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Old 19th August 2004, 05:15 PM   #1
EE is offline EE  Germany
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Question Need help designing "special" amp!

Hi all,

sorry if this is off topic, but somehow it is related to audio amplifiers... I need to switch a solenoid...

Input is a logic level signal, 0V = off, 5V = on. Output should be:

Option (1), differential mode

If input = 0V -> "connect" output to - rail
If input = 5V -> "connect" output to + rail

Option (2), standard mode

If input = 0V -> "connect" output to ground
If input = 5V -> "connect" output to + rail

Rails will come from adjustable power supply, +/-5V..+/-24V.

Can this be done with a simple audio amplifier? MOSFET maybe? I'm totally lost... How would I go about the input interface? Inverters controlling MOSFETS?

Any help is appreciated!!! Thank you very much
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Old 19th August 2004, 06:31 PM   #2
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Hi,
Sounds quite achievable. Not sure an audio amp is the solution, but something simpler.
More info is needed. How will the options be chosen - do you want a circuit for each or one circuit that will do both?
And what current will the solenoid demand at each output state?
BAM
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Old 19th August 2004, 06:36 PM   #3
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Standard 7400 series or 4000 series logic and relays seems like a much simpler solution to me than a power amp.
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Old 19th August 2004, 07:46 PM   #4
EE is offline EE  Germany
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Thanks for your help so far.

The solenoids are very fast. The switching frequencies will be between 0.1 Hz and 200 Hz. But resolution in time must be better than for 200 Hz because there are 8 channels with different phase and duty cycle - adjustable to 1°. It is for an aerodynamic test bench. That's why I thought of audio amps.

Current demand is about 100mA per channel but I suspect I need up to 1A per channel on a short-term basis.

As for chosing the two options:

I plan on providing two output connectors, one for option one and one for option two. So the user can select the mode by plugging the cable in the right connector.
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Old 20th August 2004, 09:17 AM   #5
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Default more questions

What direction is the current flowing in? I would assume the other side of the solenoid is connected to 0V. So in option 1 the solenoid is either off or positively energized, in option 2 the solenoid may also be negatively energized.

The output needs to be able to change state every 5mS. When the state is changed there will be some delay - what is the max delay between input change and output change?
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Old 20th August 2004, 11:46 AM   #6
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Try these.....
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Old 20th August 2004, 11:47 AM   #7
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Circuit #2
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Old 20th August 2004, 02:17 PM   #8
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Hi EE,

I think it's pretty easy to achieve! Why not to use power mosfet to work in a switching mode.... high current, high speed, low driven require....

If you want to have a good EMC performance, you can use a gate-drive circuit with slew control, it's a bit complex but can meet most need......


Wenye
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Old 20th August 2004, 04:23 PM   #9
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Here is a good circuit. It can draw up to 800 mA, input is 0-5V, output
swings to rails, so there are no losses anywhere, and the power supply can be anything from 5V to 24V.

The frequency in this version can be DC to several kHz, if you want to go higher, simply use some B-E resistors on the BC808 and BC818. 1k will do nicely for up to 50 kHz.

:-) Lars
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Old 20th August 2004, 10:27 PM   #10
EE is offline EE  Germany
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Default Re: more questions

Thanks for all your comments, very helpful!

Quote:
Originally posted by traderbam
What direction is the current flowing in? I would assume the other side of the solenoid is connected to 0V. So in option 1 the solenoid is either off or positively energized, in option 2 the solenoid may also be negatively energized.
Exactly. We will drive two different types of solenoids, one with a spring-loaded mechanism, so they only need a "pull" output while the other type needs "push-pull".

Thinking of it, a single circuit will be enough when it connects the output either to V+ or GND. Then I can provide a "half V+" output and connect the solenoid either to OUT and GND or OUT and "HALF V+". I think I'll do it like this.

Quote:
Originally posted by traderbam
The output needs to be able to change state every 5mS. When the state is changed there will be some delay - what is the max delay between input change and output change?
Lets see. Max freq = 200Hz, min. "on time" = 1° -> 13.9µs
Delay should be of course lower than this.

Quote:
Originally posted by Lars Clausen
Here is a good circuit. It can draw up to 800 mA, input is 0-5V, output swings to rails, so there are no losses anywhere, and the power supply can be anything from 5V to 24V. The frequency in this version can be DC to several kHz, if you want to go higher, simply use some B-E resistors on the BC808 and BC818. 1k will do nicely for up to 50 kHz.
Thanks Lars, looks simple enough for even me to understand. Say, any chance to use this circuit with MOSFETs? I have a bunch of IRF510 and IRF9510 which I ´have no other use for.
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