# Proportional Thyristor Control

Discussion in 'General Electronics Discussion' started by rob_croxford, Aug 27, 2010.

1. ### rob_croxford

262
0
Aug 3, 2010
Hi all,

I have been working on this one for a while... basically the problem is as follows:

I have a mainly analouge circuit that takes a variable 0-10V DC input and creates a linearly increaseing (frequency) square wave. This square wave is used to trigger the gate pin on a thyristor (through an optical isolator) which is used as a switch to control an electronic heater. The entire project works as a proportional controller for a heating system.

I have converted the variable DC input into a square wave by using a Voltage follower a MUX and a Comparator. (i can include a circuit diagram of this if it is needed)

So,

I have recently been trying to figure out a way of converting this process to a digital system. Unfortunatly Digital electronics is not my strong point.

My question is: Does anybody have any idea of how i can create a linearly increasing square wave (frequency) from a 0-10V variable DC input. Or can anyone give me a nudge in the right direction for a solution to this.

I have tried using a 555 timer however it is not reliable where linearity is concerned. I have also looked into using a Volt/Freq converter IC however they are to expensive.

Any help would be greatly appreciated.

Cheers,

Rob

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,270
2,718
Jan 21, 2010
The traditional approach is to have something that generates a triangular or sawtooth waveform and a comparator to compare this with your input voltage.

All of these methods are analog, as is using a 555, it's an analog device.

To do it digitally, you could use a PIC, sampling the input voltage on one pin and generating a variable duty cycle square wave on one of the output pins.

This is a fairly trivial exercise using a PICAXE.

Note that a PICAXE operates from a 5V supply, so you need to use a resistive divider to halve the 0-10V signal to 0-5V.

3. ### rob_croxford

262
0
Aug 3, 2010
cheers,

That was my first aproach so output x and y of the MUX (X creating the Sawtooth and y the compared voltage) control the "on" and "off" secions of the square wave. This proves to be a robust and reliable solution.

I have dabbled a little bit in C and C# programing however i would not say that i am at all proficient. So what would i need in order to be able to use a programable IC? and would it be cost efficient??

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,270
2,718
Jan 21, 2010
Google PICAXE -- there's lots of info out there.

Will it be cost effective? It's hard to say.

If you need flexibility to have a weird non-linear response, or any other smarts (other sensors perhaps) then it will be. For the straightforward case, maybe not.

You will end up with a lower package count, and that may be a good thing

5. ### rob_croxford

262
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Aug 3, 2010
ok thanks for the info steve

6. ### Militoy

180
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Aug 24, 2010
The V/F converter sounds like the smoothest way to handle the circuit - what's your idea of too expensive? I use a TC9400 to set up a linear pulsetrain from an analog signal for a robotics steering application. Avnet has a few in stock for \$4.14 right now; Farnell in the UK is listing them for just over 5 pounds; Digikey and Newark for \$8.71 and \$7.76 respectively.

If you're building a whole lot of production units these prices might seem high - but these are single piece prices - and should come down significantly if you buy in quantity.

7. ### rob_croxford

262
0
Aug 3, 2010
I agree that this will be the best method and is the one i will probably go with. however i was just interested to see whether a simple circuit would do the job. I am only designing in the range of 1-50 Hz for this project.

The problem is the solution i am currently using (mux and comparator) is more cost efficient than buying a V/F converter. however i will look into buying in bulk and see what the cost will drop to.

Cheers

8. ### rob_croxford

262
0
Aug 3, 2010
Ok ive had a look into the V/F converter however the data sheet confuses me. what i would like to know is;

Is the output proportional to the input voltage? for instace if only 10% of the input voltage is used ( i imagine this is known by using a referance voltage ) will the output frequency be "on" for 10% and "off" for 90%?

I would just buy the IC ( TC9400 ) and hook it up but as you said you had worked with them before i thort it may be wiser to ask before i buy.

Rob

( also if this is the case does the V/F converter work from 0-100% control??)

9. ### Militoy

180
0
Aug 24, 2010
The part is kind of a legacy device - they have been around a long time - because of their versatility and ease of use. By changing parts values, you can scale the output frequency range, analog input range, etc. So - if you want 0 to 400 Hz output corresponding to 0 to +5VDC, or 1KHz to 5KHz, corresponding to -2 to + 2 VDC, no problem. Output amplitude stays constant (unless you're at the area of signal scaled to no output) while frequency changes with your analog input. The chip can also be configured as a frequency-to-voltage converter. I'm currently using one in a circuit that converts a biipolar DC control signal (-5 to +5 VDC) to a active low pulse train, to microstep a high power stepper motor in single-step mode. The signal into the TC9400 is actually 0-5V positive for full range 0 to 500 Hz - but I'm running the bipolar analog through an absolute value amplifier, and using the polarity of the input signal to set motor rotation direction. If you look over the datasheet, you'll find some ap circuits that are pretty good - though you'll likely have to fiddle with the cap values to calibrate to the exact range you need, and zero out the pulse frequency (if you need it to go to zero).