Karl's Power Supply SCR question?

They adjust the phase angle to set the output voltage.

Which is just a fancy way of saying adjusting the voltage level it turns on at.

When a capacitor is used it is phase angle.

They are often used as switches. They can be used to be turned on. They can't be used to turn it off if DC as a source, but with an A.C. source the Anode turns off with the frequency.

So with AC it can be a binary gate. On-Off.

The 'unused' wire might have been to switches.

Mart> >

Sigh.

Depending on the design they either turn on the scr at the '0' crossing of the AC input or configure it as a phase angle system. The zero crossing system works like a hit and miss engine firing for a full half cycle until the control system indicates the proper voltage and then it cuts off. The phase angle firing system usually uses an oscillator that is fed through a gate that is turned on at the desired phase angle. The gate is hit with a burst of hi freq pulses which will turn on the scr. Once the scr is turned on it will remain on until the ac voltage goes negative. The pulses are much more effective for turning the scr on at the proper phase angle. A toroid transformer is used to isolate the gate from the pulse driving ckt.

If you just want to use it as a rectifier diode you can pull a control voltage from the anode but make sure the cathode to gate voltage is not exceeded and also you will need a reverse diode across the gate to keep it from going negative. You will have to figure the limiting resistor/voltage divider to match the scr that you use.

I would try to use the firing circuits that Karl pulled out of the machine since they are already set up and matched to the scr's. If I knew the model of the drive he has I may have a manual for it. That scr stack sorta looks like part of an old Fuji drive.

John

Roger, did you get this email?

Karl

Roger, if you are done with all that overtime, could you double check me? The numbers on my SCR are 71RC80 S5-5 9L11057 I can't tell who is the manufacturer So, I went to digikey and found what looks the same.

The way I read the specs., it takes 3 volts on the leads to turn on and 6 volts shouldn't be a problem. Is this correct? if true, I'll put four SCRs in series and connect to 24 VDC logic power supply. There are twelve total SCRs so I would have three sets. The way I read it, I could go with two sets of six SCRs in series also. I don't know what I'm doing here, so I'd like another opinion. Karl

A similar circuit to what I think Karl needs:

The third circuit down shows an AC supply with a SCR and a diode and switch going to the gate of the SCR. To use the SCR as a rectifier just connect the diode to the like the diagram shows but you don't need the switch. Connect eacy SCR that way and they will be rectifiers, no other power supplies or control circuitry needed. But like I mentioned, test at lower voltages first to make sure everything is working right. Might not be fun to flip on 230 and find out you have a wiring mistake or a bad SCR!

Also, before I mentioned 1N4004 diodes, I looked them up and they are only rated for 400V, I would probably look for 1N4007 that can handle 1000V reverse, since I know you can easily get double your peak voltage in a rectifier circuit with capacitors charged.

RogerN

He's not likely to be able to find any other power supply to give him a

240VDC source (not much higher and "adjusted" down), than a single rectifier with a single-phase 240VAC source.

I know about ripple, but LC filtering can minimize ripple.. I dunno if if filtering will minimize it to a level that fits the application (but likely will).

The "adjusted" part is more complicated than most imagine, when the desired current capacity is about 100 amps peak (as mentioned earlier).

The full wave bridge rectifier approach will increase the voltage level to an excessive level.

The proposed used power supply approaches (SCRs, VFD) to attain a reliable

240VDC power supply will likely result in a lot of wasted time, and isn't particularly safe for someone who doesn't understand isolation, for example.

He's not likely capable of utilizing parts of this SCR bank, or a VFD to attain a 240VDC supply. Others certainly have the knowledge required for such projects.. but the task isn't an "all-ya gotta do is" project.

One aspect of a lot of speculation is, that many are quick to assume, is that the aged, used components are all in good working order.

I've worked with large/huge industrial DC supplies (motor drives, plating rectifiers), and they're typically designed for specific applications.

