Newbie (ignorant) tech question

[Dan Beck]

Hello all,

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?

Thank you for any and all responses!

Regards,
Dan

 

[Tom Biasi]

Hello all,

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?

Thank you for any and all responses!

Regards,
Dan

You can control the motor with a relay and control the relay with many
different circuits.

Tom

 

[petrus bitbyter]

Hello all,

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?

Thank you for any and all responses!

Regards,
Dan

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.

petrus bitbyter

 

[Eeyore]

Hello all,

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?

That TIP can only control DC.

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.

Graham

 

[default]

Hello all,

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?

Thank you for any and all responses!

Regards,
Dan

No, you don't typically use a transistor to switch AC - it can be done
but it is perverse.

Use a relay powered from some other circuit that will provide a
voltage to the coil of the relay when you want the motor to turn on.

The voltage you have and relay coil voltage must be compatible.

Some Solid State Relays will switch 24 VAC to 240 VAC and only require
a DC voltage of 3.5 to 12 for the "coil"

 

[jasen]

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?,

look at using a TRIAC or relay to swith the motor.


Bye.
Jasen

 

[Homer J Simpson]

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?

Might you be better off running the motor at a slower speed?








--

 

[BobG]

Just unplug it from the wall. That'll stop the darn motor from turning.

 

[Dan Beck]

Hello all,

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.

Thank you for reading!

Regards,
Dan

 

[BobG]

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.

============================================
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.

 

[John Popelish]

Hello all,

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.

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.

 

[Dan Beck]

Hi John,

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.

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.

When you say to add some resistors, what would be your best guess for
rating? 2k ohm? 4.7k ohm?

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.

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...

Regards,
Dan

 

[John Popelish]

Hi John,

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?

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:
http://www.datasheet4u.com/html/S/N/7/SN74175_TexasInstruments.pdf.html
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:
http://www.datasheet4u.com/html/M/P/S/MPS-U45_MotorolaSemiconductor.pdf.html
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.

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...

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):
http://www.irf.com/product-info/datasheets/data/irl520ns.pdf
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.
http://www.irf.com/product-info/datasheets/data/irld024.pdf
(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.

 

[petrus bitbyter]

Hello all,

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.

Thank you for reading!

Regards,
Dan

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)

VCC=5V
+ +----+------+------+-- plus
| | | | | power
| | | | |
| - _|_ | |
.-. ^ |_/_| | |
680E| | | | | |
0.5W| | | | | |
'-' | | | |
from | ___ | |/ | |
------+-+-|___|-+--| | |
74175 | 150E |> _|_ -
| | |_/_| ^
| | | |
| | +------+
| | |
| | |
| ___ | |/
+-|___|------)----|
150E | |>
| |
| |
--------------------+------+------+--
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


petrus bitbyter

 

[[email protected]]

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)

VCC=5V
+ +----+------+------+-- plus
| | | | | power
| | | | |
| - _|_ | |
.-. ^ |_/_| | |
680E| | | | | |
0.5W| | | | | |
'-' | | | |
from | ___ | |/ | |
------+-+-|___|-+--| | |
74175 | 150E |> _|_ -
| | |_/_| ^
| | | |
| | +------+
| | |
| | |
| ___ | |/
+-|___|------)----|
150E | |>
| |
| |
--------------------+------+------+--
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

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.

 

[petrus bitbyter]

<[email protected]> schreef in bericht

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)

VCC=5V
+ +----+------+------+-- plus
| | | | | power
| | | | |
| - _|_ | |
.-. ^ |_/_| | |
680E| | | | | |
0.5W| | +----+ | |
'-' | | |
from | ___ |/ | |
------+-+-|___|----| | |
74175 | 150E |> _|_ -
| | |_/_| ^
| | | |
| | +------+
| | |
| | |
| ___ | |/
+-|___|------)----|
150E | |>
| |
| |
--------------------+------+------+--
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

| 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.

petrus bitbyter

 

[petrus bitbyter]

<[email protected]> schreef in bericht

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)

VCC=5V
+ +----+------+------+-- plus
| | | | | power
| | | | |
| - _|_ | |
.-. ^ |_/_| | |
680E| | | | | |
0.5W| | +----+ | |
'-' | | |
from | ___ |/ | |
------+-+-|___|----| | |
74175 | 150E |> _|_ -
| | |_/_| ^
| | | |
| | +------+
| | |
| | |
| ___ | |/
+-|___|------)----|
150E | |>
| |
| |
--------------------+------+------+--
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

| 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.)

petrus bitbyter

 

[Dan Beck]

Hello all,

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.

Regards,
Dan

 

[John Popelish]

Hello all,

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

Emitter ===> Source
Base ===> Gate
Collector ===> drain

 

[BobG]

Hi Dan. Remember... the arrow points North