 2022, 12:14:20 am
content: Rob77: If using PWM on a coil constantly turning it on and off would one need to worry about back EMF?Glotedom: I really appreciate the offer to send one of the ‘Command Access CRU2’ but after reading the pdf I’m not sure things fall within the specs. While the Vent Solenoid draws only 0.68 Amps at 12 volts, well within the 1 Amp rating, the coils resistance is 17.5 ohms. The CRUS states that anything below 18 ohms should not be used and I don't know if that half an ohm would make a difference ruling out its use.

author: Glotedom
author_rank: Newbie
timestamp: October 05, 2022, 09:52:34 pm
content: I went digging and actually found a "Command Access CRU1," which appears discontinued. The bad news is its specs say 20 ohms coil resistance minimum. A copy of the packaged instructions is attached to review. You are welcome to test it out if you like. If you release the Magic Smoke from it, there is no loss as long as it doesn't harm your solenoid coil. PM me your address, and I will drop it in US Snail Mail for you.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 06, 2022, 08:25:28 pm
content: Solenoids have a ratio of 3:1 pull in vs hold current or more.You only need to supply the full current for the time it takes to pull in. Set RC accordingly.A simple RC and a mosfet to suit your max load will do. It saves using a big polarised cap.The mosfet only carries current during pull in so you can derrate it power wise.Need to be low Rdson compared to coil. That won't be hard.Don’t forget check Rhold's power rating is high enough.You might start at Rhold= 3x Rcoil and experiment until its reliable.There’s no free lunch, if you don’t burn power in the coil you have to burn it elsewhere.... or go PWM on its butt. Say from 100% down to 20% after a few mS, or whatever works best.Solenoids want a low frequency.There must be a trashy 555 solution, surely?

author: james_s
author_rank: Super Contributor
timestamp: October 06, 2022, 08:54:41 pm
content: If you want the PWM route then a microcontroller seems like the obvious choice. This is exactly the sort of thing something like those 3c Chinese micros would be good for, but in small quantities you may as well use a 40c attiny.