Protecting BJTs from overvoltage

I'm building an electronic load and I need to protect the transistors from overvoltage. This is the over-simplified version of it:

simulate this circuit – Schematic created using CircuitLab

I can't use a relay, because what if voltage changes after the relay is connected? Aside from that, I couldn't find a relay with contact voltage rating higher than 30 VDC.

I can't use any transistors because it's a 200W load with maximum current of 10A, any transistor there would be fried without a proper heatsink that I don't have. (I only have a big heatsink with fan for the main transistors.)

Can varistors help or are they too slow? How do I protect the transistors from over voltage?

Electronic load spec: 50 V / 10 A / 200W

2N3055 datasheet.

• Varistors are for shunting transient voltage spikes, not sustained current shunting. Really you protect the transistors from over-voltage by not applying over-voltage. What is the purpose of this circuit? Are you testing power supplies or what? Nov 14 '20 at 14:59
• @Transistor Yeah, it's just a "dummy load" for testing PSUs and batteries etc. So there's no way to protect the input from possible errors? Nov 14 '20 at 15:01
• What would happen if the power supply feeding the switch (called Input Voltage) were shorted out by a crowbar circuit? Would the power supply survive i.e. is the supply self-protecting even if it can generate more than 50 volts? Nov 14 '20 at 15:09
• But, what would happen like how much current could the input voltage deliver if it were shorted out? Nov 14 '20 at 15:21
• That's what I'd consider using and a fuse - choose one that uses the TL431 and, of course, do a simulation! Nov 14 '20 at 15:30

Here's a half decent-looking site for an explanation of an over-voltage crowbar protector. It has various circuits but ends up talking about the MC3423 crowbar protection IC: -

And, although I've never used this part, it appears to tick all the boxes. Alternatively use a TL431 device (commonly available) like this circuit shown in the link at the top: -

I think this might have components chosen for 5 volt over-protection so some value adjustments will need to be made for a voltage above + 40 volts. Simulation is going to be your best buddy here.

That site then moves on to describe a version that has improved di/dt protection for the triac/SCR/thyristor used. The site also gives some other links to useful pages.

Bottom line is that you will need to have a fuse in series with the 50 volt supply and the crowbar circuit works by detecting an over-voltage and shorting out the supply in microseconds hence, it relies on the fuse blowing.

• Last question; Can I use an op amp to trigger the thyristor/triac? Nov 14 '20 at 16:02
• Can I use an op amp to trigger the thyristor/triac? 1) You shouldn't be using an opamp but a comparator. 2) Why would you want to use an opamp/comparator when this circuit does the trick? 3) Don't ask: "Can I..." because when in a "silly" mood I could answer: "I can but I don't know if you can." or "Not if you don't have an opamp." A better question would be: "I would like to use an opamp to trigger the thyristor, are there any (dis)advantages in doing that?" Focus on why instead of "can I". Nov 14 '20 at 16:35
• @Bimpelrekkie I want to use a comparator because TL431 can only set the voltage up to 36V, I have the supply in the circuit that is independent of the input voltage to supply the comparator, any advice on that? also should I use triac or thyristor? thanks. Nov 14 '20 at 16:41
• You can implement a potential divider that protects the TL431 at higher voltages or use a zener diode to act as a shunt as shown in the link earlier on. Nov 14 '20 at 17:05
• should I use triac or thyristor? Look up the difference between an TRIAC and a Thyristor. Then search for circuits: "TRIAC circuit" and "Thyristor circuit" and pay attention to what these devices are used for. Hint: it has to do with AC and DC. Stop "blindly" asking "should I use ..." and "can I use ..." without doing ANY prior research. We're not here to explain you the basics that can be found easily if you just look for them. If you show you've done your (re)search you'll get much better answers. Nov 14 '20 at 21:26