You say you want the signal across R9, which is an usual way to use the signal from such a circuit since that requires some sort of differential circuit to detect the signal.
Normally the signal is taken from the transistor collector to ground.
How are you going to use the signal across R9? Where is it going?

What do you want the output signal to do with a -15V input?

 

Mike, could you post the .asc file for the second circuit? I'd like to see how you created a variable load at the output. Thanks!

 

As I tried to point out in post #16, this circuit has the ability to sink current, up to β*V(in)/10K.
However, it sucks at sourcing current, due to the ability of the 10K pull-up resistor to source 30/10K, only 3mA!

 

You say you want the signal across R9, which is an usual way to use the signal from such a circuit since that requires some sort of differential circuit to detect the signal.
Normally the signal is taken from the transistor collector to ground.
How are you going to use the signal across R9? Where is it going?

What do you want the output signal to do with a -15V input?

ok. this is why i'm asking. if there is a better way please show me.

disregard the -15V. we said we aren't going to do that.

i'm trying to use a transistor to switch the 30V on and off. that is it. this 30V on and off will go to turn a relay on and off.

the control signal will be anywhere from 0-15V

 

Is this signal going to directly control the relay?
If so then the relay coil would replace the collector load resistor R9 and you don't need R9.

It's always easier if you tell us the whole story of what you are trying to do so we can avoid a game of 20 questions.

 

Is this signal going to directly control the relay?
If so then the relay coil would replace the collector load resistor R9 and you don't need R9.

It's always easier if you tell us the whole story of what you are trying to do so we can avoid a game of 20 questions.

ok. sorry about that.

yes direct control.

i found these examples:

 

i'm trying to use a transistor to switch the 30V on and off. ...

That might be the source of some of your confusion. Technically, your circuit switches the ground connection on and off, not the 30 V. The 30 V is connected to the load at all times. Probably nothing wrong with that, and with an NPN switching transistor that's the standard way of doing it. You are switching the return or the ground for the load power, not the source of the load power.

ak

 

That might be the source of some of your confusion. Technically, your circuit switches the ground connection on and off, not the 30 V. The 30 V is connected to the load at all times. Probably nothing wrong with that, and with an NPN switching transistor that's the standard way of doing it. You are switching the return or the ground for the load power, not the source of the load power.

ak

sorry to throw a monkey wrench in this, but could i swap out this transistor for an optocoupler? anyone know a decent one that would work with my original signal specs?

thanks for your patience. i'm new.

 

Opto-couplers generally have a low current rating but you could use one to drive the transistor in your circuit.
You could alternately use a DC type solid-state relay which would have a greater current rating but they are usually more expensive than an opto-coupler transistor combination.

 

Optomos is either a high current optocoupler or a low current solid state relay. 10 to 20 mA input LED current, 1 A output current switching capability. Clare LBA and LCA series. More convenient and more money than Wally's suggestion in #29.

ak

 

I'm checking out all these solid state relays. I really like them with their isolation which seems really needed, because the control is from synthesizers, and I don't want any risk of damage.

The circuits for these SSR's seem pretty straight forward, except I need a little help with the control values and control circuit setup.

Since my control will come from different synthesizers at different times depending on what I hook up. Sometimes +15V DC, sometimes +5V DC, sometimes +12V DC...etc. Is there anyway to scale these volts down to something uniform every time no matter what the input is? These spec sheets seem to want very specific volts and amps to control the SSR. Also a negative volt protect would be good too.

Ideas?

 

Which relays are you considering?

 

If you are using a mechanical relay, then you do not need opto-isolation; the relay provides it...

 

Which relays are you considering?

maybe something like this :

http://www.clare.com/home/pdfs.nsf/www/LCB710.pdf/$file/LCB710.pdf

or

http://www.irf.com/product-info/datasheets/data/pvdz172.pdf

really anything that is isolated and the load voltage around 60V and load current around 1 amp. these are over what i need, but a safety margin is good.

 

oh...maybe a schmitt trigger? never used one, but would that take all my different possible control voltages and convert them to a standard control signal for the SSR?

 

SSR's normally have a wide range of input operating voltage.

 

SSR's normally have a wide range of input operating voltage.

yaa, i see those. they are a lot more expensive and so much larger. i'd probably be better off conditioning my control signal to something small and standard, i would think.

 

oh...maybe a schmitt trigger? never used one, but would that take all my different possible control voltages and convert them to a standard control signal for the SSR?

A Schmitt trigger is an extension of a voltage comparator. If you make a table listing all of the relay-off control voltage ranges and all of the relay-on control voltage ranges, hopefully there is a voltage band that is in the middle and not a part of either column. The middle of that band is a good starting point for an input conditioning circuit.

Another way to go is a wide range constant current circuit that limits the SSR LED current to a maximum safe value no matter what the input is. Some SSRs have this built in, and do not require a current limiting resistor or other signal conditioning.

ak