Key fob only works on battery and not power supply. What's going on here?

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
Yesterday the batteries died on my key fob and I didn't have the appropriate batteries to replace them. I thought I could use my power supply to operate the key fob to open the car door but it did not work. I thought that it may be caused by a noisy power supply at the higher frequency range of 432MHz but when I got my hackRF to investigate the waveform with the battery and then with the power supply, I was shocked to find no signal at all. I have since replaced the batter and everything works great, but as a recently graduated EE I am at odds with what is going on. Can anyone help me figure out this mystery. Here is a youtube link of the issue
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
Maybe you applied the wrong voltage to the fob. What type were the original batteries?
The batteries are two CR2016 stacked in series. They are each 3[V] so 6[V] in total. 6[V] is what the power supply is set to and I also double checked that with the multimeter.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
That's what I'm thinking, but I did expect to get something out of it. I scanned up and down the frequency range from 300-400 Mhz looking for a signal thinking it maybe shifted or something but nothing.
 

Delta Prime

Joined Nov 15, 2019
1,191
Hello there :)Check your power supply settings. From the video could not see the power supply as you pressed the momentary push button switch. If your power supply is on constant voltage, turn your current knob fully clockwise. If it has that capability. Quite frankly!The only thing retained from your video is LED has nothing to do with indicating data is being transmitted,that you can tell from the video, it lights either way.;)
Check your power supply power Leads I always make my own I know exactly what the voltage drop is, because of the lead resistance itself.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
Hello there :)Check your power supply settings. From the video could not see the power supply as you pressed the momentary push button switch. If your power supply is on constant voltage, turn your current knob fully clockwise. If it has that capability. Quite frankly!The only thing retained from your video is LED has nothing to do with indicating data is being transmitted,that you can tell from the video, it lights either way.;)
Check your power supply power Leads I always make my own I know exactly what the voltage drop is, because of the lead resistance itself.
I double checked that the current knob was cranked up and also tried with another adjustable power supply I have. Both failed to produce a signal. I tested the voltage at the end of the leads and it was correct. Also there is negligible voltage drop as the unit doesn't use much power.
 

Delta Prime

Joined Nov 15, 2019
1,191
I double checked that the current knob was cranked up and also tried with another adjustable power supply I have. Both failed to produce a signal. I tested the voltage at the end of the leads and it was correct. Also there is negligible voltage drop as the unit doesn't use much power.
Very well. Make, model ,of FOB. :)
Edit: that's messed up
 

Delta Prime

Joined Nov 15, 2019
1,191
Specifications: 1) Model (output voltage & output current): a) HY3005F-3 (2 x 0-30V, 2 x 0-5A,5V/3 AMP FIXED OUTPUT) 2) Display type: 4 LED with 2 colors 3) Input voltage: 104 to 127V AC (60Hz), or 207 to 253V AC (50Hz) 4) Ripple and noise: CV 0.5mVrms 5)

You can reduce the output ripple by connecting Earth ground with chassis. it's something you can do real quick just put a jumper black to Green out put on your power supply.
Edit ;Twist power leads
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
I also posted this on the physics forum and a user over there had some great insight. They noticed that there were not very big buffer capacitors. This would be fine for battery use, but given that I am sourcing power from a supply over long wires, the inductance wont allow the fast pulses of current for the transmission. Here is the quote

"The thing has no sufficient buffer caps. Through a battery this is not a problem, it can support some silly big current peaks on short term: but a PSU wiring will prevent going through any HF current, so no power for transmit.
Add some 100uF tantal (or: ceramic - a miracle that you can have these in ceramic!) to the battery terminals when it's with a PSU. I think it'll work with that." (https://www.physicsforums.com/threa...on-battery-and-not-power-supply-what.1006402/)

This seems very logical. Thanks everyone for you input! I dont have any tantal capacitors to try this but possibly I can pick some up or find some from other equipment.
 
Last edited:

Wolframore

Joined Jan 21, 2019
2,399
I agree with the capacitors but I doubt it will need to be so large. Ordinary electrolytic should be fine and a smaller ceramic parallel. I will have to try a similar test to see if my bench power supply acts the same way. Usually when we test RF circuits we bypass them but never tested a fob designed for battery use.

it is not a lot of current but it needs to be quick and inductance would slow it down due to di/dt.
 

MisterBill2

Joined Jan 23, 2018
9,838
At that high a frequency every part on the PCB is part of the circuit and so opening the case and connecting wires changes everything. So of course it would not function from an external supply.
In an emergency those button cells can be recharged a bit, but it is very easy to burst them by overdoing it. JUst a bit more than the rated cell voltage and only for a few seconds at a time. then use a meter to verify that the cell is recharged.
And be sure to wear eye protection while applying the recharge power.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
At that high a frequency every part on the PCB is part of the circuit and so opening the case and connecting wires changes everything. So of course it would not function from an external supply.
In an emergency those button cells can be recharged a bit, but it is very easy to burst them by overdoing it. JUst a bit more than the rated cell voltage and only for a few seconds at a time. then use a meter to verify that the cell is recharged.
And be sure to wear eye protection while applying the recharge power.
THANKS!!
 

Delta Prime

Joined Nov 15, 2019
1,191
I cannot come up with options for all contingencies given the information in hand you're a recent graduate an electrical engineer that makes you my Superior. This is why I love AAC I learned something new everyday egos aside please keep us up to date to your findings so others as well as myself can acquire this knowledge! Thank you sir.
 
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