Hi Folks

I've been running around the internet for quite a bit trying to find the best solution for the system I am building. It is based around DC solenoids powered by 24vdc.

Parameters:
Load - 24vdc solenoid, 40ohm resistance
Relay - 24vdc actuated mechanical 240v AC max 10A max (resistive load)
Source - 24vdc
RCD Snubber to be placed parallel with load

My understanding is i would be best to implement an Resistor Capacitor Diode circuit to:
- reduce contact arcing on relay
- reduce emf
- reduce backward flow of current to other components.

I have investigated snubbing diodes, but my understanding is that they are slow to dissipate the energy so might impact closing time of the solenoid. I need the switching off of the solenoid to be as fast as possible.

Any help with choosing the right components would be much appreciated.

Basic schematic attached.

Many thanks

 

The diode across the coil can delay the drop out, but in a small relay it is insignificant, only a concern in large contactors or brakes and clutches etc.
Max,

 

If the dropout speed is really a concern, a resistor in series with the diode will increase the rate of power dissipation.
Of course that will also increase the inductive kickback.
For example, a resistor in series with diode equal in value to the coil resistance will give a kickback voltage equal to the supply voltage.

 

Looking at your pdf, I would use a diode across the solenoid coil to prevent excessive back emf and arcing of the relay contacts.
I would also put a diode across the relay coil to do the same thing. RC snubbers are generally used across contacts of relays controlling AC loads.

 

Also, a zener in series with a 1n4004 across the coil would take care of the back EMF and speed up the release. Orient the zener so that it's back biased when the 1n4004 is conducting.

 

Thanks everyone.

Yes speed is a consideration, as the faster the solenoid closes, the better the result in this application.

I am using the same power supply to also power a transformer. The solenoids and this transformer are operated at the same time, and have found that when switching off the transformer and solenoids simultaneously, the transformer cause the solenoids to be slow closing (even when controlling each device separately via their own relay).

Transformer - Primary winding: 24vdc 3.5A, secondary: Centre tapped

Will an RC snubber in this instance dissipate the unwanted current?

Again, please see rough schematic attached.


Cheers

 

Are you sure the solenoids selected are capable of the operating speed you need?
There are fast acting versions as used in precise fluid dispensing apps etc.
Max.

 

Yes speed is a consideration, as the faster the solenoid closes, the better the result in this application.

Then the fastest drop-out time will be obtained with the diode-zener combination.
You select the zener voltage to the highest value that can be tolerated by the driver circuit.

 

Various companies I've worked for made suppressor networks for relay manufacturers. These are just as described, a diode and a zener. We used a 1N4006 with a 36V zener.

Such a combo will put the 24V supply in series with the 36 zener and the diode drop for around 61 V on the coil driver. Plan accordingly.

 

Are you sure the solenoids selected are capable of the operating speed you need?
There are fast acting versions as used in precise fluid dispensing apps etc.
Max.

Hi Max, there is a notable difference in solenoid closing speed when I remove the transformer from the system. Standalone, the speed of the solenoids are fit for purpose.

 

Hi Max, there is a notable difference in solenoid closing speed when I remove the transformer from the system. Standalone, the speed of the solenoids are fit for purpose.

Remove the transformer and how are they fed 'stand alone' ?
Max.

 

Remove the transformer and how are they fed 'stand alone' ?
Max.

As per my first diagram. Either through battery source or 240ac to 24Vdc power supply

 

Hi Folks

I've been running around the internet for quite a bit trying to find the best solution for the system I am building. It is based around DC solenoids powered by 24vdc.

Parameters:
Load - 24vdc solenoid, 40ohm resistance
Relay - 24vdc actuated mechanical 240v AC max 10A max (resistive load)
Source - 24vdc
RCD Snubber to be placed parallel with load

My understanding is i would be best to implement an Resistor Capacitor Diode circuit to:
- reduce contact arcing on relay
- reduce emf
- reduce backward flow of current to other components.

I have investigated snubbing diodes, but my understanding is that they are slow to dissipate the energy so might impact closing time of the solenoid. I need the switching off of the solenoid to be as fast as possible.

Any help with choosing the right components would be much appreciated.

Basic schematic attached.

Many thanks

@JimmyChristo
Consider using a diode with a resistor in series with the diode. A very low resistance will give same result as diode alone; a very high resistance will give same result as no diode connected. A useful resistance will reduce the flyback transient sufficiently while still giving fast relay opening--you have to be the judge. Viewing the contact voltage with an oscilloscope is very helpful in determining an acceptable resistance. Viewing the contact opening with the human eye is not very useful unless really large sparks are being generated.

Hi Folks

I've been running around the internet for quite a bit trying to find the best solution for the system I am building. It is based around DC solenoids powered by 24vdc.

Parameters:
Load - 24vdc solenoid, 40ohm resistance
Relay - 24vdc actuated mechanical 240v AC max 10A max (resistive load)
Source - 24vdc
RCD Snubber to be placed parallel with load

My understanding is i would be best to implement an Resistor Capacitor Diode circuit to:
- reduce contact arcing on relay
- reduce emf
- reduce backward flow of current to other components.

