Help, my fuse doesn't work

CDRIVE

Joined Jul 1, 2008
2,219
Darkstar, I thought you might like this. It's a plot of Ron's circuit. I didn't have the FET that Ron used so I had to substitute.

Ron, please elaborate on the purpose of Q3, it's eluding me.
 

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Ron H

Joined Apr 14, 2005
7,063
Ron, please elaborate on the purpose of Q3, it's eluding me.
Q3 limits the output current when reset is pressed while the output is overloaded. This will limit the current to about 2.5A when Rs=0.27Ω. This can still cause damage to the FET if the switch is depressed for very long. Without Q3, in the case of a short across the load, the current can destroy the FET and/or Rs. In the case of only minimal overloads, you will be better off without Q3.

See attachments. Note that I changed the value of R5 to 100Ω. It was 10k.
 

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CDRIVE

Joined Jul 1, 2008
2,219
Q3 limits the output current when reset is pressed while
the output is overloaded.
Ron, thanks for clearing that up. Your circuit has a very high clever quotient. ;)

Prior to you posting this circuit I was working up something that was based on a discrete SCR also. Unfortunately I didn't have the foresight to gate it from the PNP, as you did. My second failing was not considering using PMOS as you did.

Chris
 

Ron H

Joined Apr 14, 2005
7,063
Here's a version that doesn't run the risk of damage if the reset is activated during overload. The reset is differentiated before being applied to the latch, so the "fuse" will re-trip immediately if still overloaded. The re-trip time is a function of the filter capacitor, which can be added to eliminate nuisance trips due to transient load currents. With Cfilter=0, the re-trip time should be less than 10usec. With Cfilter=1uF, the simulated re-trip time is about 5 milliseconds, still short enough to avoid needing a heat sink (I think).
 

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CDRIVE

Joined Jul 1, 2008
2,219
Hey Ron, it just hit me that your circuit would make a nifty stalled motor detector. Kind of handy when a limit switch doesn't do what it's supposed to!;)
 

Ron H

Joined Apr 14, 2005
7,063
Hey Ron, it just hit me that your circuit would make a nifty stalled motor detector. Kind of handy when a limit switch doesn't do what it's supposed to!;)
Yeah, detector and protector, all in one! The relatively large value sense resistor might be a problem, though, as it will tend to compromise speed regulation and torque.
 

castley

Joined Jul 17, 2011
31
Try using a pnp power transistor connected with the collector to the anode of the scr the emitter connected to the supply and rs connected between the base and emitter of the pnp.
 

CDRIVE

Joined Jul 1, 2008
2,219
Just a suggestion but it's usually good practice to read all the way through a thread before machine gunning out suggestions. :rolleyes:

BTW, your PNP hookup description is wrong.
 
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