Converting from 30V to 5V or less

Discussion in 'Electronic Basics' started by [email protected], Sep 19, 2008.

1. Guest

I'm using a computer to gather data using LPT1 port. The input pins of the lpt
accept 0-5V.

I want to know when three utility motors using 30V start and when they end. When
a motor starts it gets 30V. Actually they should get 24V but occasional bursts to
30V are normal. The motors themself can handle the extra voltage.

What is the simplest way lower the voltage to 5V for my LPT port? I've done
simple electronics but a simple schematic would be great. I want to know when
each of the motors start.

2. JamieGuest

Using a Photo coupler is the safest method that I know of.
Just pull one of the input pins of your port using the photo output
transistor.

Since you stated 30 V's maybe the max at times I would use ~ 2 thirds
of the LED (If) which is ~ 45 ma.
R = (30-1.2)=(28.8)/0.045 = 640 ohms.
W = 0.045*1.2 = 0.054 watts.

http://www.qsl.net/n4xy/PDFs/Semiconductor_Data_Sheets/4n35_37.pdf

P.S.
Might be advisable to use a zener diode of around 30 volts so
that it can be protected from reverse voltage exceeding (Vr)
6 and forward voltages exceeding 30 volts.

If this is a brushed motor, it can generate inductive energy that
isn't safe even for the coupler. A cheap zener diode helps this out in
minor cases and the photo coupler helps in major cases to protect the
computer.

http://webpages.charter.net/jamie_5"

3. JamieGuest

Yes, I made a mistake there. got my 1.2 mixed up with my 28.8.No, I didn't miss it.

I just wouldn't subject the PC port
to a motor circuit. Although you
have covered all bases as far as over voltage
and reverse voltage, you didn't account for
current loops on the ground path between the devices which loves to
make PC's miss behave.
Motors are notorious for generating such currents if
improper installation is performed.

To each, his own.

4. Jasen BettsGuest

Just changing the voltage is possibly not enough, things like ground
loops cour damage your pc or stop the other equipment from working.

the easiest way is use a 24V relay, or to use optocouplers.

The parallel port inputs have internal pull-ups so you only need to
connect them to the pc's digital ground or disconnect them - you don't
need to provide a 5v signal.

Bye.
Jasen

5. Jasen BettsGuest

He asked for that, but what he wants is a way to get 0V to the printer
port. the input pins have interal pull-ups.

For it to read zero the motor must have sufficiently low voltage
drop to ground the data pin through your 1K resistor...
If it's a (for example) a brushless DC motor the voltage drop
may be sufficient to present as logic high with the DC supply
disconnected.

I'd add a resistor in parallel with the zener to provide a reliable
path for the off-state signal current, 470 ohms would be a good choice.

Bye.
Jasen

6. TestGuest

Thanks guys, I'll try either way.

7. TestGuest

What is a "pull up"? How would I connect this to my printer pins? The 30V is btw
AC (just found out).

Right now I have John Field's:

1/2 WATT
30V----+--[1000]--+------>4.7V
| |
[MOTOR] [1N5230B]
| |
GND>---+----------+------>GND

Just to be safe I'll do this:

30V----+--[1000]--+------+ +---------+
| | +---+ +----> pin? in LPTport
[MOTOR] [1N5230B] | 5V relay|
| | +---+ +----> pin? in LPTport
GND>---+----------+------+ +---------+

My aim is to find out when the motor is powered up. In above the voltage is
dropped to <5V for the relay not to fry.

What about the current? I don't know what the current is.

8. TestGuest

Thanks. I was rather surprised for the AC. I was testing teh first simple setup
with another transformer that was DC.
I'll have to rethink the system over again.

9. JamieGuest

This is what you need to make it simple and still safe for
AC work. You only too use a cap on the port side so you
don't get a pulse on the pin.

http://www.toshiba.com/taec/components2/Datasheet_Sync//206/4215.pdf

This is an AC input version much like the one's used in the front ends
of PLC cards that accept AC as the input type.

Found at digikey.. as

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=TLP620GRFT-ND

http://webpages.charter.net/jamie_5"

10. Robert MonsenGuest

I would not bet on the fact that your motors are referenced with the
same ground. If you connect the wrong wire to your PC parallel port,
and you have ground issues, you can destroy your mainboard.

You can test this by using a DC voltmeter between the ground at the
parallel port and the ground at the motor.

FYI

Regards,
Bob Monsen

11. Jasen BettsGuest

a resistor used to increase the voltage at a point in a circuit whenever
it's not connected to any input signal.

what this means is that you don't need to provide 5v to the printer
port, you need to take it away from the printer port.
I had suspicion it might be...
you'll just need to change that a adding a rectifier so
that the relay gets DC, and a capacitor to stop it switching off while
the AC voltage altenates, and reduce the resistor so that the relay
gets enough power....

An opto-coupler will use less power than a relay and is less fussy
about how much power it gets which makes the design easier (cheaper
and more reliable too)

1N4002 .------.
--+-------->|---+---[3300]--[|o \/ |] PC
| | | |
| 10uF=== .---[| 4N25 |]-- pin 10/12/13
30VAC (M) 50V |- | | |
| | | [| |]-- pin 20
--+-------------+-------' `------'

the diode can be 1N4002 to 1n4010, the capacitor should be rated
for atleast 50V (more than 50V is fine)

with those pins (pick one of 10/12/13) it'll come up as the bit
6/5/4 on port 0x379.
which current?

Bye.
Jasen

Thanks.