Hi all,
is there a way to configure a 317 as a CCS that can under near short circuit conditions allow it to continue limiting at well above it's in-out 40V limit?
I want to use dual polarity supplies of between +-45V to +-60V with adequate smoothing AND to pass bias currents upto 400mA.
The two CCS to be set to a limit just above normal bias current.
I do not want excessive volts drop across the CCS when within the "normal" bias range.
In normal operation the 317+resistor does this duty quite well on it's own.
But in short circuit accident mode the 317 will go overvoltage.
is there a way to configure a 317 as a CCS that can under near short circuit conditions allow it to continue limiting at well above it's in-out 40V limit?
I want to use dual polarity supplies of between +-45V to +-60V with adequate smoothing AND to pass bias currents upto 400mA.
The two CCS to be set to a limit just above normal bias current.
I do not want excessive volts drop across the CCS when within the "normal" bias range.
In normal operation the 317+resistor does this duty quite well on it's own.
But in short circuit accident mode the 317 will go overvoltage.
too simple.jackinnj said:
If I am drawing 100mA of bias @ +-50V.
Set the CCS to 120mA.
Now short the output (a slipped probe).
The resistor has to drop ~10V to keep the 317 below 40V.
The resistor will require to be ~83r.
Now back to the "normal operation with CCS passing 100mA.
The +-50V input (from the PSU) will be ~ 50 - 83*0.1 - 1.4V(317Vdrop) - 1.25V ~=+-39Vdc.
Most of that loss is down to the 83r protection resistor.
I'll download the TL783 datasheet, it may help.
I tried a two transistor CCS,
2n5401 & 2sc1943.
When passing ~80% of CCS rated current it drops only 700mV.
As it approaches 100% of CCS rated current the voltage drop rises rapidly.
This is far lower than the Vdrop across a 317+resistor.
That two transistor version is what I will build up to insert into the PSU output leads, after the smoothing caps.
Ensure you have sufficient current bypassing the load to keep the 2n5401 well away from saturation. Even Ic=30mA @ 1.3Vce is not a problem for the 2n5401.
Why, you are asking?
Because I blew yet another output stage when a probe shorted the driver's C to E legs and my lab supply only goes upto +-31Vdc
2n5401 & 2sc1943.
When passing ~80% of CCS rated current it drops only 700mV.
As it approaches 100% of CCS rated current the voltage drop rises rapidly.
This is far lower than the Vdrop across a 317+resistor.
That two transistor version is what I will build up to insert into the PSU output leads, after the smoothing caps.
Ensure you have sufficient current bypassing the load to keep the 2n5401 well away from saturation. Even Ic=30mA @ 1.3Vce is not a problem for the 2n5401.
Why, you are asking?
Because I blew yet another output stage when a probe shorted the driver's C to E legs and my lab supply only goes upto +-31Vdc
accident damage limitation
It works!
The 51.5Vdc supply loses 0.5V through the protective CCS when the load is 200r (255mA)
Short out one of the 50r resistors leaving 150r and the CCS starts to limit, 45V across the load (~300mA) and 5.5V across the CCS.
Short out the whole load and the CCS drops all 51V.
The 8C/W sink, over a couple of seconds, does not even get warm.
No sparks, no overheating.
All returns instantly to normal on removing the short.
I wish I had built this up a long time ago.
Now for the negative supply, using 2n5551 & 2sc5200
It works!
The 51.5Vdc supply loses 0.5V through the protective CCS when the load is 200r (255mA)
Short out one of the 50r resistors leaving 150r and the CCS starts to limit, 45V across the load (~300mA) and 5.5V across the CCS.
Short out the whole load and the CCS drops all 51V.
The 8C/W sink, over a couple of seconds, does not even get warm.
No sparks, no overheating.
All returns instantly to normal on removing the short.
I wish I had built this up a long time ago.
Now for the negative supply, using 2n5551 & 2sc5200
I have a Variac, A three output Lab supply, and a light bulb tester.
When testing, measuring, setting up the bias and checking bias stability, none of these protect the amplifier from my stupidity.
I am not reproducing any signal at this measuring stage. The input is shorted.
The normal PSU can supply +-51Vdc or +-59Vdc.
But as has happened on quite a few occasions before, PCBs without test nodes built in require probing direct to components.
a To126 and To220 have collector right next to emitter. That dead shorts power rail direct to the emitters output
Adding A CCS (protective current limiter) between the PSU and the power amp will avoid the catastrophe.
I need low voltage drop across the inactive CCS so that the amp sees it's normal supply voltage and I need a foolproof current limit for the few seconds/deciseconds of hamfistedness. It works much the same way as the lab supply Voltage output with a Current limit but at almost double the output voltage.
Post 1 describes this.
When testing, measuring, setting up the bias and checking bias stability, none of these protect the amplifier from my stupidity.
I am not reproducing any signal at this measuring stage. The input is shorted.
The normal PSU can supply +-51Vdc or +-59Vdc.
But as has happened on quite a few occasions before, PCBs without test nodes built in require probing direct to components.
a To126 and To220 have collector right next to emitter. That dead shorts power rail direct to the emitters output
Adding A CCS (protective current limiter) between the PSU and the power amp will avoid the catastrophe.
I need low voltage drop across the inactive CCS so that the amp sees it's normal supply voltage and I need a foolproof current limit for the few seconds/deciseconds of hamfistedness. It works much the same way as the lab supply Voltage output with a Current limit but at almost double the output voltage.
Post 1 describes this.
I don't need another regulated PSU.
I simply want protection for the amplifier connected to a conventional PSU when I do something stupid that tries to discharge the PSU caps through the output devices.
I will add the two CCS limiters to my PSU and couple up an amp by the weekend and report back.
I doubt I need foldback limiting (JLH's reg+foldback does this admirably) since the usual short circuit is of short duration and in prolonged testing the pass Q heatsink did not even get warm. I reckon it could supply a full short circuit for at least a minute or so without overheating.
I simply want protection for the amplifier connected to a conventional PSU when I do something stupid that tries to discharge the PSU caps through the output devices.
I will add the two CCS limiters to my PSU and couple up an amp by the weekend and report back.
I doubt I need foldback limiting (JLH's reg+foldback does this admirably) since the usual short circuit is of short duration and in prolonged testing the pass Q heatsink did not even get warm. I reckon it could supply a full short circuit for at least a minute or so without overheating.
Is this what you have done?
An externally hosted image should be here but it was not working when we last tested it.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.