SCR Power Control Questions

[gar]

190317-1414 EDT

Some measurements. 120 V 60 Hz AC line as source, with a 230 ohm load resistor.

An OAC5 is a solid state relay. Probably uses a Triac as the AC switch. Uses some sort of isolated driver to provide continuous excitation the gate. Does not have fractional cycle switching capability such as an SCR or Triac alone would have. AC RMS measurements --- 118.2 Vac rms input, 117.3 Vac rms load. Average DC across load 0.034 V. Quite symmetrical. In the off state AC across load is 0.12 Vac rms. This results from their snubbing circuit.

A Lutron CTCL-163-P dimmer. 120.1 Vac rms input. Output range 112.9 Vac rms down to 17.0 Vac rms. At 20 V ac output the DC output average was 0.04 V. Over the entire output range the DC output reached a maximum of 0.1 V.

An older inexpensive two terminal dimmer. 120.0 Vac rms input. Output range 118 Vac rms down to 0.6 Vac rms. Over the entire output range the DC output reached a maximum of 0.33 V near the low voltage end.

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[StarCat]

Interesting...

Interesting...

Interesting Gar,
It was my intial take that a current probe would be useful as a start point for heater loads going in cold. Then to verify the integrity of the elements by resistance.
Looking around there are some devices made specifically for the task apparently of DIAG. on these units, and no doubt some different maneuvers depending on the OEM configuration to make troubleshooting do faster.

 

[RumRunner]

190317-1414 EDT

Some measurements. 120 V 60 Hz AC line as source, with a 230 ohm load resistor.

An OAC5 is a solid state relay. Probably uses a Triac as the AC switch. Uses some sort of isolated driver to provide continuous excitation the gate. Does not have fractional cycle switching capability such as an SCR or Triac alone would have. AC RMS measurements --- 118.2 Vac rms input, 117.3 Vac rms load. Average DC across load 0.034 V. Quite symmetrical. In the off state AC across load is 0.12 Vac rms. This results from their snubbing circuit.

A Lutron CTCL-163-P dimmer. 120.1 Vac rms input. Output range 112.9 Vac rms down to 17.0 Vac rms. At 20 V ac output the DC output average was 0.04 V. Over the entire output range the DC output reached a maximum of 0.1 V.

An older inexpensive two terminal dimmer. 120.0 Vac rms input. Output range 118 Vac rms down to 0.6 Vac rms. Over the entire output range the DC output reached a maximum of 0.33 V near the low voltage end.

.

Hi Gar,

None in your data sheet regarding OAC5 lends itself to the desired control effect of a controllable speed/dimming output.

All I can see is a solid state state relay that could easily be substituted by a garden variety isolation relay. . .which could possibly be either-- an electro-mechanical or solid state relay. The optical coupling is the most noticeable feature though. . .a worthwhile lab work.

The two-terminal dimmer employs the same principle as the three-wire variety.

The main difference between the two is, the dimmer that has three-wire uses a separate control source compared to the two-wire variety.

The three-wire variety has its gate signal that comes from an isolated source [and also within the unit] which is controlled by a variable resistor that determines the desired charging level and timing for a capacitor to provide the correct signal to fire the TRIAC's gate.

Both models that include the two-wire version have power that are provided [continuously] to DIAC and TRIAC.
Since the DIAC can only conduct [with the help of a capacitor]-- and therefore trigger the TRIAC according to the level of charge of the capacitor. . . the TRIAC remains non-conducting. . . until the signal from the capacitor to conduct is received.

Because of the sinusoidal nature of AC, the capacitor charges and discharges at every cycle. . . hence-- no need to trigger every half cycle like you would an SCR.

The distinguishing attribute between SCR and TRIAC is:

SCR only conducts on the half positive region of the sinusoidal wave form while the TRIAC can be triggered in either direction. It is bidirectional as opposed to unidirectional.

Bidirectional function can also be accomplished by a back- to- back SCR arrangement.

Now, in terms of your findings-- which indicate that you came up with an infinitesimal adjustment of almost [immeasurably small voltage]—I would recommend the type of TRIAC control system called:

TRIAC PHASE CONTROL using variable resistor in series with a capacitor.
This is the one I called out with a DIAC. . . as opposed to the two-speed/dimming capability using a HI and LOW setting for motor speed and dimming incandescent lamps..

