Not a perfect switch

Thread Starter

Martino Chiro

Joined May 1, 2015
102
A load absorbs 250mA at 12V and i need to generate voltage sags of 800ms on the 12V power supply.
With a NE555 as a monostable, i get a positive pulse of 800ms.
This pulse is the input of a NPN BC639 that reverse its polarity on its collector, connected to the 12V with a 220 Ogn resistor.
The pulse is then applied to the base of a BU810 (NPN).
The 250mA load is connected between +12V and the BU810 collector, used as a switch.
The emitters are connected to 0V (GND).
I expected to have a good switching on the collector of the BU810, from saturation to interdiction,
but with a scope on the collector i see something different.
As the picture shows, the ON OFF transition is very smoothing: something is not working well
but i don't understand what.
The load is an electronic board with some electrolytic capacitors on the +12V power supply (120uF).
It could be schematized as a resistor with a capacitor in parallel.
 

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BR-549

Joined Sep 22, 2013
4,938
Ok.....what do you think? Does that look like a good switch to you? Draw us out a schematic of your circuit. And while your drawing that out.........have you any ideas on what might be going on here?

You mentioned the load was like an R/C circuit. What property does a R/C network have, that a resistor doesn't have?

Does the delay in voltage cause problems for your pcb board, or system?
 

Thread Starter

Martino Chiro

Joined May 1, 2015
102
Ok.....what do you think? Does that look like a good switch to you? Draw us out a schematic of your circuit. And while your drawing that out.........have you any ideas on what might be going on here?

You mentioned the load was like an R/C circuit. What property does a R/C network have, that a resistor doesn't have?

Does the delay in voltage cause problems for your pcb board, or system?
upload_2017-5-3_8-2-47.png
 

Thread Starter

Martino Chiro

Joined May 1, 2015
102
Hello BR-549,
is my schematic in post #6 clear enough ?
Yes, when in place of the "RC" load i put a resistor (50Ohm 10W) the scope shows a clear and sharp pulse, as shown in my
previous picture in post #4.
Reading again your answer in post #5 I think I understand, now. Yes, with a little delay.
 

BR-549

Joined Sep 22, 2013
4,938
I am comparing your circuit description in post #1 to the schematic.
What do you mean when you say "voltage sags" ?
How often do you want the voltage sags? Why do you want the voltage sags?
What is on the PCB?
Tell us exactly are you doing.
 

Thread Starter

Martino Chiro

Joined May 1, 2015
102
I want simulate a loss of power supply (voltage sags are interruptions are of power)
on an electronic board (the LOAD) as shown here (ISO 7637-2 http://www.linear.com/solutions/7719).
Now i can generate a loss of power of T2 duration but i don't know how to generate the negative spikes
shown here (-100V of amplitude).

upload_2017-5-4_21-4-26.png
 

BR-549

Joined Sep 22, 2013
4,938
ok....Martino......thanks. I wasn't familiar with the voltage sag term. And I have no experience in simulations or power supply transients for that matter........except on the receiving end.

But there are many sim operators here. Good luck with your project.
 

#12

Joined Nov 30, 2010
18,210
how your circuit can generate the negative spikes shown in my post #9 ?
Do you want 100 volt negative spikes?
Forget about my circuit.

I did not suspect that automotive components would need to survive reversed power polarity at 100 volts.

Signed: Number Twelve
 
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