I got the email and sent a reply. The bad thing about using a 24V supply is you have to make sure it's isolated an you may need a separate power supply for each bank of SCR's.

If you use these SCR's as rectifiers and use filter capacitors you should have a bus voltage around 340VDC. If that is too high you may need to do something to lower the voltage, maybe control the SCR gates, if voltage too high, turn off SCR's, if too low, turn on SCR's. Or you could use a step down transformer to get the voltage where you need it. Look at some of the sample SCR circuits, they show examples of firing the gate using the voltage that is supplied to the SCR itself, no extra power supply needed.

I'm done with overtime until Sunday, I got scheduled! :-(

RogerN

[ ... ]

The problem is that the gates will each be referenced to a different voltage if you have the SCRs in series (*way* more than 24V apart), and as normally wired, you have the cathodes of multiple SRCs tied together, so you *can't* connect them in series. The gate signal voltage is gate to cathode IIRC. So -- you will want resistors in series to limit the voltage applied to the gate. Look up the minimum current needed to turn on the gate, and the maximum allowed current in the gate, and calculate the resistor for something perhaps half-way between -- subtracting the gate's forward voltage (probably something like 0.7V or so) from the 24V from the wall wart.

It would help if we knew what the wiring of the SCRs is like on the heat sinks. Are they all connected via the studs to the metal of the heat sinks, or are there insulating washers preventing that? Are all of the SCRs the same part number? There might be versions with anode to stud, as well as cathode to stud.

That much is clear, I fear.

Get *lots* of information and advice before you fry those nice SCRs.

Good Luck, DoN.

There's pics in the original thread. All SCRs are same part number. There are four on each phase bolted right to the heat sink for that node. The heat sink design is so nice, its the reason I'd like to use this component. Otherwise, a three phase rectifer is only $60 (no heat sink)

The power supply is the only part of this refit that has me apprehensive. With the help of RCM, I'll get through it.

Yesterday, I cleaned out the original computer cabinet to make room for the five servo amps, this power supply and a half dozen other components. I'll lay everything out today and start mounting.

I ended up with a box of more than 50 DPDT 24V relays. Anybody need them?

What's the sigh about Martin? Where am I wrong? with the "basics".

With leading phase commutation, the basics are you turn the SCR on at different points ov the rising portion of the sine wave. - Which in practical terms means when the instantaneous rising emf or voltage reaches a particular value - or voltage. (or phase angle - all different terms to explain the same process) Leading phase commutation is the only scheme that works with SCRs, and is noisy because of the rapid rate of rise of the Current (di/dt) (time rat change of current) at turn-on. Trailing phase commutation can be done with MOSFETS and other devices which can commutate both on and off and is NOT noisy because the rate of rise (di/dt) is more or less limited by the sine wave frequency.

Mike said they used a variable resistor "to fire the SCR sooner or later in the rising + portion of the sin wave." Roger seemed to dissagree - stating "They adjust the phase angle to set the output voltage." To which I replied "Which is just a fancy way of saying adjusting the voltage level it turns on at."

To which you "sigh"

Sorry Martin - That last question should have been directed to Mike

- not fully awake.

I'm no expert, but I don't THINK you can but the SCR gates in series .

Don't think things work that way. The firing voltage is the voltage difference across the junction.

That weren't me disagreeing but I didn't take that they were disagreeing just another way to explain it. With a variable resistor you can adjust the SCR firing in the rising portion of the sin wave. By phase shifting you can get the rising portion to extend beyond the peak and fire the SCR later. Or you can use more electronics to fire the SCR anywhere you want.

RogerN

What is the part number?

The numbers on my SCR are 71RC80 S5-5 9L11057 I can't tell who is the manufacturer So, I went to digikey and found what looks the same.

Who is Martin?

71RC80 is an old International Rectifier part. 800 volts, 70 to 110 amps.

IR doesn't have it on their website so it must be obsolete, but I have some old IR databooks in the basement. I'll look later.