I have a constant duty AC (28 VAC) motor that runs in one of my pinball machines. I wish to control power to the motor such that it only runs during gameplay. There are "recipe" circuits that do this for DC motors that do the same thing, in pinball machines. The heart of this "recipe" circuit is a TIP125 transistor. Will this transistor work in an AC voltage circuit?, Or, is control of AC motors more complicated than I am anticipating?
"Dan Beck" schreef in bericht news: snipped-for-privacy@corp.supernews.com...
Dan,
No and no. A tip 125 definitively cannot switch an AC circuit and control of an AC motor is not more complicated than DC.
First of all, how is the motor controlled now? Is it controlled anyhow? Or is it (or its 28VAC power source) directly switched by the mains? You need to find or to make some signal telling the machine someone is playing. That signal can be used to switch a relay or a triac which in turn switches the motor. Take into account that the motor will need some time to be fully operational after switching on. After switching off, it has also come to full standstill before it is safe to turn it on again.
However, if you interpose a bridge rectifier in series and connect the TIP across the + and - of the bridge then it will work with certain reservations about grounding that circuit and how the on-off control is derived - it may need opto-isolating. I could only advise if I saw your application circuit.
thank you for all your technical (and whimsical!) responses. To answer a few questions posed:
This motor runs directly off of the 28 VAC lugs from a transformer inside the game, and would run continuously while the game is powered up. Originally when this pinball was operated on location this motor drove a spinning wheel in the upper box (where the digital scores are); this spinning wheel would be considered an "attract" feature, in order to lure more players and hence earn money. In the home setting this is unnecessary, in my opinion. Furthermore, I think it's a neat effect to have the wheel start spinning once you initiate a new game.
There is a signal in the game that I could apply to the base of a transistor controlling the motor; because it is an AC motor I think I would have to use a relay to control the AC. This would achieve the effect I am looking for. As a matter of fact (after reviewing the schematics) there are similar applications for different functions in this particular game I am talking about, that use a transistor to control relays.
I do have a question though about the particular signal I was referring to above. This signal emanates from a 74175 TTL chip, and drives a Darlington transistor, which in turn permits voltage/current to a small relay solenoid. What I would be doing is essentially splitting that signal to a SECOND, identical Darlington permitting current to an identical relay solenoid. Will that signal from the 74175 have enough "oomph" to drive the two Darlingtons, whose part number is MPSU45? MPSU45 crosses to an ECG272, according to my ECG cross reference book.
============================================ Sounds like a job for a small solid state relay. AC (output) side goes to the fan, dc (input) side goes to any signal anywhere in the machine that goes hi when the carbon based units start actuating the controls.
Almost certainly. Darlingtons are used because they have very high current gain, so the input current is very small, compared to the output current. If there is no resistor between the 74175 output and the darlington base, you should add a pair, to divide the output current equally to the two bases.
You could also replace both darlingtons with logic level N-channel mosfets, which draw essentially zero DC on state gate current. Then no resistors would be needed.
thank you for your thoughtful and informative message. May I be so bold as to indulge you a few more questions? They are interspersed with your message, below.
When you say to add some resistors, what would be your best guess for rating? 2k ohm? 4.7k ohm?
How would I match specifications for the mosfets, with respect to the Darlingtons I would be replacing? Certainly I can find the Darlington specs in the book, but in my hands finding the most appropriate mosfet would be akin to finding a needle in a haystack... :-)
Is there any resistor between the output and the base, now? TTL family chips don't pull positive nearly as well as they pull negative. See VOH (output high voltage spec) on page 4 of:
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The guaranteed high output voltage is typically 3.4 volts while the output is pulling positive with an 800 uA current. That is what you have to turn on your two darlingtons. Those MPSU45:
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need about 2 volts base to emitter (base emitter saturation voltage on page 2). I don't know what current your relay coils draw, but if it is about a half ampere, the darlington minimum gain is at least 15,000, but you can't get very good turn on saturation voltage at that. Lets say you need about
500 mA / 5000 = .1 mA base drive to be turned pretty thoroughly. But the collector saturation curves on page 3 show what happens when the base current is 1/300 of the collector current, so I would take that as a recommendation of about what base drive is needed to really get these things turned on well. So you should be thinking in terms of about 1 mA base drive. This means that between the flip flop output voltage of 3.4 volts (which you won't quite get, because you are talking about t2 mA of load current and that spec is based om .8 mA of load current) and a base voltage of 2, the resistor has about 1.4 volts to waste while passing at least 1 mA. That means it should work with a resistor or no more than 1.4 / 1 mA = 1.4k. If you use a very low value, or no resistor at all, the base with the lowest voltage drop will suck up the majority of the current and the other may not be turned on enough. So, any resistor around 1k should work. Expect the flip flop output voltage to be less than 3.4 volts.
