Hello
I am trying to design a pulse generator circuit with the profile as shown in attachment. It consists of a 25V pulse of 180sec, followed by 35V pulse of 0.1sec and so on. Please give me the tips on how to build it.
Regards

Attached Files

Pulse Profile.doc (28.0 KB, 25 views)

 

Hi unlv007,

your sketch indicates a steady +25V level and +35V pulses.

Please clarify.

If so you could use the +25V as ground connection and apply +35V as supply voltage. The difference seen by a timer circuit (astable) would be a 10V supply voltage, well within limits of an NE555 timer chip.

Boncuk

__________________
Proper Planning Prevents Piss Poor Performance

 

Yes, i need a 35V pulse riding on a steady 25V, but i was in doubt that at the instant when the pulse hits for 0.1 sec, will my power supplies etc be in normal stage and in equilibrium- so that kirchoff voltage law will hold, (which is 25V +pulse voltage) or will there be any oscillations due to the sudden pulse?
Thanks a lot for your help
Regards
unlv007

 

You haven't given enough information.

What's the supply voltage? For example, if it's 48V then it's easy.

How much current do you require?

What is it you're trying to achieve.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

I have a supply voltage that can provide upto 60V and drive upto 10Amps of current. I am applying the pulses to trigger a chemical reaction and for all electronic purpose, we may model it as non linear resistor. The reaction usual draws <40mA current when i apply a constant voltage of 35V. Please explain how the current drawn by load will affect the state of affairs esp stability considerations when the pulse hits.
I hope that answers your question.
thanks

 

The easiest way would be to use two power supplies and a PLC but you might have problems with the 0.1s pulse - PLCs are quite slow.

A harder option would be to use two power supplies a microcontroller controlling a pair of MOSFETs.

If you don't have two power supplies then you could set the supply to 35V and use an LM317 circuit to generate the 25V.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

Hi unlv0007,

here is your circuit. I doesn't require a PLC, hence no programming. Use you adjustable power supply to supply 35V to the circuit. The circuit includes an adjustable voltage regulator LM317T which you should set to an output voltage of +25V.

Use the CMOS-version of a dual timer IC to make an astable oszillator with a duty cycle of 80%. Time out will be exactly 179.939 seconds. When timing out the astable will trigger a monostable which outputs an exact 100ms 10V pulse.

Do not connect the ground connection of the IC to circuit ground. The ground is floating on +25V. Relative to circuit ground the output, when low is +25V, your desired low level. When triggered the output swings 10V higher and again relative to circuit ground the resulting output voltage will be +35V.

Just in case the parts values are unreadable in the schematic here are the values: R1=240R, R2=82M*, R3=18M*, R4=2K7, R5=1K, R6 910K2*, P1=5K,
C1=100n, C2=1µF/35-40V, C3=2µF2/16V*, C4=10nF, C5=10nF, C6=100nF*

Values marked with an * determine timing. For C3 you might choose a tantalum capacitor.

Hans

Attached Images

PULSING.jpg (21.4 KB, 30 views)

__________________
Proper Planning Prevents Piss Poor Performance

 

Just read the data sheet of the TLC556. It can just supply 10mA output current. So you'll have to add an additional output transistor.

Hans

__________________
Proper Planning Prevents Piss Poor Performance

 

I don't see how that 555 timer circuit is supposed to work.

The output will never go below 25V and the LM317 can't sink current so the output voltage will rise and might even destroy it.

A more sensible option would be to run the 555s from 12V and use PNP transistors to switch the 25V and 35V lines.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

The output of the LM317 isn't supposed to go below 25V, but maintain a constant output voltage. Using the output as virtual ground and +35VDC input as positive supply the chip will "see" just a supply voltage of +10V, nothing to destroy the chip. Of course it will also sink current, because it has a ground connection, which is just not used to supply the timer circuit.

Why switch a line which is already there? The OP wants a constant +25V voltage level, interrupted by 100ms +35V pulses at intervals of 180 seconds.

Hans

__________________
Proper Planning Prevents Piss Poor Performance

 

The LM317 can't sink current because the output stage is a darlington which can only source current. Some current will be sunk through R1 but it will be subtracted from the minimum load current required by the LM317.

When the output of the 7555 is low it will not be able to source current. Look at the internal schematic of the 7555 it has a complementary pair on the output stage. When the output is lo QH will be off and QL will be on. QL is an N channel MOSFET and won't work when the source is more positive than the drain, in this case it's on a CMOS IC which might go into self distruct mode if you try to use it in this manner.

The circuit probably worked on your similator because it doesn't accurately model the internal components of the ICs. This is why it's important to actually understand what is going on inside those little black boxes when you design a circuit.

Sorry I thought he wanted something different, I've only just read the attachment.

Attached Images

	LM317 internal.GIF (5.6 KB, 9 views)
	7555 Schematic.GIF (12.4 KB, 12 views)

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

I guess the TLC556 doesn't care if it is supplied with +35V for VDD and +25V for ground. It will "see" just +10V at its VDD pin.

I've seen it with MCUs where VCC is connected to ground and VSS connected to -5V. The MCU "sees" a positive supply voltage of 5V.

To have a minimum load at the LM317s output a 1K2 resistor might be connected to ground.

__________________
Proper Planning Prevents Piss Poor Performance

 

I'm not disputing that. The 7555 doesn't care if the negative rail is 25V and the positive rail is 35V, it will still see a potential difference of 10V.

What I am telling you is that the 7555 will not drive a load connected to a lower voltage than 0V or a higher voltage than Vdd. Trying to drive loads connected to voltages outside the power supply voltage range is a recipe for disaster and will destroy the 7555.

In this circuit, you can either connect the load from pin 3 to 25V or 35V, anything outside this voltage range will cause it to smoke.

The proper way of doing this is to use transistors to control the external loads connected to voltages outside the power supply range.

That would work but it would be easier to just reduce the value of R1 to something like 62Ω instead of 120Ω.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

Well, may be you're right. I'll try the circuit and will see what happens.

Just thought of an easy and cheap way to just switch from 25 to 35V for lousy 100ms.

Hans

__________________
Proper Planning Prevents Piss Poor Performance

 

Before you try, I strongly recommend looking at the internal schematic of the 7555 with a load connected from pin 3 to a lower voltage than VSS.

I don't want you to waste a 555 unless of course you're got 1000s of them at your disposal.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez