Introducing a New Member

Hi Von,

you will find the schematic and calibration procedure part way down this thread: https://www.vintage-radio.net/forum/showthread.php?t=64165

Regards,

Frank

Today John and I will be looking at fixing my power supply. I've been searching the internet for schematics and not finding anything. Anybody know of a site that might help me find schematics for a:

b&k precision dynascan 1650

Today John and I will be looking at fixing my power supply. I've been searching the internet for schematics and not finding anything. Anybody know of a site that might help me find schematics for a:

b&k precision dynascan 1650

Hi John,

That's really great to see! I hope you guys have fun experimenting, and looking forward to hearing how it goes.

Hi Shabaz,

The circuit that you posted for Von looked very interesting to me and so I did a facsimile bread board of it and ran signal and looked at the output with the scope. I say facsimile as I used two IRF9520s for the P Ch FETS as this is what I had convenient and I used 2N222 NPN general purpose transistors to substitute for your SMD transistors. Here is what it looked like:

Channel 1 Yellow is input and channel 2 Blue is the output. I ran a Square 1 kHz 5 volts PP signal into the input and this is what I got on the unloaded output:

The Vdd was set at 19 volts and you can see that the circuit performed just as you indicated it would. I switched over to a sine wave input and the output continued as a quite nice clean square wave.

Finally I decided to load the output. Some preliminary tests have indicated that real world loads may be in the range of 200 to 400 ohms so I used a 330R resistor and as would be expected the loading of the circuit through the capacitor modified the output signal to this:

Von is coming over at 2:30 tomorrow PM and I have left the circuit set up on the bench in case he wants to run any experiments on it. Thanks for your help and the fun I had tonight playing with this circuit.

John

Thanks Gene.  That is very interesting.

Thanks shabaz.  That is very interesting.

Hi Von,

Here is my attempt, it took a few more transistors that I anticipated once I examined the document a bit more. It uses a similar scheme to genebren, in fact you can reduce the design components by swapping out Q2 and Q3 to become the parts on the right side of his diagram (I used a bipolar transistor for the lower one, but the design can become smaller by using a MOSFET in one half of a package instead).

As for the rest of the circuit, Q1 is there to block reverse current when the electrode is off, in case the electrode sees power (i.e. from the other electrodes when they are energised). Q5 is used to accelerate the turning off of the two MOSFETs, because although the pulses are needed at a slow rate (5msec), the pulses are very narrow, down to less than 0.3usec according to the doc. So basically Q5 allows the gate voltage to quickly rise. Q4 drives it. R1 is there so that if the board is unplugged from the logic side circuitry, the electrode outputs are off by default.

The circuit will have a peak of perhaps 50mA, but the pulses are very narrow and only one channel is on at a time, so hopefully this is acceptable, since the average current will be much lower (less than 1mA). R5 and R6 by the way also act as a potential divider when Q3 switches on, so that the gate isn't pulled as low as 0V (there is a bit of headroom anyway, since the small MOSFETs can have up to -20V for VGS max, and I think your design needs support up to 19V).

Anyway this is just one possible approach, another could be to have a push/pull system rather than the design here. Also as mentioned the component count could be reduced by using Gene's dual MOSFET implementation for the output.

The +18V is the desired voltage for the electrode, so if you want to have it adaptable, then maybe a small patch panel type area could be used, where (say) there are three supply voltages and any of them can be wired to a channel, with jumpers, e.g. those two pin jumpers that were popular on old PC motherboards.

Hi Von,

I think that shabaz is on the money with a MOSFET solution that would provide greater currents, with little to no wasted power.  Attached is a circuit that I use in several different designs, that are High side drivers (2), built from a two dual MOSFET devices (1 N-channel and 1 P-channel).  The part are pretty inexpensive (BSD223 are $.22 in quantity of 100, and the NTJD512NT1G are $.18 in quantity of 100), yielding a driver cost of ~$0.20 per channel.

The inputs are driven by either 3.3V or 5.0V and the outputs are capable of driving 20V at ~390mA.  The outputs would need some load (not shown in this example, but >10K should work fine).

I hope this helps.

Gene

Hi Von,

Thanks for the info, it is clear now. I think your design will work, however the amount of current that can be sourced is low, because the 74LS06 pulls low, so the only control you have is through the pull-up resistors. You'd be able to source more current (and have it more efficient) by having high-side drivers, instead of low-side.

The only problem with high-side is that there are not so many options in terms of ICs. An example IC is LMD18400, however it has only 4 channels and is expensive, and also has all four channel voltage source connected together, so it's not possible to set some channels to 19V, and some to (say) 17V and so on.

Others could have solutions I may have missed, but I currently believe a discrete solution to replace the 'LS06 may be better, and it could be done with about three MOSFETs per channel, and some DIL patch panel type area for selecting what voltages you want to assign (out of say a few discrete setting like 19V, 18V and 17V) to each channel. It would be cheaper, and wouldn't be too large, because they don't need to be power MOSFETs. Surface mount could be used, e.g. SOT-23 sized which is easy to hand-solder.

If you think that's the way to go, I can sketch out the topology for a channel (it would just be repeated for all channels) and it would then connect to your 'LS154.  'LS is quite ancient, you may find it easier to obtain HC or HCT parts. Owing to the number of channels, I think surface mount is definitely the way to go (John also has experience with SMD) and the larger SMD parts could be chosen to keep it very easy to assemble.

Also if it is designed so there is separation after the '154, e.g. a separate PCB, using header pins to connect for instance, then at a later date you could swap out the controller board with a microcontroller, if (say) you wished to adapt the output pattern.