This is a very frequent question, and very interesting subject to think about.
If we have a current flow of y amps and 20 volts, there is no known method to coniniously just let 10 volts through while not without affecting the difference in some way. If we would of had something like that, we would not have efficiency issues with different power sources. We do tend to think at amps and volts being different things, but its just one phenomena behind it even if it has two sides of its sort of same coin, so things are tied to each other in certain way. A linear regulator or a resistor divider would just let 10volts y amps through, and just dissipate the difference in heat for y amps x remainder of volts. The switched mode power supply employs pwm-ing the input by principle of volts*sec balance or amps*sec balance to determine the output from the input and pwm filling factor. But still smps is complicated (e.g. many failure modes) and while has better efficiency its far from being a great. This is why this idea of controling the sine wave to accomplish something similar is very appealing.
A light dimmer contains a triac that is used to start the conduction at an angle. E.g. from that entire input sinusoid only lets a part of it through. It doesnt change the output at a time, it just starts to conduct at a certain point, after which it fully conducts. But as for the light bulb example, the systems runs in good operating point / parameters while its fully on and the dimmer just goes down from that e.g. it "eases" operating points. This is why you cant use a dimmer for the heated bed like in the OP issue. It still gets same voltage peak as the initial sine, what it drops is the initial lowest parts while the voltage is low. Sort of speaking the ones using a diac to trigger would drop at least like 20-30 volts from zero, and conduct in the rest, including peaks of 308volts. Same thing the optocouplers for triacs like mocxxx types also dont switch perfectly on zero, but more or less around that, again some +/- 15v or something. Also its only 50hz so if one would use it like so to for a voltage conversion, then would need a huuugeee bank of capacitors and still have significant ripple. And those capacitors would need to be like 400v rated, which means extremely big and expensive. And few other things. But on short, the solution is not feasable for voltage conversion.
What one could do practically, is to use a type of dimmer circuit with a triac that is only 3 quadrant (because the 3quadrant ones can work ok with inductive loads), and put it at the input of a transformer and you could get that trafo to lower its output down from its normal value, because only parts of sine gets into the trafo. A normal light dimmer probably does not contain a 3 quadrant triac so its better not to try directly just like that, but use a dedicated circuit for the job. Also can do something similar on the output of a transformer, there are also some solutions that employ triacs in the middle of rectification bridge to control the exact form of the output, more to cut down the peaks instead, and again that would be non-standard, and for other purposes a proper syncron rectification would be generally more accomodating.
Its a nice idea to be able to use only parts of the sine and to get a different output, it works for incandescent bulb (light dimmer) or other cases, but it has its limitations. Sort of speaking, if the heated bed would have like at least ~60-90++ ohms resistance, then one could put a light dimmer on it, sort to control it down from its normal operating point. But it needs a high resistance, enough that it would work nice on full sine first. So wont work with that pcb heated bed because the peaks would be way too huge. Very similarly to what the op asked, there are integrated circuits that do the exact thing to control temperature, e.g. a circuit that "fires" the triac at different angles (proportional control), e.g. takes different parts of the sine depending on the feedback coming from a temp sensor. Most of us have one circuit like that in our homes, its quite commonly used in fridges, where it does exactly this thing with the feedback from a thermistor, but it just controls it the other way like we would in a heated bed, e.g. makes the temperature go down. Otherwise something like that could be easily done for a heated bed running on mains, and have the temperature on proportional control. It would be good like that, with a uC compatible on-off input. But ... But i dont think its generally advisable to promote heated bed running on mains voltage for our users. My bed runs from mains with a traic and a moc_xx, and its a aluminium plate with 8 power resistors of 50w each and runs fine. But i do not want to promote that for the regular user, which is less aware, to have something like that to be used in homes with children, with exposed wires, on the desktop, etc, on short i dont see good things coming from that, and as long as there are atx supplies those should be safer.
If we have a current flow of y amps and 20 volts, there is no known method to coniniously just let 10 volts through while not without affecting the difference in some way. If we would of had something like that, we would not have efficiency issues with different power sources. We do tend to think at amps and volts being different things, but its just one phenomena behind it even if it has two sides of its sort of same coin, so things are tied to each other in certain way. A linear regulator or a resistor divider would just let 10volts y amps through, and just dissipate the difference in heat for y amps x remainder of volts. The switched mode power supply employs pwm-ing the input by principle of volts*sec balance or amps*sec balance to determine the output from the input and pwm filling factor. But still smps is complicated (e.g. many failure modes) and while has better efficiency its far from being a great. This is why this idea of controling the sine wave to accomplish something similar is very appealing.
A light dimmer contains a triac that is used to start the conduction at an angle. E.g. from that entire input sinusoid only lets a part of it through. It doesnt change the output at a time, it just starts to conduct at a certain point, after which it fully conducts. But as for the light bulb example, the systems runs in good operating point / parameters while its fully on and the dimmer just goes down from that e.g. it "eases" operating points. This is why you cant use a dimmer for the heated bed like in the OP issue. It still gets same voltage peak as the initial sine, what it drops is the initial lowest parts while the voltage is low. Sort of speaking the ones using a diac to trigger would drop at least like 20-30 volts from zero, and conduct in the rest, including peaks of 308volts. Same thing the optocouplers for triacs like mocxxx types also dont switch perfectly on zero, but more or less around that, again some +/- 15v or something. Also its only 50hz so if one would use it like so to for a voltage conversion, then would need a huuugeee bank of capacitors and still have significant ripple. And those capacitors would need to be like 400v rated, which means extremely big and expensive. And few other things. But on short, the solution is not feasable for voltage conversion.
What one could do practically, is to use a type of dimmer circuit with a triac that is only 3 quadrant (because the 3quadrant ones can work ok with inductive loads), and put it at the input of a transformer and you could get that trafo to lower its output down from its normal value, because only parts of sine gets into the trafo. A normal light dimmer probably does not contain a 3 quadrant triac so its better not to try directly just like that, but use a dedicated circuit for the job. Also can do something similar on the output of a transformer, there are also some solutions that employ triacs in the middle of rectification bridge to control the exact form of the output, more to cut down the peaks instead, and again that would be non-standard, and for other purposes a proper syncron rectification would be generally more accomodating.
Its a nice idea to be able to use only parts of the sine and to get a different output, it works for incandescent bulb (light dimmer) or other cases, but it has its limitations. Sort of speaking, if the heated bed would have like at least ~60-90++ ohms resistance, then one could put a light dimmer on it, sort to control it down from its normal operating point. But it needs a high resistance, enough that it would work nice on full sine first. So wont work with that pcb heated bed because the peaks would be way too huge. Very similarly to what the op asked, there are integrated circuits that do the exact thing to control temperature, e.g. a circuit that "fires" the triac at different angles (proportional control), e.g. takes different parts of the sine depending on the feedback coming from a temp sensor. Most of us have one circuit like that in our homes, its quite commonly used in fridges, where it does exactly this thing with the feedback from a thermistor, but it just controls it the other way like we would in a heated bed, e.g. makes the temperature go down. Otherwise something like that could be easily done for a heated bed running on mains, and have the temperature on proportional control. It would be good like that, with a uC compatible on-off input. But ... But i dont think its generally advisable to promote heated bed running on mains voltage for our users. My bed runs from mains with a traic and a moc_xx, and its a aluminium plate with 8 power resistors of 50w each and runs fine. But i do not want to promote that for the regular user, which is less aware, to have something like that to be used in homes with children, with exposed wires, on the desktop, etc, on short i dont see good things coming from that, and as long as there are atx supplies those should be safer.