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Shoog 28th February 2006 10:59 AM

my SY CCS is only giving me 15mA
I have built a version of SY's two transistor CCS. It seems to work. However as I adjust the top pot I only seems able to get it to deliver a maxumum of about 15mA (testing the voltage drop across the pot. I need 22mA for my 5687
I have two red LEDS as the voltage reference to a 2SA722. The second transistor is a 2SA1104 with a 2K2 reference resistor. The final resistor in the chain is 47K.
Is this the limit of this circuit or have I done something wrong.


SY 28th February 2006 11:22 AM

Can you sketch out a schematic with parts values? You should have no trouble adjusting it to currents high enough to fry the transistors unless you've inadvertently put in a limitation somewhere...

Shoog 28th February 2006 11:32 AM

I have just looked at the circuit again and done some measurements, The voltage drop across the 47K resistor suggests that there is 2.7mA current going through it. The voltage drop across the 2K2 resistor suggests there is only 0.6mA going through it(?). The voltage drop across the two LEDS is only 1.6V.
It all points to the possability that there isn't enough current through the reference chain to bias it up correctly. I have a +B of 158V which would indicate that the current through the reference should be about 3mA which is just about right. I think I will have to drop the 47K resistor to 22K. Will this effect the final anode voltage.

If you think its something else I will sketch up the circuit for closer inspection - though it is effectively the same as the one draw by yourself elsewhere with different transistors and the 2K replaced by a 2K2.

All help is appreciated.


SY 28th February 2006 11:36 AM

It was unclear to me whether this is an NPN or PNP load. It still is. And parts values chosen is important. Yeah, sketch it up and we'll whack away at it.

Shoog 28th February 2006 04:54 PM

I haven't had a chance to sketch the circuit yet, but probing points to the first transistors having blown. I changed the 47K resistor for a 22K with no improvement. The 22k resistor is passing 6mA but this doesn't appear to be passing through either the LEDs or the 2K2 resistor, which are only passing 0.6mA.. I can't picture which way the current flows into the junction of a NPN. The main pass transistor is still OK.

Will do that sketch.


SY 28th February 2006 05:22 PM

A bad transistor was the first thing that occured to me. With decent high hfe units, you can assume that current flowing into the base is very, very low. Make sure you're operating within the HV transistor's SOA- the delicate low voltage one is rarely a problem unless the Big Boy pops.

If the LED isn't lit, you almost certainly have some bad silicon in there.

Shoog 28th February 2006 09:15 PM

Problem solved. I missinterpreted the datasheet for the 2SA733 and inserted it the wrong way round. Dead transistor, but I had a few spare.
LED's now light but voltage drop across the CCS is to low at just 10V. I will have to rejig the power supply to give me higher raw +B. Still I will have to run the 5687 at a lower plate voltage and current to make this work.
What the PSRR like with this CCS, i'am just thinking - what ripple can I tolerate on the +B.


SY 28th February 2006 09:24 PM

Are you using this as the tail of a diff amp?

Shoog 28th February 2006 09:53 PM

No i'am just making a simple 5687 preamp. Unfortunately I am using up a preamp power transformer that only supplies 200-0-200v.


SY 28th February 2006 10:53 PM

Is this a plate load? NPN or PNP? Anything? I'd like to help customize this, but I really kinda need to know what you're trying to do.

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