Isolated PWM control of 120 VAC load with 4-20mA signal

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tdziubak
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Isolated PWM control of 120 VAC load with 4-20mA signal

Post by tdziubak » Sat Jul 17, 2004 8:26 am

Can anyone point me in the right direction for a circuit or IC for a low cost 120VAC PWM from a 4-20mA control signal. Need, say 15A supply to resistive or inductive load with ISOLATION from the 4-20mA input control signal. Thanks, your help is greatly appreciated.
Tom D

josmith
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Re: Isolated PWM control of 120 VAC load with 4-20mA signal

Post by josmith » Sat Jul 17, 2004 3:48 pm

PWM is used for controlling DC. For AC it's usually phase control.<p>For isolation Newark has power components with the isolation built in or you can make you're own using opto isolators.<p>You could use any micro with an analog input to intrepert the input and provide the modulation.

toejam
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Re: Isolated PWM control of 120 VAC load with 4-20mA signal

Post by toejam » Sat Jul 17, 2004 5:59 pm

+if you need isolation, you need a transformer.The advantage of using pwm in isolation circuts delivering corrent at any voltage is the fact that transformers run at high frequencies much more efficiently on a cost per pound basis than at low frequeccies like 60 cps.To proprely utilize pwm, you must first rectify the low frequency ac to smooth dc than change it to high frequency ac. which can be done with reletivly inexpensive solid state compomants than run that through a small transformer and rectify and filter it if you want dc.Better yet unless you want lots of them, buy a power supply designed for that purpose.The pwm part is for controling the watts the unit puts out.

tdziubak
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Re: Isolated PWM control of 120 VAC load with 4-20mA signal

Post by tdziubak » Sat Jul 17, 2004 9:42 pm

Thanks Guys.....
Any particulars on IC's or discrete devices that would make this fast and easy... i.e. something like the Melexis MLX90804 IC driving a Triac or perhaps Fairchild's D94FR4.. I think they are called IGBTs (Insulated Gate Bipolar Transistors). <p>I don't need the high performance of a VFD, I only need single phase, and I don't want to spend the $ on the high current rectifiers and filter caps. <p>I was thinking maybe using a sensing transformer (or an optoisolator) to pass the timing of the zero cross to the control side. Then once the delay is added in pass the trigger signal back across via an optoisolator to the IGBT to control the 120VAC. Is this approach practical or am I putting in to much time delay to have it work right???<p>Also, on the control side, I was hoping for a non PLC conversion from the 4-20mA signal. Is there an IC around that would get me a signal that I could use to increase/decrease the Triac/IGBT "turn on" delay. The MLX90804 has the ability to do the direct conversion, but I am unsure how I pass the 4-20 mA across the isolation and still guarantee the isolation!!!! I could use a chopper and a coupling transformer but that looks like a long way to go... surely there is an easier way!!! Also, I guess I have the feeling all of the control circuit should be on the low voltage side of the isolation. <p>How practical is this??? Should I just bite the bullet and buy or build a VFD unit? Does anyone have a source for a suitable comercial (low performance and hence low cost) unit. <p>Lots of questions!!! Surely someone else has been down this road and has learned the hard way!<p>Thanks for your comments and thanks in advance for any further guidance.<p>Sincerely,
Tom_D
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rshayes
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Re: Isolated PWM control of 120 VAC load with 4-20mA signal

Post by rshayes » Sat Jul 17, 2004 11:46 pm

This can be done with phase control of SCRs in series with the load. Once an SCR is triggered on, it remains on for the remainder of that half cycle. The trigger pulse needed is only a few microseconds wide and can be coupled through a very small transformer.<p>Examples of these transformers and some simple circuits can be found on the Vishay web site (www.vishay.com). Look under Dale and transformers. The PT-10, PT-20, and PT-50 were designed for this application.<p>General Electric published an SCR Handbook that described phase controlled circuits with SCRs. This was published in the 60's, but there may still be copies in some libraries. It may also be available as a used book. You might try [url=http://www.abebooks.com,]www.abebooks.com,[/url] and see if any are listed.<p>More recently, you might try "The Encyclopedia of Electronic Circuits" by Graf and Sheets (5 or 6 volumes). These are fairly recent collections of circuits and may be easier to find in either a public or technical library than the SCR Manual.<p>Slightly less recent are the McGraw -Hill books by Markus- "Handbook of Electronic Circuits", "Electronic Circuits Manual", "Guidebook of Electronic Circuits", and "Modern Electronic Circuits Reference Manual". These are most likely to be in a technical library, such as at a college or university.

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