Electronics > Projects, Designs, and Technical Stuff
SMD alternative to TIP31 and TIP32
T3sl4co1l:
Haha ew, double crossover distortion. LM324 plus an unbiased (class C) emitter follower!
--- Quote from: floobydust on November 10, 2019, 05:27:30 am ---If the servo motor can jam or get overloaded, a SOT-23 BJT would exit in a puff of smoke.
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Well obviously, you don't get much power through a SOT-23 regardless. You wouldn't dissipate it in the first place -- use a class D driver. At which point you can plop in a cheap 8-pin audio amp, with self contained oscillator and biasing, and you don't even need to add an RF filter if the motor leads are short and shielded. Bridged amps are also abundant so you might use 5 or 12V instead of +/-15. :-+
Could even just use a LM393 for osc/PWM and a gate driver (TC4428ish?) if you can be reasonably sure the load won't go short circuit or anything. But any discrete solution will be hilariously larger than a self-contained amp, it really is a very good option here.
Tim
rsjsouza:
It seems an overcomplicated alternative to an on/off operation. Do you have links or rough drawings to what you are saying?
(Of course, my interest is purely curiosity as I saw myself in the same corner in the past and opted for the well known to me TH option).
T3sl4co1l:
This plus a gate driver gets you a solid amp or so of output drive:
Add on a feedback error amp (to match the above servo example, it'd be the 2 x 82.5R current feedback and U?C error amp) and you've got a class D amp with constant current output and no dead band (give or take if you replace the crusty old LM324 with a nicer e.g. TLV2372 :) ).
Something like TC4420 has a 6A peak output capacity, from a few ohms output resistance; it would be fine at 100s mA continuous duty, if maybe not an ampere or more. It's actually rated for reactive current of similar magnitude (500mA without causing CMOS latchup). Else you can add any inverter you like (last example I used, TPS28225 + SIS932EDN).
And as power goes up and dissipation rating goes down, you can consider other current sense or fault protection mechanisms.
Again, you can get various chips that basically integrate all this functionality into a little amp, all you need are a couple bias resistors (if that), a bypass, and some filtering (which you may be able to waive).
Even for something like a solenoid valve, this approach isn't completely off the wall. Higher voltage drive can be used to accelerate response time; a constant-current driver should then be used to maintain holding current. You can actually get higher performance at lower overall power consumption this way (depending on how close to minimum holding current you want to run at, and how often it's switched). When this is done with switching sources, you can get even better improvement. There are relay/solenoid drivers/controllers for this purpose. (I don't know any part numbers offhand, though.)
Tim
v8dave:
--- Quote from: rsjsouza on November 10, 2019, 11:58:27 am ---It seems an overcomplicated alternative to an on/off operation. Do you have links or rough drawings to what you are saying?
--- End quote ---
It is not ON/OFF as you have with solenoid valves, it is a servo so it is a linear output to drive the servo from closed to partially to fully open in both directions. +-5 with 0 for off. 2.5V would open the valve midway.
v8dave:
--- Quote from: T3sl4co1l on November 10, 2019, 01:35:45 pm ---This plus a gate driver gets you a solid amp or so of output drive:
Even for something like a solenoid valve, this approach isn't completely off the wall. Higher voltage drive can be used to accelerate response time; a constant-current driver should then be used to maintain holding current. You can actually get higher performance at lower overall power consumption this way (depending on how close to minimum holding current you want to run at, and how often it's switched). When this is done with switching sources, you can get even better improvement. There are relay/solenoid drivers/controllers for this purpose. (I don't know any part numbers offhand, though.)
Tim
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Thanks, Tim, but this is to drive a servo valve which can be positioned part open to control the flow rate. I have to get 16 of them onto a board that is VME form factor along with the feedback and the ADC/DAC etc.
As for solenoid, I've used PWM before to reduce the total current when the valves are held in the on state. I used 100% to pull the valve in and then reduce this to under 50% to the point where the valve stays open and this reduces the overall current. Ideal when you have 24 of them with about 10 possibly on at the same time.
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