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Constant current sources (CCSs) provide isolation between the power supply and a circuit, and provide equivalent resistance much higher that practically available with passive parts. Here is a simple one:

I have a printed circuit board (PCB) for four simple cascode CCSs, each using a pair of N-channel depletion-mode MOSFETs. For convenience, the board can be cut in half to provide two pairs of CCSs. The PCB is 1" x 4", and is mounted with two 6-32 screws spaced 2" apart. The MOSFETs with the highest dissipation are located at the edge of the board to make it easy to share a heat sink (NOTE: the TO-220 tab is connected to the input high voltage, so take care around the heat sink and insulate it from the tabs.) The input and output wire pads are sized for 18 AWG wire.

Let me know if you want to buy a PCB or two!

The current adjust resistors (U1 - U4) can be replaced with fixed resistors once the desired current output is determined. An easy way to set the current is to connect a 1K resistor from the output (O1 - O4) to ground, and a power source from the input (IN1, IN2) to ground. Apply at least 20V DC to the CCS and adjust the appropriate current-adjust resistor to the desired current output. For example, to adjust for 30 mA, there ill be 30V across the load resistor and at least 20V across the CCS, so a minimum of 50V DC is required from the power source. Other load-resistor values can be used.

PARTS:
(4) U1-U4: Trimmer Resistor, 12-turn 1K sealed top adjust: Mouser 81-PV37W102C01B00, $1.25 ea, $5.00 for 4
(8) R2-R9: Stakpole Carbon Film 1/2W 2% 1K: Handmade 8-06051001, $0.10 ea, $0.80 for 8
(4) M1, M3, M5, M7: SuperTex DN2540 MOSFET 400V, 25 Ohm, TO-220: Mouser 689-DN2540N5-G, $1.60 ea, $6.40 for 4
(4) M2, M4, M6, M8: SuperTex DN2540 MOSFET 400V, 25 Ohm, TO-92: Mouser 689-DN2540N3-G, $0.87 ea, $3.48 for 4
(1) PCB: $5.00 ea
TOTAL: $20.68 for 4 CCSs.

ESTIMATED CURRENT-ADJUST RESISTORS:

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- Guy

Save some for me!! I'll need at least four quad boards!!!

Slick

High Trans, you know they're yours!

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- Guy

Hi Guy. I'm interested in buying some PCB's.

Scott

I am interested in some too.

Guy,

I also would be interested in a few boards. It sure beats me wiring them up on perfboard like I have been doing for years.

David

I am also interested in a few boards. Thanks.

Anyone interested in buying a PCB should drop me a private mail (PM) letting me know how many PCBs and an address for mailing them.

Once we get a few of them out, we'll get feedback to see if they help!

Thanks!

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- Guy

I forgot to post a schematic of the PCB, so I'll rectify that in this post. Here is the schematic:



Remember that you can bend the DMOS leads up or down to adapt the board to your heat-sink mounting requirements.


(Wow, those pictures came out really big!)

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- Guy

Guy, why did you go with a 1k gate resistor? A lower value should make the circuit work better at high frequencies because of the input capacitance of the DN2540.