Linear power supply on USB isolator board.

Linear power supply

Linear power supply

Several days ago I received an e-mail from Larry Owens, my fellow Coloradoan. Larry is Hi-Fi enthusiast; he built USB isolator kit and made very elegant and clever modification to it – a linear power supply. Larry’s design fits on existing board footprint.

Here is a quote from Larry’s e-mail:

Thought you might like to see what a little creativity can yield — did not fancy a SMPS’s noise, but wanted to be able to handle various battery sources. Since the off-board supply would be rather quiet, just selected a plain old 7805C (found in drawer, able to dissipate plenty of heat w/ 12v+ supplies and full 500mA USB loads). Thought a power-up LED would be nice too… …the challenge was to find existing pads for all components without having to use any cut/jumps… do consider that outcome lucky at the least…

Take a look at the title picture (click on it to make it bigger). TO-220 next to the barrel connector is old trusty 7805 linear regulator. Small bypass capacitor is soldered between output and ground pins of it. Input and output pads are occupied with electrolytic caps, 100 ohm ( @100 MHz ) ferrite bead sits on inductor pads. Finally, a small LED is mounted in place of 3300pF cap with current-limiting resistor across LT1376 pins 7 and 8.

This is it – simple, elegant and very useful. Thanks again, Larry for sharing the design and very nice and clear picture!


22 comments to Linear power supply on USB isolator board.

  • Daniele

    Hello Oleg, I have purchased the USB isolator full kit. Though it can be equipped with an on-board DC-DC converter, my understanding is that an external 5V power supply is recommended for low-noise applications, please comment.
    I would use the USB 5V line to provide power to an SDR receiver which needs about 200mA. I think I could provide 12V from a quiet power supply to an additional linear regulator, as suggested in your building notes. Any suggestions about the post-regulator circuit to use? Heat dissipation?
    Best regards

    • You can use any low-noise regulator, really. LDOs from Linear are my favorites. Don’t worry about heat, the isolator consumes less than 10uA.

      You are using this isolator to isolate SDR from a PC, correct? In this case, using USB power form a PC to power it is not a good idea.

  • Daniele

    Hello Oleg, tank you for the prompt reply. Yes the problem is to isolate the receiver from the PC, both on USB data lines and on the USB 5VDC which is user to power the radio (no separate power connector on it). As I will have to use an external (linear) 12VDC power supply for an active antenna and a preselector, it seemed sensible to me to derive the USB 5VDC from it. I was worrying about heat because I expect at least (12V-5V) x 200mA = 1.4W dissipation on the 5VDC regulator. I think a classic LM317 would become nicely hot under the same conditions, without an heatsink. Anyway: I have had a look at LT LDO’s and found that LT1763 (500mA) and LT1962 (300mA) probably would be suitable for my application, what’s your opinion?
    Best regards

  • hmmm

    Hello Oleg,
    Being a hi-fi enthusiast myself, I am interested in this development as well. Any plans on updating the current store inventory to include this? Or has it been already? Thank you.

    • I’m currently prototyping low noise linear regulator for new “modular” USB isolator. The new isolator will be available with switching, linear, or USB-powered isolated supply. No ETA yet.

      • Henno


        I’m using Everyman USB headset (purchased from Skype webshop) and there is a serious 50Hz hum from the mic when the laptop is plugged to a wall socket.

        I purchased your original USB Isolator to get rid of that hum but unfortunately it did give any effect. I’m hoping that this new version you’re prototyping would work for me. Any ETA now?

        Do I understand correctly that this new version can be powered from the same USB port and thus not needing an external power supply?

        • If USB isolator you’re using now is not helping with 50Hz hum the new one won’t be helping either; likely, your mic somehow picks up line frequency and transmits it with the signal.

      • Bent

        I’m interested in this as well!

      • I am definitely interested in this “modular” USB isolator that you speak of – are you still working on it?

  • Rod

    I have an Optus mini WiFi modem. It acts as a WiFi router for up to 5 devices and also serves as our primary internet connection. It has inbuilt rechargable batteries and only comes with a USB connection. No complaints except that when the PC to which its connected is shut down for a while the batteries go flat so no one else can access the internet. I am looking for a device that can power the modem all the time in isolation from the PC its connected to. This device need not do anything with the USB signal except provide pass through.
    Does the device you have developed achieve this? As I said I am not so much after USB isolation as power supply isolation.
    Kind Regards

    Rod Smart

  • Yang Zhou

    I also made a linear USB isolator of my own but decided to use a TO-263 part to stay nice and low profile.
    What do you think?

  • Hello,
    is possible to use only D+ and D- from output?
    I would like to connect this output to my DAC with his own low noise power supply.
    Of course i will connect 5V DC for isolator output site.

    Thank you

  • matteusz

    I don’t know is still someone here, but I try post 🙂
    Got question — “In order to reduce interference to the minimum, isolator is powered from oscilloscope’s 5V regulated power supply. Also, the peripheral cable is kept as short as possible. I recommend using similar arrangement for high-end audio applications – to obtain best results, don’t use isolator’s own power supply, use power supply of the peripheral, a battery or super clean power source made for audio.” I have DAC try to connect to my laptop and i should power isolator external by 12v power and by running a wire from “Vin” to “Vbus” this both should be done?

  • matteusz

    Hi Oleg,
    thx for reply. So make connect like on this picture ?? ?? barrel connector and ” Vin” to ” Vbus” ?

  • matteusz

    I am a complete layman, sorry. So supply 5v isolator or use 5v to my DAC ?

  • matteusz

    Ok, thx Oleg. For Help 🙂 greet.

  • c3474

    In regards to Larry’s linear mod – could someone please assist some suggested values and part numbers (i.e. digikey)? I’m a soldering veteran but usually purchase kits so my circuit design skills are not good. I checked out digikey but ran in to little issues like amperage values on the ferrite bead or the physical size of bypass cap to ensure it can reach from pad to pad (I’m not familiar with surface mount sizing).

    Look like the linear mod parts list goes something like;
    7805 Regulator: digikey# 296-13996-5-ND ?
    Bypass cap 1: ?
    Bypass cap 2: ?
    Ferrite bead: ?
    LED: ?
    Limiting resistor: ?

    Any help is much appreaciated!
    Any ETA on the Linear version? 😀