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Dose anyone know how to build a voltage controller for an sstc using scr’s I understand the basic principle but I’m not shore how to build it also is this even a practical way to control an sstc or would it be better to build a buck converter (I would like to keep it as simple as possible) I ask this now because I recently got two high power scr’s from a junk yard (they test ok) I haven’t been able to find a datasheet on them but I will post the part number when I get home tonight.
I have already searched the forum and Google but haven’t found anything useful on this topic but if I missed anything please post a link
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
There would be problems with peak currents and ringing because of the snubber cap on the bridge. The best way is to make the interrupter work in "phase angle" mode (delayed turn-on with mains sync).
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I have found if you use SCRs with a largish filter cap you can control the voltage on it rather easily. A simple 555 circuit even works fine. Steve C has some great info on his website about a 555 based SCR controller. I built one a while back using a MCU for use with my DRSSTC and it worked rather well. Basically 2 SCRs and 2 caps (2000uF I think) in a basic doubler configuration. It is controlled similar to how Dr. Dark Current was explaining, the turn on of the SCR is delayed in reference to the mins cycle.
No delay = full power full delay (delay of a full half cycle) = no power
The only thing you have to watch out for is the power factor. It will be horrid at any place other than full power and even then you will be lucky to see 0.7 or 0.8 because after all it is a diode charging a big cap.
If you are interested here are some photos of my SCR based controller
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Goodchild, the way you describe it is very hard on the SCRs and filter caps, because of the huge peak currents present. The normal way of dealing with this is to add an inductor in series. However this would be dangerous for a SSTC running on unfiltered mains, because the inductance together with the snubber cap capacitance could create some oscillations and the bridge could see dangerous voltage peaks.
What I was describing is to control the actual interrupter of the TC (enable pins on gate drivers).
Thanks for the useful information I think I might try steve C's controller would using PFC on the ac output help at all? Also dose anyone have any thoughts on using a buck converter
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Destroyer of mosfets wrote ...
Thanks for the useful information I think I might try steve C's controller would using PFC on the ac output help at all? Also dose anyone have any thoughts on using a buck converter
The only real advantages with a SCR supply is that it's robust and simple. Other than that they kinda suck.
If you used a PFC you may as well just use the PFC or a buck although this would be rather complex. Personally I stick with precharging systems and use my interrupter to control the output (PWM type control). A variac although heavy is also a great option.
Dr. Dark Current, yes it can be hard on the SCRs but it's no different than what a bridge rectifier sees charging a bus cap. SCRs are very rugged like diodes and as long as the correct rating is chosen it's not a big deal.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Goodchild wrote ...
Dr. Dark Current, yes it can be hard on the SCRs but it's no different than what a bridge rectifier sees charging a bus cap.
It indeed IS different. A non-controlled rectifier, even if we disregard mains impedance, draws current pulses of finite length and amplitude, but the pulses through a SCR controller with no additional impedance (inductance) are infinitely short and large. The mains impedance and the ESR of the cap are the only things limiting the pulse current.
Thanks for all the replies I will go with Steve c's driver and this is one of the scr's I have it is an scr you just cant see the gate pin from the angle the pic was taken Also I just have one more question I'm not 100% shore how the power section should be set up should it look like A or B
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Dr. Dark Current wrote ...
Goodchild wrote ...
Dr. Dark Current, yes it can be hard on the SCRs but it's no different than what a bridge rectifier sees charging a bus cap.
It indeed IS different. A non-controlled rectifier, even if we disregard mains impedance, draws current pulses of finite length and amplitude, but the pulses through a SCR controller with no additional impedance (inductance) are infinitely short and large. The mains impedance and the ESR of the cap are the only things limiting the pulse current.
Yes this is true but also applies just the same to a regular rectifier. During the short period of time that the voltage goes above the voltage across the cap you are charging, you get a very small very high current pulse limited only by the ESR and line impedance.
This is the same with SCRs because they are in fact just controlled diodes in the same configuration doing the same operation. The difference is the placement of this short spike in relation to the mains voltage. Because the SCR is delayed in turn on this pulse can happen any place on the cycle making for a very poor power factor and higher peak currents (due to the voltage on the cap having to fall lower before it is charged) At full power (no delay) it is acting exactly like a regular uncontrolled rectifier and the current spike is situated some what in the center of the cycle like a regular rectifier.
Not saying an inductor in series with the input line is a bad idea, on the contrary it can help improve the PF by spreading out the short pulses of current. However if you only use an SCR supply to precharge a system up to an operating voltage and then use it like a regular rectifier this inductor is unnecessary.
If you plan to use the supply like a variac to control input power then the inductor should be used in series with the input.
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SCR power control for an SSTC any ideas?
