Need help desiging a Simple and Cheap AC regulator
Need help desiging a Simple and Cheap AC regulator
(OP)
Gentlemen,
I am not a power guy and I'm struggling trying to find a solution to my problem. I need a circuit that will allow me to adjust its power to a fixed resistive load. The input MUST be capable of 100-240V AC and the output power must be consistent throughout the input range.
My solutions so far:
I used a very simple SCR for phase angle firing. I can adjust the resistor that feeds the gate to change the total power output of the controller (higher voltage). This works perfect BUT it does not work on both 110V and 220V without changing the resistor values.
Second solution was reverse phase angle firing; Use a P-channel mosfet or IGBT to control the load. The gate would be connected in parallel to a capacitor and resistor connected in series across the AC line. This works because when the voltage is rising the fet is on until it is able to charge the capacitor up enough to turn it off. This also works very well. This is nice because it is on at zero crossing and turns off at a voltage determined by the resistor/cap value. The problem with this is that it is not consistent between 110 and 220V.
This controller heats up a heating element so the quality of the output is not important. It also doesn't matter that the output is only half wave. The biggest challenge is that the heating element must reach the same temperature regardless of its input voltage (given the input is 100-240V)
Have I missed any possible solutions? I need something simple as it is a disposable item and must be inexpensive to manufacture.
Thanks!
I am not a power guy and I'm struggling trying to find a solution to my problem. I need a circuit that will allow me to adjust its power to a fixed resistive load. The input MUST be capable of 100-240V AC and the output power must be consistent throughout the input range.
My solutions so far:
I used a very simple SCR for phase angle firing. I can adjust the resistor that feeds the gate to change the total power output of the controller (higher voltage). This works perfect BUT it does not work on both 110V and 220V without changing the resistor values.
Second solution was reverse phase angle firing; Use a P-channel mosfet or IGBT to control the load. The gate would be connected in parallel to a capacitor and resistor connected in series across the AC line. This works because when the voltage is rising the fet is on until it is able to charge the capacitor up enough to turn it off. This also works very well. This is nice because it is on at zero crossing and turns off at a voltage determined by the resistor/cap value. The problem with this is that it is not consistent between 110 and 220V.
This controller heats up a heating element so the quality of the output is not important. It also doesn't matter that the output is only half wave. The biggest challenge is that the heating element must reach the same temperature regardless of its input voltage (given the input is 100-240V)
Have I missed any possible solutions? I need something simple as it is a disposable item and must be inexpensive to manufacture.
Thanks!





RE: Need help desiging a Simple and Cheap AC regulator
What temperature?
What power level?
What thermal time constant for the element?
What time-period requirements for 'stable'?
Your voltage range sounds suspiciously like the worldwide range of consumer voltages - from certain parts of Japan (100VAC) to Europe (240VAC). So is it a variable voltage, or a worldwide market?
If the goal is stable element temperature (and assuming temperature and power are in the right ranges), then aren't there PTC devices that might do the job in one step? But not likely for many reason. But if you're lucky, then one component.
Another approach is to have a small low power circuit that mirrors the high power circuit.
Another approach is to simply regulate the variable voltage down to (say) a stable ~90 volts.
Or allow a temperature sensor to control the gate of a triac or SCR. This should be 'light-dimmer cheap' if your element can average out several AC cycles durations.
RE: Need help desiging a Simple and Cheap AC regulator
The power level is approx 35W. The time constant is a few minutes as its rather large.
My voltage is the worldwide range, I just neglected to mention it for reasons unknown to me.
This is a controller and is mounted a distance away from the element. Additionally a PTC is not an option because I cannot add or modify to the element.
A mirror is a new suggestion however I'm not exactly sure how to apply it to my application or if it would be cost effective. Can you elaborate on this?
Regulating the voltage would definitely work but it would have to be regulated down to about 48V (DC or RMS) to properly power the element. Anything less and the element does not get to temperature in a timely manner. The voltage doesn't have to be perfect however too much more and you'll heat the element too far. The problem I have is how to regulate the voltage down affordably. At 48Vdc, I need up to 1.5A available.
