I have a 5V boost transformer that im using to get 5V out of a lipo battery pack that is rated at 5V. I need the 5V out because the sim900 i have is incredibly finicky and works only at 5V. But Ive got it plugged in and the lipo outputs 4.96V but the other side of the boost transformer is only outputting 4.95V.
Shouldnt it be outputting 5V?
What 5V boost transformer?
4.95V ~= 5V
.05V (50mV) less
"works only at 5V"
Get something else.
Could you suggest something? Lm7805 maybe?
Can you provide details of the battery pack.
5V is an odd voltage for a LIPO battery, so it sounds like some kind of LIPO based power supply.
The Sim900 can draw as much as 2 amps when transmitting, so your power supply must be able to source this much current without the voltage sagging.
Hi,
You mean a power bank, if it gives out 4.95Vdc, that is well within spec for USB powered devices to run.
If your sim900 is incredibly finicky and works only at 5V, then it sounds like your power circuit for it may be a problem, have you got bypass capacitors on its power pins?
Tom... 
I don't, dunno if the module has them. So you're saying I should add some capacitors? I'm not an electronics engineer and have little experience but from what little I understand, you mean I should add some capacitors so that I have a "backup" pool of energy for when the sim needs it?
How would I calculate what size caps I would need?
Hi,
you mean I should add some capacitors so that I have a "backup" pool of energy for when the sim needs it?
Basically yes, put in two caps 10uF and 0.1uF.
At the gnd and 5V pins of the SIM900.
Tom....
Hi,
Yes..
Tom...
Well that didnt work. It doesnt make a difference in the voltage, its still 4.8-4.9V. The lipo is outputting 5.08V without load. I was on a breadboard so I moved the leads closer on the power rails and sim is working properly.
Must be a crappy rail connection in there.
TomGeorge:
Hi,Basically yes, put in two caps 10uF and 0.1uF.
At the gnd and 5V pins of the SIM900.Tom....
Whoops - didn't do the maths did you?
For GSM transmission you need more like 22,000uF to sustain 2A for several ms without much voltage drop,
so its much more practical to get a supply that can hold up under 2A load.
That power bank is probably simply unable to source 2A at all.
Could you explain the math to me.
2A for a 5ms transmission timeslice is 2 x 0.005 coloumbs of charge, ie 10mC
The voltage change across a capacitor = dQ/C, 0.01C / 0.022F = 0.55V, which is still
quite a large voltage droop across the duration of the timeslice (Although the power supply
will provide some of this current, so its more likely to be 0.3V or something).
GSM transmission is done in short timeslices, normally for just 12% of the time, so the ~ 2A peak
current could average at 0.25A, yet a supply of 1.5A won't work at all (unless such a large
capacitor is employed to bolster it up).
Multimeters only see average, so can be deceptive with this sort of load - they will see the
supply voltage droop a little, but not indicate the transmit slices are dropping out completely.
