Hi,
Yesterday and today I worked on the circuit to ensure that the power supply is switched on and off properly.
This is not all that is needed for an impeccable behavior of this power supply when switching it on and off and using the enable button.
If this part I'm showing here now works well, I'll look at what else might be needed so that no abberation occurs during a power cycle and enable use.
The first schematics were equipped with auxiliary circuits that used the compensation connection of the LM723.
If you pull this connection to Vs from the LM723, the power section will not get any current anymore and there will be no voltage on the output.
Well a little bit, this because the power source at the output brings the shootky diode over the output in alignment and there is max -0.2 a -0.3V on the output.
But I didn't think that was a big problem and I don't know if the circuit I'm showing here does a power off without abberations.
That is one of the things I still have to test.
OK, this circuit, which is visible below, is intended to prevent the switch-on abberations from occurring.
The second function is an enable switch which ensures that when the power supply is switched off with this switch, the output cannot become negative due to the 25mA current source.
Earlier I had already indicated that the "voltage loop" should be kept closed for good behaviour.
When using the compensation connection of the LM723, you interrupt this voltage loop and this always results in more abberations that you can not solve in all circumstances,
see my pictures earlier in this topic.
On the right a simple version of the LM723 schematic, I did this in order to make the changes more visible.
The power transistor is now drawn like a simple Darlington and can for know be ignored.
The diagram is drawn when the power supply is switched off and the Enable switch is on.
On none of the power lines is voltage at that moment.
If we now look at the right part of the schematic, we can see that the relay contacts are also drawn at rest.
The +input of the LM723 is connected via the 22 Ohm resistor to the "0" of the power supply.
The reference output is also connected to the power supply "0" by a 4K99 resistor, but there is still no voltage on the power supply lines.
This doesn't seem very useful, but I will explain the function of resistor (R7) later.
Now let's switch on the 230V mains voltage!
The LMM723 and the Blah-1 Power transistor get their power now quickly, I will measure later, how fast the buffer electrolytics have 90% of their charge.
While building up the voltage for the LM723 and the Power transistor, the +input of the LM723 is still at the "0".
At the moment that the LM723 starts working linear, this is from 9V supply voltage, then the +input of the LM723 is still connected to the "0" of the power supply and the output voltage is also "0 "volt.
But, where is the problem now? The point is if the LM723 is not yet linear, what does it do with the steering of the power transistor...
If I use the original uirgang of the LM723, then a resistor from connection 10 to ground will almost certainly help here.
If I am going to use the compensation connection to control the power section I will have to use an extra active component on pin-13 to get a nice behavior, we will see how it behaves.
If C1 in the left part is sufficiently charged and the SW-1 switch is closed, the relay contacts are switched over.
RE-1a switches from R7 to the potentiometer P1 and the 22 Ohm resistor is disconnected, If the potentiometer P-1 is not turned counterclockwise,
output voltage will appear at the output terminals of this power supply.
The output voltage will rise "slowly" due to the Potmeter P1 value and the R8 resistor.
C8 always filters the noise from the reference output and C8 has so two functions.
If the enable switch is turned off, the potentiometer P1 is switched off and the reference is now loaded with the resistor R7 which has the same resistance value as the potentiometer P1.
Also, C4 is now quickly discharged by the relay contact RE-1b via the resistor R9 of 22 Ohm.
This 22 Ohm resistor protects the contacts of the relay.
OK, why draw R7 in the schematic.
This is to keep the dissipation in the LM723 IC as Constant as possible, this reduces the DC drift.
The contact on the relay was free for this application, so why not!
Now let's go back to the left part of the schematic, the enable switch has been removed and C3 shows that there is almost no bounching possible, this works together with the hysteresis of the relay.
I use the current through the relay to also control a red or green LED to see if the enable is on.
The relay used is a DIL-16 type of low power, that is completely closed.
This means that the service life will be very long.
The brands I usually use for these applications are: OMRON, Meisei and NEC in the 24V version.
To reduce the dissipation in the relay a series of resistors can be included, most of these relay types already work around 16V.
My minimum voltage at C9 is around 25V and maximum 34V as a resistor 4 a 5V reduction would be possible, but that is for later to see if this makes sense.
I also do not want to go too low in the relay coil voltage, as this also affects the speed at which the relay reacts.
Over the relay is a diode to counteract the EMF, but in series with the diode there is also a resistor that has about the value of the value of the coil itself, this resistor helps to accelerate the shutdown.
In the test shown below I took R2 loose and made it 1K, connected it to the output of one of my function generators and set it to 30Hz and a pulse width of 6mSec, 0 to 5V Top.
The blue trace is the pulse from the generator, the yellow trace is the 27V voltage which is also used for the relay coil and which goes through one of the relay contacts with a resistor to ground.
It takes almost 4mSec for the yellow trace to become positive, this delay is built up by the inertia of the relay and the 1K resistor together with the 0.1uF capacitor C3.
When the relay is switched off, it takes a little more than 2mSec if resistance R5 of 3K9 is included.
And this is the delay if the diode is switched directly over the coil, this delay is then 3.2mSec.
Furthermore, in this picture it is clearly visible that the contacts are bouncing.
This is especially the case in this picture at the rising edge.
The possible bouncing of the contacts has such a high frequency that it is well filtered by R8 and C4 at the +input of the LM723 and I don't expect any problems with that.
The trick with the series resistor is very useful here because the relay slows down a bit and is switched off by the C3 and R6 combination.
This means that no very high peak voltages are generated over the relay coil when the relay is switched off.
I want the power supply to go to "0V" as soon as possible when I set the enable switch to off.
A number of delays are added up to determine the final time.
That is this relay control and relay, the RC time of the potentiometer P1, R8 and C4 and at light loads, the 25mA current source, and the capacitors in the D.U.T that are located over the power lines.
And of course the biggest delay in "magic smoke" is my reaction speed...
The only piece I haven't discussed yet, is the function around Q1 which is just like Q2 a small MOSFet with the type number BS170.
When the power supply is switched off, the voltage at the negative edge of C3 quickly goes towards the "0V", this lever gate control goes up for Q1 which then quickly releases RE-1.
With this I hope to get the output as good to "0V" for the power lines for the LM723 and the power transistor have dropped completely.
The timing which are determined for this function by R3, R4 and C2 I still have to test.
The 1N4148 should protect the Gate of Q1.
R3 and R4 are also chosen so that there is no more than 18V at maximum mains voltage at the gate.
Also the RC time C2 and R4 must be so long that C1 is emptied properly.
So the fine tuning of the parts values comes later, the power on delay as it is now, is about 1.5 seconds.
Which is determined by R1, R6 and the Vgs or Q2 (R2 is too small to have much influence).
Some of you may find it even more complex with the relay, but don't forget that this is a power supply for me
and I think it's important to pay attention to the detais to get the most out of this power supply regarding good behavior.
Now for the last parts I almost forgot to mention, that is an extra resistor to pin-2 of the LM723, I always thought it was already in the IC itself.
But after going through several datasheets and looking at the diagram of the structure, this resistor was never present...
This resistor is in the base of the transistor that regulates the current limitation in the LM723.
The last part is the diode1N4148 which goes from pin-4 to "0V", this protects the -input in case of a short circuit.
In case of a short circuit, the charge of C8 which 10nF in this schematic dumped into the -input.
At slightly higher output voltages, the -input may fall below the value of the -Vs connection of the LM723, this can damage this input, the 1N4148 prevent this situation.
Enough now, this was a long session to type this here, I hope you learned something.
And I'd love to hear your comments, additions and my stupid mistakes I made.
Kind regards,
Bram