It should have plenty of drain voltage rating to handle the DC supply feeding the relays, and have a channel resistance that drops only a small fraction of that supply voltage when the gate is driven with 3 or 4 volts (the flip flop will produce a bit more on state voltage, since the gates draw no DC current.) Most mosfets are designed to be fully turned on with about 10 volts gate to source, so you will not get any where near their specified on state resistance when they are operated with 3.5 volts. MOSFETS specifically designed to switch with logic level outputs would work better.
I don't know what your coil currents are, but to be conservative, lets say they are near the maximum rating of the MPSU45 at 2 amps. I also don't know what your relay supply voltage is, but we can start with mosfets rated for the same 40 volts the darlington is rated for. If the mosfet drops no more than 1.5 volts, it will get no warmer than the darlington would, though lower drop would put more of the supply across the coil.
Looking for a cheap and common logic level deviuce that fits these requirements, I come up with IRL520N (though it has a different pinout):
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rated for 100 volts when off, drops less than 1 volt with 3 volts gate drive and a 2 amp load, less than .4 volts drop with 4 volts gate drive. (figure 1, page 3).
If your coil currents are a lot less than 2 amps, a smaller device could well work.
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(about 5 volt drop with .5 amp)
Make sure you connect a diode across each coil, to short out the voltage spike they will generate when the current is switched off, the way an ignition coil does. The diode should have the banded end connected to the positive supply end of the coil.
"Dan Beck" schreef in bericht news: snipped-for-privacy@corp.supernews.com...
Now you found the signal and want to use a relay, you have to find out what current the AC motor requires. Keep in mind that the inrush current may be a multiple of of the normal operating current. The relay you choose, needs to be able to handle the currents. This in turn sets constrains on the relay. Huge currents require big contacts which need much power to be switched and the relay coil has to provide that power. Of course the relay coil needs to operate at an DC power level you have available inside the machine.
When you choose the Darlingtons mentioned, keep in mind that they still have about 1.5V between collector and emitter when on, so a 5V relay will not be a good choice.
The 74175 will hardly be able to drive two Darlingtons on its own. It can source up to some hundreds of uA which I consider too low for even one Darlington. But the device can sink up to 16mA, so you can use a pull up resistor two provide extra current to drive the Darlingtons. (See below using fixed font)
You got one of your diodes connected wrong, but I agree that this is a good way to increase the pull up capability of TTL. Works good to increase the voltage applied to MOSFET gates, also.
| You got one of your diodes connected wrong, but I agree that this is a | good way to increase the pull up capability of TTL. Works good to | increase the voltage applied to MOSFET gates, also.
John,
Oops, I see, thanks. Original modified, see above.
| You got one of your diodes connected wrong, but I agree that this is a | good way to increase the pull up capability of TTL. Works good to | increase the voltage applied to MOSFET gates, also.
Original schematic modified.
Went to Google groups to check out all postings on this thread. An N-channel logic MOSFET will be a good choice to drive the relay but you'd better replace the existing Darlington too. A pull up resistor will still be usefull but is not critical. A 10k will do.
A solid state relay will be the most simple solution although a little expensive. The input can be connected parallel to the load of the existing Darlington. Keep an eye on the maxima (voltage, current) the input can handle. No need to say the output should be able to handle the load (i.e. the motor) like I mentioned before.
(As an aside: I went to Google groups. My ISP, although claiming to be the best in the country, considers its newsservers to be an "experimental" service. I've access to two of these servers but I nevertheless miss postings, including some of my own. I never missed one in Google groups so far.)
thank you again for your responses to my questions! They have been most helpful.
If I may indulge one more time, I need to expose my ignorance again. I am going to go the route with the mosfets. The IRL520 does come in a TO-220 package, which would be the most compatible package with the circuit board I have. I certainly know how to tell where the emitter, collector, and base are on a transistor, (using the data sheets)however I am not sure how a mosfet correlates. My guess is below:
emitter ===== drain base ======= gate collector===== source
Please correct me, if I am in the wrong here! I will follow up my progress as I obtain results.
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