Constant current source at high voltages - Page 1
Forum section: electronics / projects

author: Kush1999
author_rank: Newbie
timestamp: April 04, 2023, 06:16:07 am
content: Hello everyone,I trying to figure out constant current source using a single IC (not using op amps and FETs). My requirement is voltage up to 75V and current up to at least 100ma. Current should be settable using uC either by digiPOT or DAC. I have came across some solutions, please let me know if you have better ideas.LT3092 - 200mA 2-Terminal Programmable Current Source. (It supports voltages up to 40V)LM3402HV - 0.5-A Constant Current Buck Regulator for Driving High Power LEDs (This works up to 75V, however I am not sure how to use this as a constant current source. Also, not sure how to set the current)PSSI2021SAY - Constant current source 50ma. (This is the most suitable for my purpose, it is small, cheap and I can parallel two of them to get current up to 100ma)Other standard ICs like LM134, do not meet either current or voltage. I am not comfortable using voltage regulator as current regulator because voltage regulators at such high voltages are very expensive.Can you suggest some other ICs I should explore? Please help me as I am stuck on this for long.Thank youPS: I am working on an electrical stimulator. My Load is human skin and I am using DC-DC converter to get 75V. I am using an H bridge to switch the direction of pulses.

author: daqq
author_rank: Super Contributor
timestamp: April 04, 2023, 06:49:45 am
content: A constant current source is really simple with an op amp and a transistor - if nothing else you get to avoid the problem of dissipating 7.5W on a single IC (75V @ 100mA into short worst case scenario). See:https://www.ti.com/lit/an/snla140d/snla140d.pdf?ts=1680551702889&ref_url=https%253A%252F%252Fwww.google.com%252F, page 21 and 22.That said, 100mA into a human? Doesn't that, you know, kill people?

author: Kush1999
author_rank: Newbie
timestamp: April 04, 2023, 07:32:58 am
content: Hi daqq,thanks for your replyI went through the shared document. The only problem I see is that I may not be able to find an op amp that works up to 75V. The voltage to current converter looks promising. I am not sure if the op amp supply needs to be same as load supply (36V each in this case).I am not trying to kill somebody, I am injecting currents using metal electrodes that have pretty high impedance. Hence, I need high current. 100ma would be maximum, realistically, it would be around 40ma to 60ma. Enough research has been published to support my claim. It is safe.

author: daqq
author_rank: Super Contributor
timestamp: April 04, 2023, 09:16:30 am
content: When properly designed and selected, for Figure 2-14. you do not need an op amp that goes anywhere near the target voltage, a 5V powered op amp will easily do the trick. Try playing around with it in a simulation. Also, don't double post.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 04, 2023, 10:10:18 pm
content: Quote from: Kush1999 on April 04, 2023, 07:32:58 amI still don't get it. How 'high impedance' and 'high current' match?

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2023, 10:13:30 pm
content: 75V 100mA should be easily achievable by many different LED drivers. Have a look at ICs designed for LED light bulbs. There are both switching and linear varieties.

author: jonpaul
author_rank: Super Contributor
timestamp: April 04, 2023, 10:27:36 pm
content: Very dangerous project, skin to electrode R can be very low.Potential for heart pacemaker disruption.Get liability insurance]J

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2023, 10:35:43 pm
content: "High impedance", "high current"?I think revisiting Ohm's Law would be appropriate.But trying to inject 40...60 mA into the human body does not sound healthy to me.

author: Kush1999
author_rank: Newbie
timestamp: April 05, 2023, 03:49:20 am
content: james_s thank you, I would definitely explore LED drivers. It is more of a subject to try.To everyone who has certain reservations on what I am doing, I am an entrepreneur developing a FES device.Please see this paper. Page no. 11 Appendix A. It is a review of specifications of all the devices available.https://doi.org/10.1016/j.clinbiomech.2014.11.007Yes, this device can’t be used on heart patients, patients with pacemaker, epileptic patients, patients with metal implants and pregnant women. Rest should be fine. It will go under thorough clinical trial before being certified.Thank you

author: BravoV
author_rank: Super Contributor
timestamp: April 05, 2023, 03:55:06 am
content: Its for human, and you're asking internet for help from the scratch.