Since the voltage that controls the timing [when the capacitor is charging/discharging] by the equation: t=RC. . . and through the amount of charge that this capacitor is capable of delivering the trigger signal-- can also be used in controlling heating elements and small universal motors.


Cheers

 

[gar]

190318-2016 EDT

StarCat:

Consider Fig. 22 of the Chromalox discussion. This is a back-to-back SCR single phase circuit. Below when I mention AC I mean with any DC component removed, as by a capacitor in series with the meter. Most electronic AC meters have this capacitor built in and not switchable. In the Simpson 260/270 the capacitor is switch selectable. With the Simpson meters you have to use Output to add a series capacitor.

In said figure, if both SCRs are turned off, then both the AC and DC voltages measured across the load are essentially zero.

If both SCRs are fully turned on for the complete cycle, then the AC load voltage will be close to the AC source voltage, the DC source voltage should be zero, and the DC load voltage less than 1 V.

If both SCRs are symetrically triggered, then the resulting output voltage should be an AC voltage between zero and full line voltage minus about 1 diode drop. DC load voltage should be less than 1 V.

If one SCR is open, does not conduct, then the load voltage is unidirectional DC with an AC ripple voltage anywhere from from zero to some larger voltage up to what would be seen from a single diode.

If one SCR is shorted, then the output is AC plus DC from a minimum of what a diode would provide up to that of both SCRs being on. Both SCRs on is near zero DC voltage.

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[gar]

190319-1529 EDT

myspark:

I found your post to be quite confusing.

None in your data sheet regarding OAC5 lends itself to the desired control effect of a controllable speed/dimming output.

I did not say it did. The point of the OAC5 data was to show how low the DC component could be with a hard driven Triac gate.

The two-terminal dimmer employs the same principle as the three-wire variety.

The main difference between the two is, the dimmer that has three-wire uses a separate control source compared to the two-wire variety.

The three-wire variety has its gate signal that comes from an isolated source [and also within the unit] which is controlled by a variable resistor that determines the desired charging level and timing for a capacitor to provide the correct signal to fire the TRIAC's gate.

I don't know what "separate control source" means. Most inexpensive dimmers don't have some isolated source.

Both models that include the two-wire version have power that are provided [continuously] to DIAC and TRIAC.
Since the DIAC can only conduct [with the help of a capacitor]-- and therefore trigger the TRIAC according to the level of charge of the capacitor. . . the TRIAC remains non-conducting. . . until the signal from the capacitor to conduct is received.

Dosen't make much sense. A capacitor is not required to make a Diac conduct. If you exceed a threshold voltage applied across a Diac, either + or -, then the Diac is triggered into a conductive state. The capacitor and resistor are used to provide a controlled time delay to the voltage at which the Diac triggers, and in turn when the device gate is triggered. A Diac or Triac is not discussed in ether the 1st or 2nd editions of the GE SCR Manual. By the 5th edition both are mentioned. A v-i curve for the Diac is shown.

Because of the sinusoidal nature of AC, the capacitor charges and discharges at every cycle. . . hence-- no need to trigger every half cycle like you would an SCR.

Nonsense. How do you expect to trigger an SCR or Triac on every half cycle unless you trigger on the desired half cycle.

The remainder of your post is even more confusing.

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[gar]

190321-2200 EDT

For you that just conjecture, do not experiment, do circuit analysis, or look at datasheets and manuals here is some data for a random GE C22F probably manufactured in 1960s.

Source is an HP variable voltage DC power supply. Current limiting resistor 350 ohms.

Anode cathode voltage vs current at room temperature after triggering and with open gate circuit once triggered.

0.81 V 46 mA
0.82 V 20 mA
0.94 V 10 mA
1.05 V 8 mA
1.16 V about 7.2 mA dropout point
Below dropout jumps to source voltage.

Note that the voltage drop never went negative or to zero before dropout. The GE specification for holding current is about 8 mA.

I suggest that you study SCRs and do some experiemts with them before you discuss their circuit operation.

Driving an inductive load with a gradually changing current will product the same result up to dropout current. After dropout the anode cathode voltage nearly instantaneously jumps determined by the inductor rather than the power supply. Inductive kick.

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