Quarter- Wave rectifier? - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: February 12, 2016, 05:51:14 am
content: 1. I have a circuit which has 220Vac/50Hz input which heats up a wire. On full mode, entire 220V is connected to wire, so wire gets heated up quickly. Next mode is half mode, in which I have connected a diode in series, so negative half is clipped & positive half is formed, so wire gets less heated.2. Is there any way so that power can be further reduced by half, by placing a simple diode or component so that wire gets less heat?

author: Ian.M
author_rank: Super Contributor
timestamp: February 12, 2016, 06:03:04 am
content: No. The next step is a phase angle controller using a TRIAC, i.e. a dimmer, which would give you fully variable control.

author: Brumby
author_rank: Unknown
timestamp: February 12, 2016, 07:20:30 am
content: ... or use a longer wire.

author: Ian.M
author_rank: Super Contributor
timestamp: February 12, 2016, 07:34:52 am
content: The other option is series/parallel. Two heating elements in parallel for full power. Switch one out for half power, and switch them both in series for quarter power, but that's only any good for bulk heating applications which doesn't help you if you need to specifically control the wire's heat.

author: tszaboo
author_rank: Super Contributor
timestamp: February 12, 2016, 07:38:25 am
content: They normally do this by placing an extra resistor in series, so the current drops half. Otherwise you can use a triac.

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 12, 2016, 01:50:46 pm
content: As mentioned above, you want a phase-control light dimmer type circuit to do this. It is considered extremely poor form to use a half-wave rectifier as that sets up a DC bias in the distribution transformer which can cause it to draw excessive magnetizing current.

author: timb
author_rank: Super Contributor
timestamp: February 12, 2016, 03:03:11 pm
content: You could also do Proportional Zero Voltage control: Apply a low frequency PWM to the input of a Zero Crossing opto-isolated triac, which would drive a larger triac (or just use an SSR module), thus varying power to the load. (So, if you had a 1 second period and the PWM was 120Hz, a 100% duty cycle would keep the triac constantly on; a 50% duty cycle would result in it being on for the first 60 ZC cycles and off for the last 60 cycles, etc.)This has an advantage over phase angle firing, as it produces much less noise. It works very nice on purely resistive loads and with a snubber can also work on small inductive loads as well. (It can cause noticeable flicker when used with incandescent lighting, but for a heating element it would be my first choice.)Here's a circuit from a Vishay datasheet:(Note, the circuit seems to have a couple of errors. I've been meaning to do a thread on it, but it's a good starting point.)

author: gautamdamodar
author_rank: Contributor
timestamp: February 14, 2016, 07:35:40 pm
content: I think next possible step is triac control. Diode is also a active component, right?

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 08:24:54 pm
content: Quote from: Vindhyachal.takniki on February 12, 2016, 05:51:14 amHiThe problem with a single diode is that you now have a DC load on an AC circuit. This puts a saturation current through the mains transformer. That tends to make the power company guys mad at you.Bob

author: pmbrunelle
author_rank: Regular Contributor
timestamp: February 15, 2016, 04:51:35 am
content: Quote from: uncle_bob on February 14, 2016, 08:24:54 pmI think this has a remedy. You can use two heating resistors in parallel. Then, for half-power mode, insert a diode in series with each heating resistor (for half-wave rectification), but install the diodes with opposite polarities. So that for one half-cycle, one resistor conducts, and then for the next half-cycle, the other resistor conducts, alternating between the branches.However, this may be a pain in the sense that both resistors would have to be well matched to each other. Probably the matching would be not bad if the resistors came from the same spool of wire?I don't know how much DC offset current is considered "OK".In general though, I prefer the schemes involving playing around with series/parallel combinations, switching between different taps... that sort of thing.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 03:05:54 pm
content: Quote from: pmbrunelle on February 15, 2016, 04:51:35 amHiIf multiple heating elements is an option, hook the elements in series for the lower power. That can be extended to 4 elements or more. There are no matching issues and it's just a switch or relay to do the adjustment.Bob

author: sarepairman2
author_rank: Frequent Contributor
timestamp: February 15, 2016, 09:49:45 pm
content: yes, there is.a differentiator connected to a pass element, so only the + or - derivative is passed.when the derivative of the signal you are measuring is positive (happy parabola), you open a gate. when it becomes negative (unhappy parabola) you close the gate. this will give you precisely 1/2 of a haversine.http://www.learnabout-electronics.org/Amplifiers/images/op-amp-diff-cct.jpghttp://www.tutornext.com/system/files/u29/study%20of%20monotonicity%20-%20first%20derivative.JPG

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 10:25:55 pm
content: HiWith a normal temperature controller, running off a normal mains connection, using normal heating elements, even the derivative signal is *long*. Trying to do a high voltage controller by having multiple pass elements is a *very* expensive way to do it. Even on small (as in tiny) controllers, a single pass element is used and the various control elements are summed ahead of that pass element.Bob

author: Anks
author_rank: Frequent Contributor
timestamp: February 16, 2016, 03:48:57 am
content: A multi tapped transformer would be a simple solution.

author: Xplode
author_rank: Contributor
timestamp: February 16, 2016, 04:51:05 am
content: The TRIAC sounds like the simplest way to do it.But I feel like I should ask, what are you using this heated wire for, and what is the actual current draw of the wire? If we're talking 20A then the solution might be different from 500mA.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: February 16, 2016, 06:14:19 am
content: Thanks for reply.I will use full & half wave only, as this is the only effective solution in my case.