Closed loop control would be great but it is just not practical in this application. I cannot mount anything on the element and it needs to be a dirt cheap solution. I'd love to explain the details but the big guy in the corner wont let me.
Thanks for your suggestions!
RE: Need help desiging a Simple and Cheap AC regulator
I think that a much better solution would be to use a switcher with 48 V DC output. There are lots of 24 V devices and probably also quite a few 48 V devices. Just google to find them. They tend to be consumer market priced. So economy should not be a problem.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Need help desiging a Simple and Cheap AC regulator
Bill
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"Why not the best?"
Jimmy Carter
RE: Need help desiging a Simple and Cheap AC regulator
RE: Need help desiging a Simple and Cheap AC regulator
My quantities are 10k+ so it is affordable to build a custom solution. Even if I go commercial, in those quantities I have significant lead times that approach the lead times I've received from my contract manufacturer on one of the initial 110V only designs.
RE: Need help desiging a Simple and Cheap AC regulator
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Need help desiging a Simple and Cheap AC regulator
The (cheap) controller regulates the temperature of a very small heater in the controller. The big heater is wired in parallel with the small heater. So they tend to track each other. It's not ideal for precision work, but the same technique is used in many consumer appliances.
Power doesn't necessarily equal temperature rise. Variable air flow over the element (for example) will change the temperature of the element.
RE: Need help desiging a Simple and Cheap AC regulator
If you want to run without a processor then you're still likely looking at some sort of analog feedback circuit phase controlling a triac or 2 SCR's.
RE: Need help desiging a Simple and Cheap AC regulator
Still, the PF at 230 V will not be acceptable.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Need help desiging a Simple and Cheap AC regulator
Gunnar: "Still, the PF at 230 V will not be acceptable."
A cheap solution to #2 is #1; slow things down and take advantage of the thermal time constant.
RE: Need help desiging a Simple and Cheap AC regulator
I am not aware of any PF requirements in the US. We have harmonic current requirements but I believe it is for applications consuming > 75W.
Can you point me to some material I can read regarding EU/CE requirements for PF and harmonic current?
LionelHutz,
You have an excellent idea. I did visit their site and they have some very nice application designs which would be perfect. The problem I see, aside from PF and harmonic current, is that it requires a much higher component count and requires programming an MCU which takes more labor. I would have to call my CM and run a BOM on the design before I could go through with it.
As of now, the best solution I can come up with that meets my cost requirements is to have two separate versions.
Thanks for everyone's input!
RE: Need help desiging a Simple and Cheap AC regulator
I would think deeply about that if I were the OP. Tolerances will not be very good. But I have a feeling that it isn't needed in this case.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Need help desiging a Simple and Cheap AC regulator
Right. Less than 75 W doesn't have any limit re PF or harmonics. So, it will probably be OK using a triac. The programming of a microprocessor will hardly be affecting the unit cost dramatically if you are in the 10k+ bracket. It will make smart solutions possible that probably makes the total cost lower at the end of the day.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Need help desiging a Simple and Cheap AC regulator
I knew I posted on here for a reason. A combination of what LionelHutz and VE1BLL said would make for a mean little controller.
With this I could even have say some kind of input (hall effect) to change the total power output for other future products or if the customer requires a different "level" of power. I say hall effect because the board will be potted. Run a magnet across the magic spot with an LED for visual feedback and you'd have a inexpensive control over temp. The reference design i saw on Microchip's site actually had an IR receiver so you could use a remote to set the brightness (was for an electronic dimmer). This would be even cooler but means I'd have to design or source a IR remote.
If they give me enough time, I will go with this solution.
Thanks again everyone. I'm very glad I posted this thread!
RE: Need help desiging a Simple and Cheap AC regulator
I'm sure you can find a Chinese manufacturer that can supply what you need
RE: Need help desiging a Simple and Cheap AC regulator
If you want fancy then you could measure the power (volts and amps) and run through a PID type control to regulate the output. If you want even fancier then do zero-crossing switching (basically a 60hz PWM) and eliminate much of the harmonics.