# Protecting output of OpAmp for a Zapper circuit

#### Trailer

Joined Jan 22, 2023
57
I have the attached setup for a zapper circuit.
Input signal is amplified through 741 OpAmp. Output is expected to feed a zapper circuit for Lab tests.
It's supposed to be running continuously for at least 2 hours with extensive shorting on the zap-net without a problem.
My question is:
- Can it be connected directly to the zapper net without damaging the OpAmp or it's feeding circuit or I must include some protection at the output?
Note: required output current no more than 200mA

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#### dl324

Joined Mar 30, 2015
16,899
- Can it be connected directly to the zapper net without damaging the OpAmp or it's feeding circuit or I must include some protection at the output?
Note: required output current no more than 200mA
The circuit won't work. 741, and most opamps, can only sink/source around 25mA. But it will tolerate indefinite shorts because it has current limiting.

What is the frequency of the square wave? Is it 0-10V or -5 to 5V?

#### Audioguru again

Joined Oct 21, 2019
6,688
With your +16V and -16V supply, the output of an ancient (54 years old) 741 opamp can go only as high as about +14V.
Then with the voltage gain of 3 times its output will be clipping. It cannot produce frequencies at full output voltage above only 9kHz.

The typical maximum output currentfrom a 741 opamp is only 25mA so when shorted it will get hot but it will not burn out.

Zappers usually produce thousands of volts but yours doesn't.

#### ScottWang

Joined Aug 23, 2012
7,398
If the voltage level of the square wave is 0-10V and the output voltage is 30V then maybe you can use an NMOSFET to drive the Zapper circuit, MOSFET provides a low Rds, so it is easier to solve the problem of heat.

If the voltage is only 30V, does the Zapper circuit will works OK?

I have some Electric mosquito swatters, they provide about 3KV~4KV output voltages.

#### Trailer

Joined Jan 22, 2023
57
The circuit won't work. 741, and most opamps, can only sink/source around 25mA. But it will tolerate indefinite shorts because it has current limiting.

What is the frequency of the square wave? Is it 0-10V or -5 to 5V?
It's not a typical zapper (like for mosquito) it's kind of research application so we have to first try with that low current.
The input is -5/+5V variable frequency from around 70 to 1400 Hz.
We just need to know if the OpAmp can withstand such excessive sorting at the output or need protection

#### Trailer

Joined Jan 22, 2023
57
It's not a typical zapper (like for mosquito) it's kind of research application so we have to first try with that low current.
The input is -5/+5V variable frequency from around 70 to 1400 Hz.
We just need to know if the OpAmp can withstand such excessive sorting at the output or need protection
Shorting *

#### dl324

Joined Mar 30, 2015
16,899
The input is -5/+5V variable frequency from around 70 to 1400 Hz.
We just need to know if the OpAmp can withstand such excessive sorting at the output or need protection
A 741, and most opamps, are poor choices for squarewave inputs. The slew rate on 741 is probably 0.5V/us. A comparator would be a better choice. Something like LM393. You can put a CMOS inverter on the output to get a rail-to-rail at higher current.

What is the frequency and duration of shorts?

#### Trailer

Joined Jan 22, 2023
57
With your +16V and -16V supply, the output of an ancient (54 years old) 741 opamp can go only as high as about +14V.
Then with the voltage gain of 3 times its output will be clipping. It cannot produce frequencies at full output voltage above only 9kHz.

The typical maximum output currentfrom a 741 opamp is only 25mA so when shorted it will get hot but it will not burn out.

Zappers usually produce thousands of volts but yours doesn't.
We don't need k's of volts as the aim is not to kill the test subjects.. 30v up to 50v is the max output required

#### dl324

Joined Mar 30, 2015
16,899
We don't need k's of volts as the aim is not to kill the test subjects.. 30v up to 50v is the max output required
You didn't mention human test subjects before. I think 50V is considered a lethal voltage.

#### Trailer

Joined Jan 22, 2023
57
A 741, and most opamps, are poor choices for squarewave inputs. The slew rate on 741 is probably 0.5V/us. A comparator would be a better choice. Something like LM393. You can put a CMOS inverter to get a rail-to-rail output.

What is the frequency and duration of shorts?
Appreciate your proposal for the LM393.. grateful if you post a circuit sketch.
As for the frequency and duration of shorts.. that's impossible to calculate.. my best guess is at least 100 shorts per second maybe more.. it's almost continuous

#### Trailer

Joined Jan 22, 2023
57
You didn't mention human test subjects before. I think 50V is considered a lethal voltage.
Oh no... no humans involved lol.. only insects

#### Trailer

Joined Jan 22, 2023
57
Oh no... no humans involved lol.. only insects
If the voltage level of the square wave is 0-10V and the output voltage is 30V then maybe you can use an NMOSFET to drive the Zapper circuit, MOSFET provides a low Rds, so it is easier to solve the problem of heat.

