# overvoltage protection circuit

Discussion in 'The Projects Forum' started by unlv007, Apr 11, 2008.

1. ### unlv007 Thread Starter Active Member

Apr 5, 2008
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I have an experimental device that produces a voltage rising from 0V to 30V over 5secs and then steady at 40V and this voltage is monitored using a multimeter.
Occasionally there may be overvoltages , like a spike of 600Volts. I want to build a overvoltage protection circuit that will be placed between the multimeter and experimental device for protection. Does anybody have any idea about how to build such a circuit using diodes capacitors etc?? The overvoltages i get from my device can be a spike of 600V or even a constant smooth voltage of 600V. I need to make a voltage limiting circuit to isolate and save my multimeter from damage.

2. ### SgtWookie Expert

Jul 17, 2007
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You could use a 2 M Ohm 1/4W resistor connected to your output, with a 50V MOV on the meter side of it going to ground, or perhaps a 50V Zener; a 1N4757 is 51V, a 1N4756 is 47V. The MOV or Zener would prevent the voltage from rising above it's rating. The resistor limits the current to 0.6mA. The maximum power dissipated by the resistor would be 152mW.

3. ### unlv007 Thread Starter Active Member

Apr 5, 2008
44
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Thanks for recommending a MOV/Zener based protection circuit.
I read from wikipedia that MOV is good for transient overvoltage, how do i protect from sustained overvoltage lasting continuously?

4. ### SgtWookie Expert

Jul 17, 2007
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The Zeners I mentioned have a 1 Watt capacity. At worst case, (the 51V Zener) with your supply putting out 600v, power dissipation would be about 30.6mW, not even close to it's capability. It won't even break a sweat

You could add in a MOV of slightly higher rating (say, 55v), just in case the Zener decided to call it a day. A very small fuse, say 1/8A, between the supply output and the 2M resistor will keep things from getting too interesting if something goes wrong.

5. ### unlv007 Thread Starter Active Member

Apr 5, 2008
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Thank you very much. My experiment consists of a constant current power supply powering my experimental device by passing a const current up to 60mA and the voltage developed across the device is measured by voltmeter. Please explain how to isolate my constant current power supply from experimental device in case of overvoltage. Do i use the same concept of zeners and resistors, like a voltage regulator circuit ,as we did for protecting the multimeter. I am a chemistry major doing research in electrochemistry.

6. ### SgtWookie Expert

Jul 17, 2007
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Well, do you really need up to 600V to maintain a 60mA current?

What is the maximum voltage that you want to expose your experiment to?

You could use a comparator with a voltage divider network to turn off the supply in case the desired maximum was exceeded.

7. ### unlv007 Thread Starter Active Member

Apr 5, 2008
44
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I dont need 600V at all. My power supply produces a constant current =60mA. But the experimental device may attain a very large resistance under abnormal conditions. We know that V=IR. Thus 60mA * large resistance produces up to 600V. Under normal conditions, the experimental devices produces smaller resistance and voltage developed across it is about 50-60Volts. The compliance voltage of my power supply is 1KV so it can drive 600V but i dont want this situation.
Thus i want to design a circuit such that power supply is exposed to a maximum of 60V and is isolated if a overvoltage of 600V occurs.

8. ### unlv007 Thread Starter Active Member

Apr 5, 2008
44
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please give me some product no. / model no. etc of comparator etc( just like you gave for zener diode -IN4757) that i could use to build the circuit. I would like to study its operation and get back to you if i have any questions.

9. ### SgtWookie Expert

Jul 17, 2007
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See the attached.

On the left is just a simple model of a 60mA current supply whos' voltage output ramps up as the capacitor charges across the resistor; you can ignore it.

On the right is a voltage clamp. The three Zener diodes are arranged in series so that basically, the sum of their voltages is equal to the voltage at which you wish to start clamping.

If there is no current flowing through the Zener diodes, R1 keeps the base of Q1 low, so that Q1 does not conduct.

When the voltage from the current supply increases the voltage beyond the sum of the Zener diodes, Q1 turns on and begins conducting current. This transition is rather sudden.

Q1 will dissipate around 3.7 Watts, so it must be heatsinked.
D1-D3 will dissipate around 580mW apiece, which is why there are three in series and not merely a single Zener.

