A small 75 VA control transformer

From what I could find, those puppies are not cheap.

 

Thanks all for your suggestions. I will analyse what fits better. One last thing is that as I said above, the design has a SMPS flyback with one output winding 12V/100mA. I could modify the transformer design to add a higher voltage output winding. Let's say I add a 80VDC output winding which is enough for the coil. Doing the math for the 290 ohms coil, that's about 22W. I can't get away with 22W in the SMPS. Maybe lowering the current and charging the proper cap could be an option. The thing is that as soon as the solenoid actuates, I need to interrupt its current flow, that's why the pulse control. Perhaps the supply can also handle the surge.

 

From what I could find, those puppies are not cheap.

Whoa, maybe I have been away too long. I figured $15 USD transformer and with bridge and fuse maybe $20 USD total. What I am now seeing is those little transformers are $40 USD for some. Not what I expected and normally I would have linked one like Hammond or Triad but this time I neglected to do so. Guessing the BOM has increased over the past 8 years or so. Wow! Even big power resistors have gone up.

Ron

 

Maybe lowering the current and charging the proper cap could be an option.

That would work.
But to minimize the capacitor size, the voltage should be some greater than 120V so the solenoid stays pulled in for the desired time.
Do you know what the dropout voltage of the solenoid is?

 

That would work.
But to minimize the capacitor size, the voltage should be some greater than 120V so the solenoid stays pulled in for the desired time.
Do you know what the dropout voltage of the solenoid is?

The capacitor voltage will HAVE to be greater than 120 volts because it will start dropping instantly when the solenoid is connected. And it will need to be higher also to overcome the inductive reactance that tries to prevent the current rise. So really it will be "firing" the solenoid with a quick blast of power. Consider that probably the watt-seconds should be the same as if it had been engaged at 120 volts the whole time. Take that number as a start, then add a safety factor. Of course, I am just guessing that rapid operation of the solenoid is important. If it is intended to interrupt fault current that is probably a good guess.

 

That would work.
But to minimize the capacitor size, the voltage should be some greater than 120V so the solenoid stays pulled in for the desired time.

Something like this:

 

Something like this:
View attachment 203703

In the second circuit, R4 & C1 form like an snubber, right?

 

In the second circuit, R4 & C1 form like an snubber, right?

Capacitor in parallel with load (L1) will be snubber, it smooths current pulse.
But in series with load, capacitor C1 narrows current pulse (anti-snubber).
Resistor R4 is burden load. 560V drops on R4 and 120V drops on L1.

 

Something like this:

Note that the peak power in the 1.4k ohm resistor is about 224W, so it will need to be hefty to tolerate that, even if it is for only 25ms.
I suspect you would need a 5 to 10W resistor to handle that overload without exceeding its operating temperature but a wirewound 1 or 2W may work.

 

Something like this:
View attachment 203703

This circuit shown may work but it is not even slightly close to what I was intending to describe. This circuit has the capacitor charging rapidly to create a current pulse, and it has a whole lot of power dissipated in a series resistor, And there is no capacitor getting charged up to dump charge into the solenoid coil. So Danko has his own idea, totally different in every aspect.

 

These Forums are hilarious at times.
Everybody making wild guesses and coming up with bizarre solutions.
All Information/Specifications/Limitations must be compiled before the first question is asked.
1) A "relay" is not a "solenoid".
2) Does this relay already exist ?, and exactly what does it control, how does it control it, and at how much current ??
3) Where did you find a relay that can reliably respond in ~25ms ?? especially if it's a large, clunky, high power, 3-Phase Contactor.
4) Nobody mentioned whether the relay must also turn off in 25ms, and how often it will be required to change states.
5) Why is a relay being used in the first place ??
6) Is this a theoretical Classroom Test Question ?, and the rest of you guys are just messing with a poor confused student ?
7) What is the high voltage 3-Phase Delta supply powering ?, and through what type of controller ?
8) Is the 3-phase power being interrupted by the "relay" ?
9) You say there is limited space, then why are you using a relay in the first place ?
10) If the relay is already a part of the equipment design, why can't the relay be swapped-out for a higher voltage rated model ?
11) Does the "Micro-Controller" already exist as part of the existing control equipment ? If it doesn't, DON'T USE ONE.
12) What type of Protection Circuit is this ?, what exactly is it protecting ?,
under what exact circumstances must the protection be triggered ?,
does the control or protection loop already exist ? how does it work ?
why does this "relay" not already have a control system in place ?, did the original system fail ?
13) What type of signal is used to initiate the fault sequence ?
a limit-switch ?, excessive voltage or current ? loss of voltage ?, short-cycling of a process loop ?, missing event ?
14) Is the "25ms" a response speed requirement ?, is it temporary and for what duration, or latched until reset ?
15) Messing around with a cobbled-up "Unusual-Solution" patch on
high powered equipment can get VERY EXPENSIVE and/or DEADLY at the drop of a hat.

 

Everybody making wild guesses and coming up with bizarre solutions.

Agree.

I have a 120VDC solenoid that I want to use in 480VAC application,

Also Mention of 12v and Micro (5vdc?). Potential safety nightmare.!
IMO there is only one way and that is to use the properly designed, typical equipment used in a project like this , especially when 480vav (3ph?) is the source.
Max.

 

These Forums are hilarious at times.
Everybody making wild guesses and coming up with bizarre solutions.
All Information/Specifications/Limitations must be compiled before the first question is asked.
1) A "relay" is not a "solenoid".
2) Does this relay already exist ?, and exactly what does it control, how does it control it, and at how much current ??
3) Where did you find a relay that can reliably respond in ~25ms ?? especially if it's a large, clunky, high power, 3-Phase Contactor.
4) Nobody mentioned whether the relay must also turn off in 25ms, and how often it will be required to change states.
5) Why is a relay being used in the first place ??
6) Is this a theoretical Classroom Test Question ?, and the rest of you guys are just messing with a poor confused student ?
7) What is the high voltage 3-Phase Delta supply powering ?, and through what type of controller ?
8) Is the 3-phase power being interrupted by the "relay" ?
9) You say there is limited space, then why are you using a relay in the first place ?
10) If the relay is already a part of the equipment design, why can't the relay be swapped-out for a higher voltage rated model ?
11) Does the "Micro-Controller" already exist as part of the existing control equipment ? If it doesn't, DON'T USE ONE.
12) What type of Protection Circuit is this ?, what exactly is it protecting ?,
under what exact circumstances must the protection be triggered ?,
does the control or protection loop already exist ? how does it work ?
why does this "relay" not already have a control system in place ?, did the original system fail ?
13) What type of signal is used to initiate the fault sequence ?
a limit-switch ?, excessive voltage or current ? loss of voltage ?, short-cycling of a process loop ?, missing event ?
14) Is the "25ms" a response speed requirement ?, is it temporary and for what duration, or latched until reset ?
15) Messing around with a cobbled-up "Unusual-Solution" patch on
high powered equipment can get VERY EXPENSIVE and/or DEADLY at the drop of a hat.

I do not recall the TS mentioning a relay, only a solenoid that somehow launched a disconnection process. Large hgh voltage system switches often use a remote-trip function to disconnect the load. And medium sized circuit breakers often have an external trip input. That was my guess after the TS mentioned that it was to disconnect the power when there was a fault.

 

The design has no access to the neutral. Seems like I'm doomed.

Does it even have a neutral?
i.e. 3ph delta transformer for e.g.?
Or has it been confirmed a earth grounded neutral is actually present?
If not, without galvanic isolation, a danger to personnel operating any part of it can become a possibility.
Max.