driving solenoids with a solid state relay 3

[linacman]

I need to pulse a pair of 12vDC solenoids at about 500 strokes per minute. I can use a 555 timer to drive a solid state relay but am concerned that voltage spikes coming back from the solenoids will damage the SSR. The solenoids draw 4amps and my SSR is rated for 10A. Are simple diodes enough the protect the SSR? Any advice greatly appreciated.

Thanks
linacman

 

[melone]

The clamping diodes should work, but check to see exactly how much energy is being shunted by the diode (measure the flyback voltage and current). Then size the diodes accordingly.

 

[xyzz]

10A rated SSRs are usually of thyristor type and cannot work on DC.
Ensure yours is of a transistor type.

 

[CarlPugh]

Maximum current through diode is same as solenoid current. (4 amp)
Maximum voltage across diode is 12 volt.
Current and voltage are under ideal conditions and generous safety factors should be used when selecting diode.

Diode will increase release time of solenoid. To decrease release time, you can
Add resistor in series with diode.-Increases voltage across solenoid and SSR.
Use zener or transient suppression diode instead of diode.-Increases voltage across solenoid and SSR.

The diode is operating at 500 hertz. This should not be a problem for the diode, but should be checked.

 

[OperaHouse]

You might need to increase the supply voltage at those speeds. Add a resistor if you have to. Remember, you are driving an inductor. A zener across the SSR would be be the fastest. Choose as high a voltage as you can that is below the rating of the SSR. You might be amazed how hot the zener gets.

 

[linacman]

Thanks for the replies from everyone:

To xxyz:

My SSR is one which runs on DC voltages. There are not many available but I found one.

To CarlPugh:

I'm not sure why the diode will change the release time of the solenoid. It will drop the switching voltage by a half a volt. The intent is to block reverse voltages from the inductors. Also, the switching speed I mentioned was 500 time per minute (~8 Hz. rather than 500 Hz.)

To OperaHouse:

I've only got 12 vDC available. I ran the solenoids with a conventional relay for a while (really toasted the contacts) switching at 500 per minute. So really my only concern is the protection of the SSR.

 

[BrianG]

Hi linacman, the diode is not in series with the coil but goes across the coil (so that it has the cathode to +12V). This therefore does not reduce the switching voltage available. As the SSR turns off the diode clamps the back- e.m.f. at the SSR/coil common point to within a diode forward drop above the +12V supply rail, so protects the SSR. However, it has to dissipate all the energy stored in the inductance of the solenoid. During this period the collapsing magnetic field causes a current to continue to flow in the solenoid via the diode. This current opposes the collapsing field, so slowing its decay and hence increases the solenoid release time.

 

[OperaHouse]

Forgot to mention that many DC solid state 4 terminal relays are a darlington output and the saturation voltage can be over 2 volts. That is a lot of power to waste. A close look at the spec sheet will find that you have to derate the relay unless you put an enormous heat sink on. Since you are down at the component level, you might as well go with a power mosfet like IXYS IXTH30N50 with internal diode.

 

[linacman]

Thanks for the explanation, BrianG. It makes perfect sense, even to me! Component level stuff is my weakness and I've been in management too long. Two strikes against me on this little project. (Did you get your premature failing magnetron sorted out?)

Note to OperaHouse: Thanks for your suggestion. I checked the SSR specs and it is described as a "MOSFET Output" device, so I don't think I'll need to change (unless I blow them up and am forced to buy something else.

regards

linacman

 

[buzzp]

Keep in mind that your solenoid may pull up to 20 times the nameplate amps when you energize it each time! These little animals are worse than a motor, on their own scale.

Eaton hydraulics has many notes on protecting electronics hooked to coils, if you can maneuver around their web site.

 

[OperaHouse]

AC solenoids do have a wide current swing because of the change in inductance from core out to in. DC solenoids do not, unless they have a second winding with an internal switch. It is interesting to note that if you monitor the the DC coil current, you can see a change in the slope when the solenoid or relay pulls in due to the change in inductance. This is a very usefull method in determining the mechanical speed of a device when there is no other practical way.

 

[OperaHouse]

For some good theory, download the National semiconductor PDF file on the LM1949 injector driver.

 

[hillwilliam]

The best way to drive a DC solenoid at this frequency is to gate a n-enhancement MOSFET (with the integral diode) using a simple signal generator. Put the solenoid (s)in as the load and there will be essentially no "arcing" as with mechanical relays. Size the fet to the load current. You may have to heatsink the fet and/or cool the solenoid

 

[frankeng9]

OperaHouse is right on target. DC solenoids do not experience the "in-rush" current that an electro-magnetic AC device will encounter. AC solenoids will experience an in-rush current due to the eddy current effect and the change in reluctance (and thus inductance) of the magnetic circuit. SSR’s can violently fail (improper sizing) due to the in-rush current on an AC solenoid, if the solenoid plunger becomes stuck. This effect is not present in DC solenoids however. A DC solenoid is a first-order system and the current rise is that of any first-order system with a very predictable time constant (t = L/R or 63% of I_max). As OperaHouse stated, viewing the current rise of a DC solenoid by running a lead wire of the solenoid through a current probe connected to an oscilloscope, will show the current rise of the inductor (the solenoid), when the solenoid plunger begins to move, and when the solenoid plunger pulls in and comes to rest against the stop. Restricting the movement of a DC solenoid plunger will not harm your SSR, power MOSFET, or the solenoid coil. When the current flow to the solenoid is turned off, the magnetic flux flowing in the circuit resists any change (analogous to inertia) and now the diminishing flux in the magnetic circuit tries to induce a current back into the solenoid coil just as a generator or transformer would. As Operahouse indicated, you can monitor the current rise and fall characteristics of the circuit, but you can also monitor the back EMF voltage spike of the circuit to view the effect of your diode and series suppression resistor. As a note, you can use this monitoring technique to observe when a solenoid will no longer operate due to excessive plunger load or extreme coil temperatures that will reduce the force developed in the magnetic circuit. Hope this information is helpful.

 

[buzzp]

Hmm, a 12VDC mfg claims that DC solenoids pull up to 20 times rated current when power is first applied (assuming no initial charge-not your case). One well known manufacturer is Eaton. Check there site. I remember reading it on there a while back. I never took the time to measure it or think about it.

 

[linacman]

Thanks for this additional input, frankeng9 and for taking the time to post it. I am amazed at the expertise available here.

I have a system working now, although I think I'll need to add two more solenoids to provide enough power to pull what I need to move. It ran for a good long test without damage to anything.

Note to buzzp:

As ou mention in your first post, I found navigating the Eaton site difficult and gave up after a while. Thanks

linacman

 

[skenn]

I suggest you use a hexfet from InternationalRectifier. They have inherant "flyback" diodes, and high voltage/low-on-resistance. I am using one (714 I think) for driving an ignition coil.