5VDC sgnl to 24VDC sgnl using a transistor

  • Thread starter dgeccles
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  • #1
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Main Question or Discussion Point

Hello,

I have two devices, one that emits a 5V DO and another that recieves a 24V DI. I wanting to interface them but am having problems.

I was searching for a small solid state device that could easily interface them but could not find any. So I have been trying to creat a circuit using a transistor that will do the job.

Is there an easier way to do it with a small component that can fit on a circuit board?

I have managed to design a signal that outputs 5VDC to 24VDC with an input of 5VDC to 0VDC respectively. While this is enough difference to act as the digital signal between the two I would rather drop the outputs minimum voltage to be 0VDC.

I am using the NPN transistor BD681. The Base is connected to directly to the 0-5VDC input signal. The Collector is connected to the 24VDC supply through a 100k Ohm resistor AND to the OUT signal. The Emitter is connected to Gnd via a 330 Ohm resister.

Can anyone help me improve this design?

Thanks guys,
David
 

Answers and Replies

  • #2
berkeman
Mentor
57,270
7,261
Hello,

I have two devices, one that emits a 5V DO and another that recieves a 24V DI. I wanting to interface them but am having problems.

I was searching for a small solid state device that could easily interface them but could not find any. So I have been trying to creat a circuit using a transistor that will do the job.

Is there an easier way to do it with a small component that can fit on a circuit board?

I have managed to design a signal that outputs 5VDC to 24VDC with an input of 5VDC to 0VDC respectively. While this is enough difference to act as the digital signal between the two I would rather drop the outputs minimum voltage to be 0VDC.

I am using the NPN transistor BD681. The Base is connected to directly to the 0-5VDC input signal. The Collector is connected to the 24VDC supply through a 100k Ohm resistor AND to the OUT signal. The Emitter is connected to Gnd via a 330 Ohm resister.

Can anyone help me improve this design?

Thanks guys,
David
You can use the 7407 or 74HC07 buffers to do the level translation:

http://pdf1.alldatasheet.com/datasheet-pdf/view/50893/FAIRCHILD/7407.html

That link is being a PITA at the moment, but you can search for other links to the datasheets. The 7407 is a buffer (no inversion) that has a high-voltage open-collector output, which you can pull up to 24V via a pullup resistor.

As always, check your source's Vol and Voh voltages in its datasheet, and check the Vih and Vil voltages in your 24V input's datasheet to be sure the interface circuit is correct for your application.
 
  • #3
vk6kro
Science Advisor
4,081
40
[PLAIN]http://dl.dropbox.com/u/4222062/OPamp%20amplifier.PNG [Broken]

I don't think you could do it with one transistor, but with 1/4 of a cheap opamp chip, you could. See above.
You would have to use this particular opamp, the LM324, because it allows the inputs to be the same voltage as the negative supply.

You also need a power supply of at least 26 volts.

This is a simulation and I haven't built it (of course), but it may be worth a try.

The 38 K is not a standard value, but 39 K would be OK if you don't need an exact result.
 
Last edited by a moderator:
  • #4
4,662
5
Does your 24-volt input expect a 24-volt current-sinking driver like an npn collector, or a 24-volt current source? If the latter, you probably should add a pnp transistor with the emitter tied to 24 volts.

Bob S
 
  • #5
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Berkeman

Thanks Berkeman, that looks like the option I am after. My electronics is very rusty I am sorry to say. The Sink would be the 24VDC digital input, yes? Would the source be the voltage supply of 24V I am using to up the signal or the 5V signal that I am upping?

vk6kro

Thanks Vk6kro. That does look like a good option, i even have a few spare op amps, however don't have a 26V power supply so I don't think I could use that option.

On a secondary note though. What program did you use to test the circuit? Such a program would be very useful!

Bob

As i said I am rather rusty on my electronics. Wouldn't a PNP transistor with the emitter tied to 24V force the gate open?

Assuming not, would I then connect the base to my 5V signal, and the collector to the 24V signal as well as ground through a resistor?


Thanks a heap for the help guys!

David
 
  • #6
4
0
Hello

I was going over the 7407 design and I don't understand the difference between VO(MAX) and VOH, likewise between VO(MIN) and VOL.

Thank you for oyur help!
 
  • #7
vk6kro
Science Advisor
4,081
40
Maybe you should explain exactly what you are doing. What is the device you are trying to drive at 24 volts?

Even logic circuits have specific drive requirements.

If a logic device has an input range of 0 to 5 volts, it may accept anything above 3.5 volts as a logic "high" and anything below 2.5 volts as a logic "low". So, it may not need the full 5 volts to count as a logic "high".

As an output, most devices cannot produce output voltages which are the same as their supply volts and this doesn't matter if they are matched up with a device from their own family which expects input that is not perfect, but adequate.

That program you asked about was LTSpice 4 which is available free from www.analog.com.
It is easy to use and you should get a copy and have a play with it.
 
  • #8
4,662
5
To possibly repeat on vk6kro, and expand:

1) Do you want an inverting or non-inverting output to drive the 24 V?

2) Is the 24V input circuit current sourcing, or does the 24 V output circuit need to be a current source?

3) How much current is required for a logic 1 (high) and for a logic 0 (low)?

4) If this is a relay coil input, can one end be attached to 24 V, or must it be grounded?

Bob S
 
  • #9
4
0
Ok, thanks for the help.

What I am doing: I am interfacing between a Quanser Q4 real time control and DAQ card and a Parker SLVD7N motor controller. The Q4 card has 0-5V DIO and the SLVD7N has 0-24V logic.

I do not know if the SLVD7N is sourcing as I cannot find the relevant information in the manual. Is there any way to test? If it is sourcing would this mean that if the DI is not dragged down it would return to 24V?

Again, I don't know how much current needs to be supplied to activate push the DI high. I cannot find it on the manual. However I can test this as I have access to a good power supply (with current and voltage control) and a good multimeter.


If a logic device has an input range of 0 to 5 volts, it may accept anything above 3.5 volts as a logic "high" and anything below 2.5 volts as a logic "low". So, it may not need the full 5 volts to count as a logic "high".
Would this be correct? In this case the:
VO(min) = 0V
VOL = 2.5V
VOH = 3.5
VO(max) = 5V

Thanks for the advice on the LTSpice 4. I will definitely Play around with it.

Thanks a heap for your help guys.
David
 
  • #10
4,662
5
You can find a copy of thr Parker sLVD manual at
http://www.compumotor.com/manuals/SBC/SLVD_UG.pdf
Read it.

The two attached thumbnails show the a) electrical connection on page 22 (it is a pnp collector output), and b) the logic levels on page 87 (0< 3 volts = logic 0, and 15 < 30 volts = logic 1).

You will need to supply an external 24 volt source for the pnp transistor. I recommend a 1k pulldown on the pnp collector.

Read the manual.

[added] You still need to consider whether you want an inverting or non-inverting driver.

Bob S
 

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