Need help on TL494 PWM magnetic levitation

Thread Starter

RobertShin

Joined Jul 29, 2011
7
Hi there people. I'm currently doing a project to make a levitating system with the use of an electromagnet and a control circuit to vary the current in the electromagnet, thus getting the object to levitate. The sensor I'm using is UGN3503 hall effect sensor, and this measures the change in magnetic flux and gives and output voltage proportional to the change. This is fed into the TL494 chip, which then gives a PWM signal based on the error amplifier. This signal then goes to a power mosfet, which turns the electromagnet on and off.

Here's what I've done:

Pin 1 - Hall effect output
Pin 2 - Reference voltage
Pin 3 - Left it open
Pin 4 - Left it open
Pin 5 - 220 nF Capacitor
Pin 6 - 2.7k Resistor
Pin 7 - Ground
Pin 8 - 12V supply
Pin 9 - Gate pin of a power mosfet
Pin 10 - Shorted to 9
Pin 11 - 12V supply
Pin 12 - 12V supply
Pin 13 - Ground
Pin 14 - Into pin 2
Pin 15 - Left it open
Pin 16 - Left it open

I am failing to get any PWM signals at all, what have I done wrong?

Here is the data sheet
http://www.datasheetcatalog.com/datasheets_pdf/T/L/4/9/TL494.shtml
 
Last edited:

SgtWookie

Joined Jul 17, 2007
22,210
Well, you are really going to have to draw a schematic, and document everything that you have connected to the IC, including part numbers where applicable, resistances, capacitances, inductance values, what your electromagnet's specifications are or manufacturer's datasheet, what voltages you're applying (the reference, Vcc, supply to your magnet) and all other items that are relevant.

A photo of how you have it laid out, or at least a drawing of the board you have it on, will also be very helpful.

Otherwise, it will take a very long time to get anywhere with this.

Schematics and layouts help us to get to the root of the problem as quickly as possible, and help greatly to avoid misunderstandings.

Also, you should tell us which manufacturer's TL494 you are using, as they are made by several different companies.
The links you provided for the TL494 go to generic datasheet search engines rather than the manufacturer's current datasheet from their website. While the datasheet sites are convenient for finding things initially, you should always go to the manufacturer's site for the part status, and download their most current datasheet.
 
Last edited:

russpatterson

Joined Feb 1, 2010
353
Eagle is the best "free" schematic editor and board layout tool IMO. It produces industry standard "Gerber" files so you can use any PCB house to get boards made. There is lots of good info, and a decent tutorial on it at Sparkfun.com.

Sounds like a cool project. I'd really like to see the schematic. Is this just an exercise or school project or what is the purpose?
 

Thread Starter

RobertShin

Joined Jul 29, 2011
7
Alright here is my schematic, and the boxed area is the area I haven't connected yet. I wouldn't have thought it necessary to connect the output part to get a PWM signal coming out of the chip anyway. The company is Texas Instruments, and the first datasheet is the one they've supplied.

Thanks
 

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SgtWookie

Joined Jul 17, 2007
22,210
OK, TI's datasheet is here: http://focus.ti.com/lit/ds/symlink/tl494.pdf

What's the thing on the far left that has 5v going in, ground and out?

It has no part number or description. It might be a regulator. It could be lots of stuff.

This is why we ask for complete schematics, as the lack of information results in more questions and takes more time for everyone - and meanwhile you still don't have an answer yet because we don't know what we're dealing with.
 

SgtWookie

Joined Jul 17, 2007
22,210
Hmm, ok.
You have no controls for 2IN+, 2IN-, yet you're using C2 and E2 - in parallel with C1 E1 no less. If you don't have any input for 2IN±, then ground them, and remove the paralleling with C1/E1.

Take a look at page 6 of the datasheet, and how they're using C1/E1. E1's grounded, and C1 has a 150 Ohm pull-up to +12v, and the collector is the output.

I tried simulating it the way you had it as an emitter follower, and I couldn't get more than 3.5v on the gate of the MOSFET.

What MOSFET are you using?

I don't know what frequency you were intending to operate at, but it's right at 2.3kHz, which is rather slow.
 

Thread Starter

RobertShin

Joined Jul 29, 2011
7
The mosfet is BUZ73L, which has a threshold of 1.6V typical. I wanted a lower frequency for a slightly more stable levitation.

oh ok so I can't just keep 2IN +- floating? I just thought shorting the two outputs C1 & C2 would effectively make it a single output anyway, so I wouldn't have thought it'd make any difference in the circuit.

I don't get the pull-up to +12V part, what does that mean?
 

SgtWookie

Joined Jul 17, 2007
22,210
The mosfet is BUZ73L, which has a threshold of 1.6V typical.
You really need to go by the Rds(on) specification instead of Vgs(th), as the former is given with a current that approaches the maximum rating, while the latter is usually given where Vgs=Vds with a minimal current (like 1mA, 250uA, etc). When using a MOSFET as a power switch, Vgs(th) is really only useful for determining how low you need to get the gate to turn the MOSFET off.

Anyway, I found a SPICE model for the BUZ73L from Siemens, so that should work OK.

I wanted a lower frequency for a slightly more stable levitation.
I'm wondering how you thought that a lower frequency would result in a more stable levitation?

Anyway, the characteristics of your electromagnet are only vaguely defined; a resistance of 18 Ohms is not much to go on. Did you make it yourself, or did you salvage it from somewhere, or did you buy it? What are its' specifications, or how is it constructed? What gauge wire is it wound with? Please try to give as much info as you can; the better you describe it, the better I can attempt to simulate it.

If the electromagnet has a relatively large value of inductance, then it would be OK to operate it at a lower frequency. However, if it has a relatively small inductance, the current would fluctuate too much.

oh ok so I can't just keep 2IN +- floating? I just thought shorting the two outputs C1 & C2 would effectively make it a single output anyway, so I wouldn't have thought it'd make any difference in the circuit.
This IC is really designed for applications where one would need two power switches, like when the primary side of a transformer is center-tapped with the tap fed with +V. C1 and C2 would take turns controlling switches (MOSFETs) that would ground the "ends" of the transformer in an alternating manner, which would induce alternating current in the secondary winding(s).

You could have used a regulator designed for a flyback converter; a one-switch output - like a UC3842.

I don't get the pull-up to +12V part, what does that mean?
You had a pull-down resistor from the gate to ground. I swapped it around so that now it's a pull-up to +12v.

Have a look at the attached. I just made changes until I got the thing to run. I haven't done anything to attempt simulating the Hall-effect sensor; I just was trying to get the simulation to run without complaining. I haven't tried simulating the TL494 before this run; I found the SPICE model on the 'net and changed the symbol to agree with the datasheet representation on page 6.

Have a look at the attached. It's not ready to be connected to your magnet; it's just a start to get something working.
 

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Thread Starter

RobertShin

Joined Jul 29, 2011
7
The electromagnet was bought, and it is a 12V, 8.5W, 0.71A electromagnet with a holding value of 150 lbs, and I don't know the type of wire it was made from. It's a round electromagnet, with an iron core and a steel outer wall with epoxy encapsulation.

I am thinking for future considerations to use possibly a progammable microcontroller.
 
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