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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Hello, So here is my problem.... I need to generate a waveform that spikes to 24V for a few ms then falls back down to 12V for the duration of the wave. The supply is 12V. This is for a solenoid valve. I ordered the wrong type (too low of a pressure rating) and the engineers say that if I apply 2x the voltage for no more than 10ms the valve will open. Once open, 12V will keep it that way. I am looking for the simplest method. I imagine it can be done with a clever inductor circuit, something similar to the boost converter action? Thanks!
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
You could put the solenoid in parallel with a capacitor and feed it the 12 v through a diode, and then use a boost circuit to add a current pulse ... but as I said, a current pulse. You get a damped sine on top of the DC voltage, not a neat square pulse.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
A clever inductor would do it, or it could be done something like this.
The two resistors charge the capacitor to 12v (eventually). When the switch closes (could be a relay or FET, or bipolar, or even a switch), the capacitor voltage is stood on top of the input voltage, sending the output to +24v, less a diode drop. When it has run down or the switch opens, the other diode maintains 12v to the output.
As it starts discharging at once, it won't meet any minimum time at 24v, in fact with the diode drop it only gets to 23v. However, it's simple enough to throw one together and see if it works.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Wow just tried it out and it gets the right waveform! Thanks! Is there a simple way that I could use a transistor as the switch and only have it conduct for a certain amount of time no matter how long the signal is? For example, a capacitor in series with the base with a large value resistor in parallel with the capacitor. Choose values so that the transistor only has enough current to the base as the capacitor charges. The resistor would then discharge the capacitor when the signal is gone. I would also have a MOSFET on the output connected to the valve that would start conducting at the same time as the switch in the pulse circuit.
The extra circuitry is so that I can send a single signal into this circuit and get the desired pulse that goes from 0V to 24V then falls to 12V.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
What we did in past for brake plate solonoids that needed a spike to get them closed, and then less to keep them closed, was connect a lightbulb in series. Bulbs have low cold resistance and high hot resistance.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Turkey9 wrote ... Is there a simple way that I could use a transistor as the switch and only have it conduct for a certain amount of time no matter how long the signal is?
Is a 555 simple enough? If so just google for 555 and monostable.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
A 555 would work but I'm going for as simple as possible here. The solenoid valve will be driven PWM from a microcontroller and I don't want to have to deal with two signals to actuate the valve. The ideal waveform would go from zero to 24V then decay to 12V when an input of 12V is applied.
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Voltage spike generator