author: mikerj
author_rank: Super Contributor
timestamp: April 05, 2023, 10:56:19 am
content: Quote from: Kush1999 on April 04, 2023, 07:32:58 amYou seem to be misunderstanding basic electrical theory, specifically Ohms law. I=V/R, if R is high then you need a higher voltage to achieve a given current. Irrespective, 75mA is quite capable of causing death in humans.

author: jonpaul
author_rank: Super Contributor
timestamp: April 05, 2023, 01:05:33 pm
content: OP...See very old TENS electric simulators, cheaply availabile in USA and EU75 V ICS,are available but costly, a simple H or,half bridge plus,IC driver's is optimized wayUse of voltages above SELV,42.2 V peak is regulated in the West.Study the types and,property of human skin electrodes, the resistance can be 10s..100s Ohms up to me ohms.Assume that the device can be misused. Then you can be held responsible.Bon courageJon

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 05, 2023, 03:22:57 pm
content: Quote from: Benta on April 04, 2023, 10:35:43 pmOh, come on! Maybe we will see someone becoming known as 'The Living OLED'!

author: Terry Bites
author_rank: Super Contributor
timestamp: April 05, 2023, 07:41:34 pm
content: A current source has a high output impedance, i barely changes with u. That's what you want from a CCS.High voltage. A CCS has a limiting "compliance voltage" ie the max I*R it can supply while remaining reliably constant.If your H bridge gets stuck or fails short somewhere, you have a continuous 100mA. Thats fatal. So you need to use a transformer and or caps to couple the current to the subject. Ther power supply can't be mains earth referenced unless you use a DC-DC converter with a high isolation barrier withstand voltage. Ideally 4kV in the 230V world. (Ignore IEC 60601 at your own risk).Its much safer to use batteries and a boost converter.Attched are some practical source and sink schematics. Clearly the sink is much simpler and will be perfectly good for generating bipolar pulses with an H bridge.It important that the output is isolated from circuit ground. It will stop the current source or sink operating correctly and presents risk to the subject.Note that the output current is set by Vin/Rsense. Rsense is the low resistance R at the MOSFET source. Easy.....

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2023, 09:51:13 pm
content: Yikes I didn't see the part about the load being human skin. 100mA? Are you trying to build a modern take on a medieval torture device?IIRC it takes only about 10mA across the chest to kill an adult human.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 05, 2023, 10:17:48 pm
content: Quote from: mikerj on April 05, 2023, 10:56:19 amYeah. It may be possible, he mentioned 75V and 100mA - an impedance of 750 ohm with appropriate electrodes and well-prepared skin is possible.Now that is obviously a major risk. He didn't really mention the application. That's the kind of stuff that you can find for muscle contraction, although 100mA even for this looks a bit much.But it does require knowing what you do, proper training, making sure to apply it to the right spots and nowhere near the heart or above, making sure the person doesn't have any metallic or active implant, etc. And designing it with all due care regarding isolation. The kind of stuff you're usually not supposed to be helping with on a forum.

author: JoeyG
author_rank: Regular Contributor
timestamp: April 05, 2023, 10:31:43 pm
content: for high voltage current monitors measuring across a resistive shunt. look at the HV7800, HV7801, HV7802Up to 450Vhttps://www.microchip.com/en-us/product/HV7800https://www.microchip.com/en-us/product/HV7801https://www.microchip.com/en-us/product/HV7802

author: Benta
author_rank: Super Contributor
timestamp: April 05, 2023, 10:33:11 pm
content: Quote from: SiliconWizard on April 05, 2023, 10:17:48 pmNot in my experience. Even with sweaty, salty skin you land at 20...50 kohm.But perhaps the electrodes are drilled into the flesh first? Then it's certainly possible.