Symmetrical triangle wave using BJT's? or?

Thread Starter

keithostertag

Joined Oct 4, 2012
55
This meter movement is 50uA, internal resistance approx 1.8K.

That's pretty high, I think most of my meter movements are lower, in the 700 ohm range, and I'd like the option of using them with this circuit as well, if that's feasible.

Keith
 

Thread Starter

keithostertag

Joined Oct 4, 2012
55
Hi Ron- thanks much!

On the schematic you show both batteries with their negatives tied to a ground symbol- is that correct?

I.e. both Vcc and Vee are +1.5v, even though you have V1 marked -1.5v. So, sorry but I am confused about that.

Keith
 

Ron H

Joined Apr 14, 2005
7,063
Hi Ron- thanks much!

On the schematic you show both batteries with their negatives tied to a ground symbol- is that correct?

I.e. both Vcc and Vee are +1.5v, even though you have V1 marked -1.5v. So, sorry but I am confused about that.

Keith
That's the same as labeling them both as +1.5V, and having one negative and one positive tied to ground.
 

Thread Starter

keithostertag

Joined Oct 4, 2012
55
OK, thanks for the clarification. You don't think the schematic is confusing? It clearly depicts both battery (V1 and V2) negatives going to ground, not "having one negative and one positive tied to ground".

I'll try putting it together tomorrow. I have two other questions:

1) what is the purpose fo the two caps on the front end, C2 and C3?

2) Can you suggest a best place to put a switch? Does this circuit require that both Vcc and Vee be switched simultaneously with a DPST type, or is there a way to use a more common SPST somewhere?

Keith
 

Ron H

Joined Apr 14, 2005
7,063
OK, thanks for the clarification. You don't think the schematic is confusing? It clearly depicts both battery (V1 and V2) negatives going to ground, not "having one negative and one positive tied to ground".

I'll try putting it together tomorrow. I have two other questions:

1) what is the purpose fo the two caps on the front end, C2 and C3?

2) Can you suggest a best place to put a switch? Does this circuit require that both Vcc and Vee be switched simultaneously with a DPST type, or is there a way to use a more common SPST somewhere?

Keith
It's confusing for a novice. It's common practice for EEs,where the battery symbol is generic for a power supply. It doesn't clearly show the negative terminal of each battery going to ground. The minus sign swaps the polarities of the terminals. Still, I would have drawn it differently if it had entered my mind. I'll try to remember that for future attachments.
Think of it as +1.5V and -1.5V power supplies.

1) The caps keep the power supply impedances low at the op amps, hopefully preventing them from oscillating. Wiring between the batteries (or, in general, power supplies) has inductance and resistance. This is undesirable. The caps bypass these parasitic impedances. They are standard practice, and should be used to bypass every IC's power supply pin on any circuit. They are also required on most circuits that don't have ICs.

2) You need a DPST between the ungrounded terminals of the two batteries and the circuit. You could use an SPST plus a transistor and a resistor.
 

Thread Starter

keithostertag

Joined Oct 4, 2012
55
Hi Ron- thanks. I had a little time this morning and put it together. Works! Yea!

A few more questions:

1) Is there a rule-of-thumb, or how to calculate the lowest effective value to use for C2 and C3?

2) What mod would I make to this circuit if I need more current output, say, 50mA? Add an additional amp stage using a transistor or beefier opamp?

3) I am surprised to see the interaction between the offset pot and the output pot. You had previously said I won't need buffers with this circuit. Would it help? Or is there something else going on?

Thanks, I am learning a lot from you.

Keith
 

Ron H

Joined Apr 14, 2005
7,063
Hi Ron- thanks. I had a little time this morning and put it together. Works! Yea!

A few more questions:

1) Is there a rule-of-thumb, or how to calculate the lowest effective value to use for C2 and C3?
For most applications, 100nF (0.1uF) works well. Are you concerned about space? If so, 10nF would probably work OK.

2) What mod would I make to this circuit if I need more current output, say, 50mA? Add an additional amp stage using a transistor or beefier opamp?
An NPN emitter follower inside the integrator feedback loop would work if the positive supply voltage were a little higher. A low-voltage, high-current op amp might be available.

3) I am surprised to see the interaction between the offset pot and the output pot. You had previously said I won't need buffers with this circuit. Would it help? Or is there something else going on?
Buffers won't help. I can't think of a way to make the controls non-interactive.

Thanks, I am learning a lot from you.

Keith
 

Thread Starter

keithostertag

Joined Oct 4, 2012
55
Hi Ron-

Another question: when the circuit is initially powered on, the output goes high for a few seconds, pegging the needle on my d'arsonval movement. Can you recommend a simple fix? Maybe a well-placed cap? Or some kind of current limiter? Or a delay circuit?

The period is about two seconds.

Thanks,
Keith
 

Ron H

Joined Apr 14, 2005
7,063
Hi Ron-

Another question: when the circuit is initially powered on, the output goes high for a few seconds, pegging the needle on my d'arsonval movement. Can you recommend a simple fix? Maybe a well-placed cap? Or some kind of current limiter? Or a delay circuit?

The period is about two seconds.

Thanks,
Keith
How are you switching the power supplies?
 

Thread Starter

keithostertag

Joined Oct 4, 2012
55
Hi Ron-

Sorry, false problem- I was having a problem with my breadboard. Using a DPST as we discussed works fine.

Thanks for getting back to me.

Keith
 
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