Ohms law applied to solenoids?

[chipwitch]

Okay, so my knowledge of electronics is limited, but while shopping for some solenoids for a project, I encountered a website that's making me doubt myself. 

I found a good supply of solenoids, reasonably priced on electronicsurplus.com.  In reading the descriptions though, I see they list the current of each solenoid, as if it's a published on a nameplate.  Some of the amps seemed kind of high to me, then I noticed they amperages posted were the same as if they simply divided the nameplate voltage by the measured resistance.  Please tell me that's wrong.  My understanding is inductors are dynamic and Ohm's law doesn't apply.  But considering this site is an electronics surplus outlet, I would expect them to know this.

Is this some kind of accepted standard of advertising for inductors or do I need to do some remedial study on inductors?

[ElektronikLabor]

The impedance of the solenoid is Z = R + j*2*pi*f*L
R ist The resistance of the solenoid
L is The inductance
f is the frequency

When The frequency is 0Hz (dc current) the inductance doesn't  have any effect on the impedance.

You Are right, that inductors habe a dynamic impedance, but it's only matters, when the current is dynamic too

[chris_leyson]

You should apply ohms law to solenoids and also to relays as well to get the correct operating current. It's the coil resistance that limits the current for a given applied voltage. Probably the higher current solenoids are specified for lower voltages, however, they could be physically larger and need more current to generate the specified operating force.

You're correct when you say inductors are dynamic, if you think of a solenoid as an R-L series circuit then it has a time constant tau = L/R. Apply a voltage to the solenoid and the current builds up exponentially, after one time constant the current is at 63% of its final value and at 5 time constants its 98%. They take a small time for the current to build up.

Don't forget to add a back EMF protection diode. I guess you already know this, in that case  . I've seen EEs oblivious or clueless to back EMF and don't fit any kind of protection 

[Seekonk]

Don't forget that the inductance changes with the position of the core.  As a simple example I can determine the speed of a relay by monitoring the current on a scope.  When I see the slope change I know the armature has moved.  For high speed applications like depositing glue dots I drive solenoids at 6 times their standard voltage rating   and then PWM the current down to keep the valve cool.  Hold in is generally 1/4 of pull in current.  In low repetition applications you will often see a capacitor in parallel with a series resistor to accomplish this.  So much of what you are worrying about is irrelevant.

[TimFox]

Also note that relay and solenoid coils rated for AC and DC use are physically different.  Of course, the inductance does matter with AC.

[chipwitch]

@medvedev  --  I had forgotten about the reactance/impedance formulas.   That must have been what I was thinking.  I (incorrectly?) thought the magnetic flux, position of the push rod, eddy currents etc increased the resistance too.

My DMM must have a poor ammeter.  I got the following results on a small dc solenoid I just tested.  These are measured results.

125 Ohm
29VDC 500mA PS. (tested with and without load to ensure no ps voltage drop under load).
125mA.

That was on the 2A scale.  I tried the 10A and got .4A

I then tested a 3W 330 ohm resistor using the same setup and got similar results (49mA on the 2A scale, about half the expected.)

Time to budget for a new Fluke.

[chipwitch]

Don't forget to add a back EMF protection diode. I guess you already know this, in that case  . I've seen EEs oblivious or clueless to back EMF and don't fit any kind of protection 

Thanks for the reminder.  I'd forgotten that.  Do you think it matters for my current project?  I'm using smallish solenoids (<.5A) and using 10A mechanical relays for switching?

[chipwitch]

Don't forget that the inductance changes with the position of the core.  As a simple example I can determine the speed of a relay by monitoring the current on a scope.  When I see the slope change I know the armature has moved.  For high speed applications like depositing glue dots I drive solenoids at 6 times their standard voltage rating   and then PWM the current down to keep the valve cool.  Hold in is generally 1/4 of pull in current.  In low repetition applications you will often see a capacitor in parallel with a series resistor to accomplish this.  So much of what you are worrying about is irrelevant.

Who said I was "worried?" 

Most of the more appealing solenoids I've found (read: less expensive) don't come with much data... no Force, for example.  So, the power used is somewhat an indicator (I know higher power doesn't necessarily mean more pull).  If I am "worried" about anything... it's having "garbage" in my brain.  I try to keep it accurate.

[ElektronikLabor]

125 Ohm
29VDC 500mA PS. (tested with and without load to ensure no ps voltage drop under load).
125mA.
That was on the 2A scale.  I tried the 10A and got .4A
I then tested a 3W 330 ohm resistor using the same setup and got similar results (49mA on the 2A scale, about half the expected.)
Time to budget for a new Fluke.

I think your Multimeter works fine. Every multimeter measures the current thru a shunt resistor. The 2A- range has a bigger internal resistance, than The 10A range, hence you get different current.
Dave already made a Video about that (just search for burden voltage)

[chipwitch]

@medvedev.  I kind of remember seeing one of his blogs on that topic.  It was the reason for him developing his micro amp meter.  I'll go back and see what I can find on it, but I thought what I'd read was concerning much smaller currents.

PS... thanks for nothing.   You just deprived me of the justification I needed for buying a Fluke! 

[Seekonk]

(I just happened to find this last week when looking for something else.  Otherwise I couldn't have found it on a bet.  This is a product I designed in late 90's for a glue applicator.  Those orange dots are test sheet where the solenoid is on for 2ms and off for 2ms, faster than many relays can switch.

[chipwitch]

@medvedev... looks like the Fluke is still justified.  At 29V for my power supply, the burden voltage is insignificant. 

[ChunkyPastaSauce]

Just to point out... solenoids are often used in intermittent operation due to heating...... wires that heat significantly wont be following ohms law. So even in the steady state case, it can behave more like a filament bulb, (but probably not as drastically)

The steady state iv curve looks like this for a filament.

[Paul Moir]

Got left out - doing something about the EMF with a freewheeling diode, TVS, MOV or RC filter will help with arcing on the relay contacts driving the solenoid, extending their life.

Page 14 here has good information:
OMRON Relay info

[danadak]

You might do a few things to check out your meter -


1) Check fuses and their holders for the two ranges have no oxidation or
battery corrosion on them. Inexpensive meters use a cheap rotary switch,
where one contact is PCB traces, the other the wiper. R variation of the
contact resistance pretty uncertain unless area is very well cleaned of all
oxides and dirt.

2) Measure the current shunts R value with your meter if you can find out what
their value is supposed to be. You can examine rotary switch to see what V range
it uses for the current measurement, then calc the value based on max I of range
and the V.

3) Look to see if meter has any adjustable pots to cal. Get a standard R and V
by using someone elses meter for measuring. Then use them to see if basic meter
function of V and R measuring reasonably well. Known wire length/diameter can
be used as a fairly accurate. For a Vref look on ebay, you typically can pick up
fairly cheap .1% accurate ref ICs.

Regards, Dana.

[chipwitch]

Paul, chris_leyson pointed that out in the third post.  I haven't yet had much opportunity to work with A/C in electronics beyond getting from mains to dc output in a ps.  Almost all my experience with A/C comes from the perspective of electrical wiring, not electronics.  Thanks for the link and page number.  Now I understand why I've seen some A/C solenoids with an integral capacitor.  Back EMF has always meant "flyback diode" to me exclusively.  So much to learn.

I understand it is good to practice back EMF protection in all circuits, but I'm curious... at some point, wouldn't it be unnecessary?  I mean, if using a small solenoid, say 50 mA and a 10 A rated relay, the contacts would be too huge, I would think, for the tiny arc generated to heat up the contacts enough to wear?  Just a hypothetical, not trying to justify not using protection.

Also, how is suppression implemented on larger scale circuitry?  I just robbed a relay out of my central A/C compressor after having the unit replaced and saw nothing across the load.  Did I miss it?  How about larger industrial scale?  Generally, we're talking knife switch disconnects, but there are big switching relays too, I imagine (1 kA?).  Do they just oversize the contacts and replace them more frequently?

[chipwitch]

You might do a few things to check out your meter -


1) Check fuses and their holders for the two ranges have no oxidation or
battery corrosion on them. Inexpensive meters use a cheap rotary switch,
where one contact is PCB traces, the other the wiper. R variation of the
contact resistance pretty uncertain unless area is very well cleaned of all
oxides and dirt.

2) Measure the current shunts R value with your meter if you can find out what
their value is supposed to be. You can examine rotary switch to see what V range
it uses for the current measurement, then calc the value based on max I of range
and the V.

3) Look to see if meter has any adjustable pots to cal. Get a standard R and V
by using someone elses meter for measuring. Then use them to see if basic meter
function of V and R measuring reasonably well. Known wire length/diameter can
be used as a fairly accurate. For a Vref look on ebay, you typically can pick up
fairly cheap .1% accurate ref ICs.

Regards, Dana.

Thanks for the tips!  I'll try that first.  I just saw the prices on a Fluke 77 iv      I was prepared to spend a couple hundred, but geesh!  Maybe I'll look for a decent bench DMM instead (used or Vichey?).  My handheld is only an old UEI, but it's been good to me.  Had it for 20 years or so.  Everything but current has been as accurate as I ever needed.

[Paul Moir]

I mean, if using a small solenoid, say 50 mA and a 10 A rated relay, the contacts would be too huge, I would think, for the tiny arc generated to heat up the contacts enough to wear?  Just a hypothetical, not trying to justify not using protection.

Also, how is suppression implemented on larger scale circuitry?  I just robbed a relay out of my central A/C compressor after having the unit replaced and saw nothing across the load.  Did I miss it?  How about larger industrial scale?  Generally, we're talking knife switch disconnects, but there are big switching relays too, I imagine (1 kA?).  Do they just oversize the contacts and replace them more frequently?

Yes that's right, if it isn't switched very often or if the load isn't particularly high current, or the switch contacts are particularly large, then protection isn't necessary.  You said 10A contacts and I pictured ice-cube relays, which don't have very large contact areas.

On industrial panels, it's fairly rare to see contact protection and rather large contactors (contactor=large relay) are used.  (Nowadays drives are often used since they've gotten fairly cheap if there is any reason to use them at all.)  Interestingly, the control side of the contactor's drivers are usually protected without exception.  Normally a diode is used in DC applications or an RC network for older 24VAC control.  I prefer to use AC TVS because they can switch off a bit faster than a diode and you don't have to concern yourself with polarity. 

Sometimes I've found myself backed into a corner.  For example, I had to switch an 1/4hp motor with an ice cube relay due to space limitations.  The shut-off arc was obviously nasty and this relay was going to switch medium-many (~100-1000) times a day, so the contacts were not going to survive long.  A simple RC filter extended the contact's life out to many years.

For consumer gear, they just go with whatever is cheapest.  Large contact area / short lifespan seems to win out from what I've seen.