Can an SCR handle high voltage?

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Discussion Overview

The discussion centers around the suitability of using a silicon-controlled rectifier (SCR) for switching a 12V motor drawing 3A. Participants explore the voltage ratings associated with SCRs, specifically the On State Voltage (Vtm), and the implications for circuit design, including the challenges of forced commutation and the potential need for alternative solutions like relays or MOSFETs.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant expresses concern about the maximum On State Voltage (Vtm) of 3.7V for the SCR, questioning its applicability for a 12V motor.
  • Another participant clarifies that the On State Voltage refers to the voltage drop across the SCR when it is conducting, suggesting that the effective voltage for the motor would be reduced significantly.
  • There is a discussion about the possibility of misunderstanding the voltage parameters, with some participants suggesting it might refer to the maximum gate trigger voltage instead.
  • A later reply suggests that the On State Voltage is likely an absolute maximum rating and that normal operation would see lower voltage drops.
  • One participant mentions plans to use forced commutation to turn off the SCR, which is noted as an ambitious approach.
  • Another participant proposes considering a power supply with a higher voltage to account for the voltage drop across the SCR.
  • One participant emphasizes their goal of gaining experience with controlling high current loads using semiconductor devices, expressing a preference for avoiding relays.
  • There is a suggestion that using a MOSFET might be a more appropriate solution for switching high current loads.

Areas of Agreement / Disagreement

Participants express differing opinions on the viability of using an SCR for the intended application, with some advocating for relays or MOSFETs instead. The discussion remains unresolved regarding the best approach to take for controlling the motor.

Contextual Notes

Participants note the importance of understanding the voltage ratings and the implications for circuit design, but there are unresolved questions about the specific parameters and their meanings. The discussion also highlights the challenges associated with forced commutation in SCR circuits.

durkmusic
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I'm looking on digikey for an SCR and there is a parameter that concerns me:

Voltage - On State (Vtm) (Max)

It seems the maximum voltage rating for this parameter is 3.7V. I must be misunderstanding what this parameter means. Is there somewhere I could look up what it means?

I plan to run a 12V motor pulling 3A and use the SCR as a switch instead of a big bulky relay. But if the max voltage is only 3.7V, then I must look for an alternative solution.
 
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Well that scr is not going to be much use to you.

The on state voltage is the voltage across the device when it is on ie conducting.

So if you SCR drops 3.7 volts you will only have 8.3 volts left for your motor.

further the scr will be loosing 3.7 *3 = 11 watts.

By contrast the contact resistance of a good relay is measured in milliohms so pretty well all of your 12 volts will appear across your motor.

Page 8 here

http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF
 
Last edited:
You sure it's not the maximum gate trigger voltage, Vgtm, they're referring to?
 
You sure it's not the maximum gate trigger voltage, Vgtm, they're referring to?

That's VGM in my reference.
 
Actually, your reference for gate trigger voltage is VGT

The gate dc voltage required to produce the gate trigger
current.

Edit: You're right for the max. Sorry.
 
durkmusic said:
I'm looking on digikey for an SCR and there is a parameter that concerns me:

Voltage - On State (Vtm) (Max)

It seems the maximum voltage rating for this parameter is 3.7V. I must be misunderstanding what this parameter means. Is there somewhere I could look up what it means?

I plan to run a 12V motor pulling 3A and use the SCR as a switch instead of a big bulky relay. But if the max voltage is only 3.7V, then I must look for an alternative solution.

It sounds like that is probably an absolute maximum rating at absolute maximum current. Assuming that the current rating for your SCR has been correctly chosen, then the actual on-state voltage under normal operation will probably be in the 1.0 to 1.5 volt range.

BTW. Is this a 12V DC motor? How do you plan to turn the SCR off?
 
it is a 12V motor and i planned on turning it off by forced commutation.
 
durkmusic said:
it is a 12V motor and i planned on turning it off by forced commutation.
That's getting ambitious, but would make an interesting project. :smile:

You could search for an SCR with a lower voltage drop, or maybe you could change to a power supply of say 13.5V on the assumption that you'll lose 1.5V or so across the SCR?

Is your goal here to get electronics experience, or to construct a reliable working circuit for a larger project? If the latter, then I side with those suggesting a relay as an off-the-shelf solution.
 
This is for experience. The goal is to control high current loads with a micro controller. Motors, large DC light bulbs, etc. Basically all with semiconductor devices (aka no relay). A relay is the obvious choice but I've already completed operating these loads using a micro controller and a relay. My goal is to do this without using a relay. It seems to be very tricky to do but I like challenges. I'm not getting much support on this project and I don't know why but I will continue on! :D

Here is what I have as a basic idea:
3501lk1.png


I don't know how to size the cap or r2 but r1 will be the load. the SCR I am experimenting with now is a S6020L and the transistor I am experimenting with is a MJH6287. It's a PNP. I meant to order an NPN version but I got em mixed up. I want high true but pnp gives me low true. Not really a problem just more of a slight inconvenience to the way I normally think of things. Anyways. Is my though process on the right track here or am I trying to do something impossible?

Basically a positive pulse from the microcontroller will turn the circuit on and a different pulse from a different pin of the microcontroller will cause the transistor to close and force current the opposite direction of the circuit for a second causing the scr holding current to go low enough therefore turning it off.
 
Last edited:
  • #10
That's the basic idea, and interrupting 11 mA in a resistive load should be no problem. :smile:
But what happened to the 12V 3A motor?
 
  • #11
Oh. I'm sorry about that. What I was trying to say is that R1 will be REPLACED with whatever load I want. All 12V loads ranging from 3A-10A or more. Idk yet. The goal is the 12V 3A motor. Sorry about the confusion. I guess I should have just put a motor schematic symbol instead of the load, flyback included. I just threw down a basic idea but yes that r1 will be REPLACED by a real load.
 
  • #12
Bump :(
 
  • #13
durkmusic said:
Bump :(

Just use the correct tool for the job (switching 10A @12V DC), a MOSFET.

If you're doing this to learn about SCR's and forced commutation then that's fine (though you'll need to beef up your commutation circuit to switch 10 amps), but let us know what the purpose is here.
 

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