Instead of a 2N3055, use a 2N3902. I didn't have the 2N3902 in my PSPICE library, which is why the 2N3055 is shown instead. The 2N3902 has a far higher Vce rating than the 2N3055 does. As shown, the 2N3055 is being operated right at it's absolute maximum ratings, and would not last long thus being unreliable.

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10. ### unlv007 Thread Starter Active Member

Apr 5, 2008
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how did you calculate the power dissipated in the transistor= 3.7W? In what mode (active/ saturation) is the transistor when it turns on ? for active mode power dissipated= Ic Vce + Ib Vbe where Ic/Ib =beta. What beta value did you use?

11. ### SgtWookie Expert

Jul 17, 2007
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The simulation indicated slightly over 3.6 Watts dissipation between the collector/emitter and base/emitter current combined.

Ohm's Law:
P=EI
P=60 x 0.06
P=3.6 W
The small remaining power is due to the base-emitter current of around 266uA - and because the be junction raises the trip voltage by around 0.6.

It's active. 266uA is not near enough to saturate the transistor. Vb ~=593mV
Actually, I'd just used the preexisting library model for the 2N3055. Now that I look at it, the ideal Beta is listed as 360 with a roll-off of 250m, which of course is absurdly optimistic.

A 2N3902 with an Ic around .06A at room temp has an hFE around 17; this rises dramatically with an increase in temperature, so keep the heat sink small

60mA/17 ~= 3.53mA
So, power dissipation by all three Zeners combined is ~= 212mW, or under 71mW each. Forget about what I said before about using the multiple Zeners to dissipate the power. Even a single 1W 60V Zener won't break a sweat.

12. ### unlv007 Thread Starter Active Member

Apr 5, 2008
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the power dissipation calculation = EI=60* 0.06 appears to be for healthy state when voltage across transistor= 60V. but when abnormal condition causes const current power supply to produce 600 and still constant current= 60mA what is the power dissipated at transistor? that must be the power rating needed for transistor.

13. ### SgtWookie Expert

Jul 17, 2007
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No.

When the abnormal condition occurs (ie: your experiment goes awry and it's resistance dramatically increases) the Zener diodes start conducting at 60v, and when 60.59V is reached, Q1 is conducting enough that 60mA current is sunk, preventing your current supply from "running wild".

Even if you set your supply to output several times that much current, the voltage would not rise above approximately 60.7v until Q1 reached saturation.

A 2N3902 is rated for 3.5 Amperes, 400V, 100 W.

As things sit now (with the three Zeners) and the above information, you tell me what the maximum output of your current supply can be before things start getting fried, and what part will fry first?

14. ### unlv007 Thread Starter Active Member

Apr 5, 2008
44
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I will read some material from the library to brush up some electronics concepts and get back to it tomorrow.

15. ### unlv007 Thread Starter Active Member

Apr 5, 2008
44
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hi,
first of all you had asked me the maximum output of my current supply : that is 80mA .
Your zener diode with transistor circuit looks very good for a continuous overvoltage protection. But my experiment can produce continuous as well as transient overvoltages (as shown in attached files "abnormal 1" and "abnormal 2" ) . How can i provide transient voltage protection? If there is a spike for 1sec of 400V, and then voltage falls below 60V, will the zeners be able to catch up with such rapid fluctuation? you talked about MOV. How do i integrate MOV in to the circuit to provide transient protection?

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16. ### unlv007 Thread Starter Active Member

Apr 5, 2008
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should there be a separate circuit using MOV for transient protection or will the zeners also provide transient protection? please let me know on how to make transient protection as per my question in above post.

17. ### SgtWookie Expert

Jul 17, 2007
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if you wish, simply add a mov of a rating about 65v across the transistor's emitter and collector.

the response of the circuit will be fairly rapid, but may not catch extremely fast rise times. you could add a 1kv 1uf cap to slow such rise times down.

18. ### unlv007 Thread Starter Active Member

Apr 5, 2008
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If the MOV is connected in paralled to my experimental device and then the zener diode circuit is put between the MOV and power supply, will that provide transient as well as sustained overvoltage protection?

19. ### SgtWookie Expert

Jul 17, 2007
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that should do it.

20. ### unlv007 Thread Starter Active Member

Apr 5, 2008
44
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please explain how capacitor slows down rise time- the concept and
how to connect the capacitor, whether
1)the capacitor is in series with MOV and together this MOV-capacitor series network is in parallel to zener circuit
2) capacitor, mov, zener circuit are all parallel to each other.