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Author Topic: Stepping up PWM to 28v and very low current  (Read 731 times)
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Hi all, got a potentially easy question (I hope!). I am using Arduinos on my homebuilt flight simulator....currently I'm using one to read a rotary switch on a weather radar, another to read the frequency setting and drive a light on a real transponder (a common aircraft radio). I also am driving a real flap indicator gauge, a vertical speed indicator (easy servo instrument), and a temperature gauge....I gotta say I am loving these boards, any of the "off the shelf" flight simulation solutions couldn't do nearly as much and would cost 10x the price!!!!

Anyways, I have a slightly new challenge. I have a fuel quantity indicator that, despite 12+ pins in the back, appears to be a simple D'Arsonval meter. Varying the voltage on one of the pins moves the needle. Trouble is, it takes about 28v for maximum deflection.

I've been using the Arduino's PWM for running similar analog instruments. So, what I need is a simple circuit to step the Arduino's 5v PWM output to 28v. I did some reading about FETs and other things that seemed to be geared to heavy loads, but I'm thinking that's all overkill since the load from this meter will be almost nothing. I just need to step up the Arduino's output. Is there a simple and safe way to do this without a complex circuit?

One thought I had was using transistors to switch the meter to a 28v source, I just don't know if a transistor would switch fast enough to keep up with the Arduino's PWM. Bad idea?

Thanks!

Matt
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I think the right way to do it would be a DC/DC converter to get the 28v supply and a small transistor switch to PWM the 28v.
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Boost regulator
http://www.pololu.com/catalog/product/799
Logic level, N-channel, Low Rds mosfet - will keep up with the Arduino just fine.
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
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The solution I would use depends on the resistance of the meter. If it is high enough, then I would use this circuit.

Does the meter have a cover that can be removed? There is probably a series resistor inside, and you could reduce the value of that resistor to make it compatible with 5V.


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What you first need to do is determine what current is needed for max deflection. If it is a darsonval meter the voltage will be of secondary concern as they are current measuring devices. Better to get a potentiometer or 2 (start with a high value one...) and a good ammeter (or milli-ammeter) and see what the device really wants. A transistor tied to a PWM output and possibly a resistor to limit the max current would be a good place to start. If you are using other aircraft devices I would think that you may already be using some thing that is using 24-28V.
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Perhaps a programmable constant-current boost LED driver chip might work if the current is in the 10--100mA range?  Just an odd thought (I've been experimenting with LED drivers today you see, everything looks like an LED!)
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