Here is the fifth post on my el-load. In this post I will discuss the complete el-load excluding the +12v and +5 volt power supply. I will cover that dual voltage power supply and the case later. Please refer to the schematic diagram in the picture below. In previous posts I covered the Over Temp Stage (OTC) in the lower left-hand of the diagram. I also covered the voltage control and output stages that appear across the top of the diagram. In this post I will cover the switching, crowbar, line driver, load on LED, and panel meter. Sorry the switch symbol is so large but that's what was available. I am just learning the open source KICAD program so there are a few things that I will need to change. I hope to also use KICAD to make the circuit board artwork.
OK let's talk about the switching. It looks a bit involved but is actually pretty simple and inexpensive. The four symbol circles represent actually just one rotary switch that cost $3.50 USD. It is a three position, four pole, non-shorting switch. This switching operation could be done with electronic switching or relays but this approach is simple and cheap. One pole is section S1A at the top of the diagram and it has three positions; 1, 2, and 3. A little confusing but starting from position 1 and going to position 3 is moving the panel knob clockwise on this diagram it is counter-clockwise. The three positions of the switch are OFF - SET - ON. In the OFF position the unit is turned off... duh. In the SET position the unit is turned on but the load is disabled. In this position you can set the current using the front panel LCD meter without the load being in operation or even connected. This is helpful because it is hard to know initially where the 10-turn RV1 is set. Moving to position 3 (ON) enables the load current and reads its value on the LCD meter. At anytime you wish to disable the output without turning off the unit, you would just move the switch from ON to SET. Then make any changes to the external circuit and then switch back to ON position to restore operation. Also as a little safety feature you can never turn the unit completely on without going through the SET mode. That way you can see the current you are about to allow before it actually happens.
Looking at section S1A you can see that it connects the green Load On led to the output of U1D. This means that the ON LED D2 will only turn on in this position and that the output of U1D must be in a low voltage state. If however there is a fault with the temperature U1D's output will be high and not allow the Green diode D2 to be on while the Temp red LED D1 is on. If the output of U1D is high, near the 5 volt rail it will bias Q3 the Crowbar E-MOSFET to saturate and pull the control line going to pin 5 of U1B near zero volts. This will turn off Q1 and Q2 and therefore IL will go to zero. Why use such a large FET for Q3. They are cheap and I had one. It is also nice for repair to have to only deal with one type. You could of course use a very low power device for Q3.
Switch section S1B grounds the input to pin 5 of U1B in the OFF and SET position so that IL is zero. In position 3 the switch couples the control voltage to pin 5 of U1B for normal operation in the ON mode.
Switch section S1C in the OFF position grounds the input to both the panel meter and the line driver U1C. I like grounding these input until the unit powers up. In the SET position section S1C connects the control voltage from the wiper of RV3 to both the line driver and panel meter. This allow you to view and set the IL current before enabling the output. When section S1C is in the ON position the voltage (related to IL) across R8 the sense resistor is sent to the line driver and panel meter for monitoring.
Section S1D is the AC power section and connects to the power supply. In the OFF position the AC line is broken and the unit is off. In the other two positions the power supply is turned on. This rotary switch control the 120 volts AC and this switch has that rating and is also fully enclosed and has a plastic shaft for safety. If you decide to use a rotary switch make sure it can handle and is designed to operate AC line voltage. For example the one I selected cannot handle the 220 V provided in other locations.
I have a close-up of this rotary switch in a picture below and it hasn't yet been wired for the AC line but all the other connection have been made for testing. I like using ribbon cable to make the wire management neat. So far I have been running test to see if when switching the system, it had transients or instability. Below is a oscope picture showing the turn off and turn on of the system and this is at a low sweep speed and looks very clean with no overshoot. I also used much faster sweep speeds and still looks good with a slight ramp up.
In this post a couple of comments on the line driver and panel meter. The panel meter is under $20.00 USD and is 3 and 1/2 digital back light LCD. It can be programmed with solder jumpers for AC/DC current and voltages. It also includes several ranges and you can set the decimal point. Right now I have it set to DC volts on the 2 V range with the decimal point 2-digits from the right. Therefore it is reading out in amps with a resolution of 10 mA. For example if the display is 12.50 that would represent 12.50 amps of IL. I am not too happy with the back light, pretty dim but it does the job, I guess. I was going to use the light from the panel meter to indicate when the unit was on but decided to add a red power on led in the power supply section. The purpose for the line driver U1C is to provide either the SET voltage or the voltage related to IL to a remote voltmeter. The back of the unit will have two banana jacks at 3/4-inch spacing so you could plug in a dual banana jack connected to a DMM. This way you could read down to milli-amps or for easy monitoring. For example have the unit under the bench and still easily view the IL value. R12 is another 270 resistor and will limit the opamp current to a safe value so that if you accidentally short the remote wires together the opamp will not be harms and since U1C is a buffer you will not upset the operation of the el-load. I plan to do a PC board, final case assembly, and performance testing but this may take a week or two. Hope this helps, John