Make a 2A pulse from 500mA

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#1
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A place where I work, have just got some 12V 5ohm solenoids, the problem is that I only can give about 700mA (and the PSU only 1A , and max 190% for a short time), so I can't just use a bigger transistor.

It should have been 24 ohm as I had for test.

 

Just a serial resistor don't work. ( need more start torque)

 

I just checked: a 4700uF cap at 12V can open the solenoid

When first open I only need about 100mA to hold it.

All the extra has to be placed outside the PCB.( after my 700mA output)

 

Any smart way to do that ?

This topic has a solution.
Last Edited: Tue. May 31, 2016 - 12:21 PM
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Is there an important reason why the supplier cannot take the wrong solenoids back and supply the 24 ohm ones instead? If it is their problem, why must you provide a technical solution?

Ross McKenzie ValuSoft Melbourne Australia

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main problem is they need them now! and I guess that it take 2-3 week to get some new. (perhaps if they want to pay about $20 they can get them day to day), I should get some (about 5) to work today, for some mechanical tests.

 

 

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Seems like you could use PWM in a half bridge, with series inductor to charge large capacitor. Ramp the PWM profile to control charge current. Solenoid driver uses additional transistor, full on for open, PWM to hold. A tiny 45 could do this.

It all starts with a mental vision.

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I didn't plan on using a micro, but I have about 250 tiny85 somewhere, if it's needed.

 

I was more thinking of: charge the cap thru a 22 ohm res., and when the the voltage reach something like 10-11V (or by time to charge a small cap) a transistor open (and stay open), until I turn the hole ting off (a charging time up to  300 ms is ok)  

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I am confused.   You should have a 24V solenoid working from a 24V supply.    With a 24 ohm resistance,  this would be 1000mA current.

 

If you have got a 12V solenoid with a 5 ohm resistance this will need great care from your power supply.

You could simply charge a 4700uF capacitor at 100mA via a 120 ohm series resistor from a 24V supply.

 

There will be 1.35 Coulombs available for 2.4A inrush current.   100mA for hold current.

 

You will need to experiment.    Is 1 Coulomb enough to pull the solenoid in?

Is 100mA "safe" for hold current?

Is your 4700uF going to be happy with a 2A discharge current?

 

Since your Power Supply could manage 500mA you can afford a faster charging current and safer hold current.

 

My apologies if you do not have a 24V supply.

If you only have a 12V supply,   I am surprised that a 24 ohm solenoid could have a comparable performance to a 5 ohm one.    You could pre-charge a 4700uF but don't have Coulombs available.

 

David.

Last Edited: Tue. May 31, 2016 - 12:05 PM
This reply has been marked as the solution. 
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I only have 12V

and I guess there is a performance difference if you pay $3 or $20 for about the same thing. (the problem is that I don't have data for the 24 ohm it just came from somewhere).

 

Now I try with a relay , 22 ohm res and a 4700uF cap, and I guess that it should work.

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ok #7  works fine

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Pre-charging the 4700uF from 12V might be enough Coulombs (0.35) to pull the solenoid in.

 

I doubt if a relay will be any different to a saturated MOSFET or NPN.    They will all be around 0.2V.    Ok,  the relay may be 0.0V when new.

 

We are spoiled in the UK.   I am sure that Farnell would get replacement Solenoids to the UK in 24 hours.

 

David.

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As #1 yes the 4700uF can make the pull.

 

The problem is that my control out is collector on a NPN, (and then the other end direct to 12V).

I could not find an easy transistor way.

And I have all the components.

 

To help the PSU perhaps I should add a reverse diode. 

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The NPN should be fine.   It will need a hefty base current to saturate.   3A transistors tend to have a poor hFE.   An AVR's GPIO pin can only provide 40mA.

 

Or you use a PNP in combination with the NPN.

 

You certainly need a zener or reverse bias diode for when the solenoid is switched off.

 

I am fascinated by 2400mA pull-in vs 100mA hold.   Mind you,  a solenoid is different to a relay.  It has a far bigger movement.

 

David.

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 It has a far bigger movement.

No not this one.

ok i need about 160mA is safe hold. (depends of spring)

But with a 22 ohm I use about the 500mA and it should be with the correct solenoid.

 

And to make it clear I have made an NPN transistor out that ground a terminal (max current 700mA), and the solenoid should go from 12V and that terminal (I have a 12V terminal next to it for the same reason) and I can't change that.

So no AVR pin's etc. can be added, and no PNP first.

 

 

Last Edited: Tue. May 31, 2016 - 02:12 PM
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What they used to do with stepper drivers was have a Big Resistor and a Big Cap in front of the coil, and hit it with 48V or something, and the cap would pull it in and the R would hold it. The Rs got hot, but the power supply was plugged into the wall, so who cares? You could rig your 12V trans from parallel to series secondaries if you are lucky enough to have split secondaries ( 2 pairs of wires). Hope this helps.

 

Imagecraft compiler user

Last Edited: Tue. May 31, 2016 - 08:14 PM