If the voltage is only 30V, does the Zapper circuit will works OK?

I have some Electric mosquito swatters, they provide about 3KV~4KV output voltages.
Yes 30V works fine.. actually this is the max.. the source have an adjustable voltage so the output is 12 to 30V.
I also looked on the mosquito swatters for ideas but I don't think can be applied here as is unless it's tweaked to suite my application

#### Trailer

Joined Jan 22, 2023
57
It's not a typical zapper (like for mosquito) it's kind of research application so we have to first try with that low current.
The input is -5/+5V variable frequency from around 70 to 1400 Hz.
We just need to know if the OpAmp can withstand such excessive sorting at the output or need protection
Correction.. input is 0-10V not -5/+5

#### ScottWang

Joined Aug 23, 2012
7,398

Yes 30V works fine.. actually this is the max.. the source have an adjustable voltage so the output is 12 to 30V.
I also looked on the mosquito swatters for ideas but I don't think can be applied here as is unless it's tweaked to suit my application
So you can find the N-type MOSFET and Its Spec as Vds>40V<80V, Rds<20mΩ, Ids>1A, G-Input, S-GND, D-Voutput(Current sink).

Here are some P and N-type MOSFETs.

#### dl324

Joined Mar 30, 2015
16,899
Appreciate your proposal for the LM393.. grateful if you post a circuit sketch.
As for the frequency and duration of shorts.. that's impossible to calculate.. my best guess is at least 100 shorts per second maybe more.. it's almost continuous
Do you want full voltage under short conditions? An opamp wouldn't have done this.

The input is a 0-10V square wave and you want the output to be 1.5 3 times that?

The output only needs to source current?

Last edited:

#### AnalogKid

Joined Aug 1, 2013
11,030
To answer the original question -

The LM741 is rated for continuous operation into a short circuit. As above, the typical short circuit current is around 25 mA. For output currents greater than this, the internal protection will kick in and decrease the output voltage.

If you really want 200 mA, there are relatively simple discrete circuits that will do this.

Separate from that, do you need an output waveform that is symmetrical about GND? That is, an output that swings from a positive voltage to a negative voltage wrt GND? If not, and a unipolar output would work, such as an output that goes from 0 V to 30 V (instead of -15 V to +15 V), there are simple non-opamp circuits for that.

ak

#### LowQCab

Joined Nov 6, 2012
4,055
The problem with this Thread, or Project, is that
we have no idea of what the end result is supposed to be.

A detailed drawing of the "Zapper" being driven would clear-up a lot of questions.
The Schematic provided is showing no DC connection,
therefore, there can be no Current flow until the Air in between the gaps becomes Ionized
and creates a Spark, which isn't going to happen without very High-Voltages.

The legs of Bugs do not conduct Electricity very well at all.
.
.
.

#### Audioguru again

Joined Oct 21, 2019
6,688
With your +16V and -16V supply, a 741 opamp cannot produce an output of +30V.

Its maximum total supply is 36V then if you have no load current and use a +32V supply and a -3V negative supply then its maximum output can be 0V to +30V, but it will burn out if its output is shorted to 0V.

#### Trailer

Joined Jan 22, 2023
57
Do you want full voltage under short conditions? An opamp wouldn't have done this.

The input is a 0-10V square wave and you want the output to be 1.5 3 times that?

The output only needs to source current?
Yes.. full voltage required under short conditions.. so if the OpAmp can't do it, any other suggestions to boost the output? If possible to boost to 40V would be even better.
As for output.. I believe sourcing current is enough.. see its still experimental.. what I know is they're gonna zap different kinds of bugs at various voltages and different frequencies and monitor reactions and mortality rates. I can't be sure how much current is required, the main thing is to make sure the circuit won't burn or get damaged. If the out current have no effect then we'll need to find a way to amplify it or come up with different design.

#### Trailer

Joined Jan 22, 2023
57
The problem with this Thread, or Project, is that
we have no idea of what the end result is supposed to be.

A detailed drawing of the "Zapper" being driven would clear-up a lot of questions.
The Schematic provided is showing no DC connection,
therefore, there can be no Current flow until the Air in between the gaps becomes Ionized
and creates a Spark, which isn't going to happen without very High-Voltages.

The legs of Bugs do not conduct Electricity very well at all.
.
.
.
I saw their zapping net, it consists of parallel thin copper wires (about 0.7 mm) spaced about 4 or 5mm apart.
Conductivity of bugs legs... who knows.. I can't tell