Resetting SCR in this circuit

[Ranger80]

I'm trying to reset an SCR.....I know you have to drop the holding current below a certain point, but i cant figure out how to do that. Right now im just cycling the power, but i need to figure a way out to reset it electrically.

I built a braking circuit that shorts the motor leads thru a resistor.
----------------------|----------|
| | R2 |
| Z | |
PS ---- | ------ |
-- -------\ / M/Generator
---- | \ / |
-- | | |
| R1 | |
| | | |
-----------|---------------------|
---
-
R = 10ohm, 100W Resistor
R = 10ohm, 1W
M = 48V PMDC Motor
Z = 24V Zener
PS = 24V Power Supply

What I'm trying to do is build a braking system so when the PMDC motor starts becoming a generator that the SCR shorts and brakes the motor thru the resistor(R2). The motor runs off the PS(24V), but under certain conditions the motor will get driven and become a generator. When this happens i want to short the scr and use the braking resistor. Ive got the circuit to work, BUT i need a way to reset the SCR. Im at a standstill with this project, any help would be much appreciated.

 

[OperaHouse]

Is the power supply always 24V? What you need is a power supply capable of sinking current also. Instead of using a SCR, try using a power transistor instead. Use a resistor in series with the zener to limit the base current. Not sure how effective this will be. Seems I remember this not too long age. Another question I have is what trigger method would you use to sense when to turn off the SCR?

 

[Ranger80]

The Power Supply is always 24V. I use R1 to limit the base current. That should work. Thats the part where im stuck, trigger method to shut the SCR off. I was thinking transistors, but im stuck on this subject. When the motor is not acting as a generator, i just want to run off the power supply. Maybe somehow i can use a transistor and another zener?.......any ideas

 

[VEBill]

Turning off SCRs typically involves cycling the power.

That's why SCRs are most often used in AC circuits where the 60 Hz waveform provides the periodic reset 120 times per second.

Using them in DC circuits is typically limited to protection functions (over voltage for example) where you'd expect the user to cycle the power switch to reset.

In other words, for your application, an SCR is probably not the right part to build the circuit around.

 

[IRstuff]

> you do not need an SCR, a transistor will work as well, and without the turn-off problem you're facing

> your circuit as drawn shows the SCR also pulling current from the power supply, making it ineffective as a braking circuit

> your circuit shows nothing that disconnects the PS from the motor

> you show nothing in the circuit that turns on the SCR

> your circuit runs the risk of autotriggering the SCR if the power supply ramps too quickly or if there are excessive transients in the circuit

TTFN

 

[MacGyverS2000]

Disregarding the fact that an SCR isn't the best way to achieve what you want, as mentioned above, and simply answering your question...

Place a relay in the "holding current" path... when you want to reset the circuit, trip the momentary relay.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Skogsgurra]

The standard way of turning an SCR in a DC circuit off is to use a reverse charged capacitor and an auxiliary SCR. The easiest way of doing it is to charge the capacitor with a small transformer/rectifier assembly and trigger the auxiliary SCR to switch the main SCR off.

Google "force commutation" for details.

Gunnar Englund

http://www.gke.org

 

[europus2002]

I agree with skogsgurra totally, forced commutation is the only method u would use in DC circuit. Use SCRs in circuit pattern of transistor based Bi-stable multivibrator....It should work. To avoid further complications for high inductive loads , don;t forget to use free wheeling diodes otherwise u will have random commuation problems....

 

[Ranger80]

thanks for the input so far, but im still stuck on how to add the capacitor/inductor/additional scr. Not sure where to put it, but so far i cannot get it to work.

 

[OperaHouse]

This is going to require a little more complexity than you may want to get into. I'd suggest using something like a LM431 adjustable zener driving a several power transistors in a darlington configuration. Basically this will be a big 75 watt zener set at 300mV above the supply. Is this supply regulated? It would have to be in order for this scheme to work. It would take at least a couple of transistors in prallel with a lot of heat sink. Most power will be dissipated in the transistors and not the resistor.