I have investigated snubbing diodes, but my understanding is that they are slow to dissipate the energy so might impact closing time of the solenoid. I need the switching off of the solenoid to be as fast as possible.

Any help with choosing the right components would be much appreciated.

Basic schematic attached.

Many thanks

@JimmyChristo
I am confused. If you are using relays and solenoids designed for DC, why do you expect them to operate identically (if at all) on AC (the transformer)?

 

@JimmyChristo
Consider using a diode with a resistor in series with the diode. A very low resistance will give same result as diode alone; a very high resistance will give same result as no diode connected. A useful resistance will reduce the flyback transient sufficiently while still giving fast relay opening--you have to be the judge. Viewing the contact voltage with an oscilloscope is very helpful in determining an acceptable resistance. Viewing the contact opening with the human eye is not very useful unless really large sparks are being generated.


@JimmyChristo
I am confused. If you are using relays and solenoids designed for DC, why do you expect them to operate identically (if at all) on AC (the transformer)?

@TeeKay6 The transformer in the system is not powering the solenoids, but being turned on and off at the same time as the solenoids. It’s a high voltage transformer, and it’s supply is also 24Vdc. The supply to both the solenoids and the transformer is the same 24Vdc source (either via batteries or via a step down power supply. Basically the system can run off mains 240vac power (via step down power supply), or directly from battery.

 

@TeeKay6 The transformer in the system is not powering the solenoids, but being turned on and off at the same time as the solenoids. It’s a high voltage transformer, and it’s supply is also 24Vdc. The supply to both the solenoids and the transformer is the same 24Vdc source (either via batteries or via a step down power supply. Basically the system can run off mains 240vac power (via step down power supply), or directly from battery.

@JimmyChristo
I remain confused. You cannot drive a transformer with DC ("...its supply is also 24VDC").

 

What is very clear is that the rest of the operation is unknown. Feeding a transformer with switched DC is seldom done, unless you are trying to create high voltage spikes, such as in a spark igniter system. And if you really need very fast solenoid operation then you may need to use a relay with much higher rated contacts and just live with the sparking.
Without any more information about the application it is unlikely that you can get much useful help, instead getting good guesses at best.
There are a lot of smart folks willing to help, but guesses based on no information are seldom the best advice.

 

Hi everyone, thank you for your responses.

So I have tried the following to reduce the arcing on the relay @JohnInTX @crutschow @ErnieM

• added a 1N4007 1000v diode and a 1N47531 36v zener in series, wired across the load. See configuration attached.

I noticed that there was no difference in spark on the relay contact. However, I removed the zener (leaving just the diode) and noticed a visible reduction. It was not eliminated, but it was reduced. I have no means to measure this kickback unfortunately. I also tried to reverse the zener polarity as an experiment, and it reduced the spark also. By doing this though, I wonder if I have basically eliminated the function of the zener? Any thoughts on this?

@MisterBill2 yes you are correct, its a high voltage spark transformer with inbuilt oscillation circuitry. I have tried putting a 1N4007 in series with the supply to this transformer, and it seems to have stopped the current flowing back and holding open the solenoids. Unfortunately I don't have much info on this transformer, and with my limited knowledge on electronics I am unsure if this is the best solution. That's why I was researching RC snubbing, as I thought it would reduce the kickback, and also dissipate the reverse current at the same time.

 

A simple thing would be to use a relay with two sets of contacts, with one set controlling your spark generator and the other set controlling the solenoids. That will provide the isolation that you need for spark suppression on the solenoid portion and also keep the high voltage transients out of the electronics portion of your spark generator. I doubt that the spark circuit has much of a turn-off spike, so that should not be a problem.
The snubber circuit values will depend on the current drawn by the solenoid valves, and how often they cycle. For a start I suggest a 47 ohm resistor and a 0.1 Mfd 200 volt capacitor in series across the contacts.

 

A simple thing would be to use a relay with two sets of contacts, with one set controlling your spark generator and the other set controlling the solenoids. That will provide the isolation that you need for spark suppression on the solenoid portion and also keep the high voltage transients out of the electronics portion of your spark generator. I doubt that the spark circuit has much of a turn-off spike, so that should not be a problem.
The snubber circuit values will depend on the current drawn by the solenoid valves, and how often they cycle. For a start I suggest a 47 ohm resistor and a 0.1 Mfd 200 volt capacitor in series across the contacts.

Using different relays, or a dual pole relay is essentially what I am doing. As the common contact is supplied by the same power source, it seems to still hold up the closing of the solenoid.

Will try that RC combination. Can you please confirm if that capacitor rating is 0.1 millifarad, or microfarad?

Cheers

 

Using different relays, or a dual pole relay is essentially what I am doing. As the common contact is supplied by the same power source, it seems to still hold up the closing of the solenoid.

Will try that RC combination. Can you please confirm if that capacitor rating is 0.1 millifarad, or microfarad?

Cheers

One MICROfarad! A Millifarad capacitor would be quite large, in fact, I have never seen any capacitor value given in millifarads.