| Electronics > Beginners |
| DIY 0-30V 0-3A ...Again |
| << < (13/13) |
| Jwillis:
I've only done in depth tests and measurements on a modified circuit using the OP445.Of course all the transistors are of higher ratings than the original circuit. As far as I can tell, when messing around with the circuit , Q1/T2 is a current dump for when the circuit is turned off. Without it, the output voltage shoots up to the charged voltage in the filter capacitors then slowly drains off . Even when voltage and current is set to 0V before shut down.In my case the out put would shoot up to 40 volts. On tests I noticed that when power is on, the base ofQ1/T2 is at a negative volltage equal to the negative rail.Around -25 volts. When power is switched off the base goes positive and dumps any voltage at the output to ground.But the filter capacitors remain charged for a period of time until they to drain off. I did put a similar circuit to also discharge the capacitors and it worked pretty much the same way, but later removed it to simplify things for further testing and design.The breadboard was getting crowded. |
| xavier60:
--- Quote from: Jwillis on November 29, 2019, 08:27:31 am --- I've only done in depth tests and measurements on a modified circuit using the OP445.Of course all the transistors are of higher ratings than the original circuit. As far as I can tell, when messing around with the circuit , Q1/T2 is a current dump for when the circuit is turned off. Without it, the output voltage shoots up to the charged voltage in the filter capacitors then slowly drains off . Even when voltage and current is set to 0V before shut down.In my case the out put would shoot up to 40 volts. On tests I noticed that when power is on, the base ofQ1/T2 is at a negative volltage equal to the negative rail.Around -25 volts. When power is switched off the base goes positive and dumps any voltage at the output to ground.But the filter capacitors remain charged for a period of time until they to drain off. I did put a similar circuit to also discharge the capacitors and it worked pretty much the same way, but later removed it to simplify things for further testing and design.The breadboard was getting crowded. --- End quote --- Have you measured the size of the current spike when the output is short circuited? |
| Jwillis:
On the original or modified version? The original I have not . I could set it up later and test it. but I can't imagine it being much higher than the 3 amps I was getting on load test. On the OP445 version it has a maximum current output of well over 20 amps shorted .But I only tested that once because it set the boards on fire and could not get an accurate measurement . :-DD .Right now its limited to just 10 amps until I can modify the bread board setup with heavier rails for current. |
| xavier60:
The concern is what the current spikes to in the first few hundred microseconds. Because output BJTs are usually used at a fraction of their maximum ratings, it might seem that the BJTs are safe even if the spike is very large. Maybe they will never fail. What bothers me is not being able to figure out how much SOA derating to allow for at high temperature. |
| Navigation |
| Message Index |
| Previous page |