Ebay bargain – DC-DC Step up module

Thread Starter

Hymie

Joined Mar 30, 2018
985
I often trawl ebay looking for bargains and came across these that work out at less than 50p each.

https://www.ebay.co.uk/itm/10-x-MT3...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649

The specification looks impressive, but having tested one, the output voltage ripple may be a deal breaker. With an input current of 2A the module soon shutdown – I’d recommend a maximum input current of 1.5A for reliable operation.

My measurements below are based on an input of 5Vdc, I have listed the input current together with the output voltage/load and ripple voltage. The ripple voltage frequency was measured at around 10MHz.

Note that the module is step-up, so the output voltage cannot be less than the input. For some reason the output voltage adjustment pot is wired in reverse.

Input 1.1A, output 12V at 400mA – ripple 1.0V

Input 0.5A, output 12V at 200mA – ripple 0.5V

Input 1.25A, output 20V at 250mA – ripple 1.5V
 

MisterBill2

Joined Jan 23, 2018
9,036
Is any of the circuit of this module visible? It seems like it is simply a very poorly filtered DC to DC non-regulated device. It might be useful depending on the application.
 

schmitt trigger

Joined Jul 12, 2010
453
I have purchased similar step up modules from Alibaba or Ebay, and have also found very high ripple voltage.
You require to add a C-L-C output post filter.
And a large decoupling capacitor at the input.
 

Thread Starter

Hymie

Joined Mar 30, 2018
985
I’ve already used one of these to replace a 9V PP3 battery, powered via a 5V USB source.
Although only powering a timer circuit (consisting of two 555 timers) probably drawing less than 50mA – the circuit works well.

Despite the output ripple voltage, I envisage using them in circuits where I need an increased voltage at low current e.g. operate a 28V relay coil from 12V control circuitry.
 

MisterBill2

Joined Jan 23, 2018
9,036
Certainly some applications can function well with power that is not pure, noise free, DC power. Relay coils mostly are included in that group. The challenge is in knowing in advance what will be satisfactory and what will not be OK.
 

Thread Starter

Hymie

Joined Mar 30, 2018
985
I tried to illuminate the ripple with a 10µF capacitor across the output.
Based on the formula V=I.t/C, with 10MHz ripple, drawing an amp would require a 10µF to reduce the ripple to 0.01V

Due to the ESR of the electrolytic capacitor it had no effect; as schmitt trigger says something more is needed.
 

MisterBill2

Joined Jan 23, 2018
9,036
Filtering at 10 MHZ is a different sort of situation. One other thing is the impedance looking back into this module, since that will certainly have an effect on filtering.
What was the load current when the 10mfd cap did not have any effect? Can the TS add a series diode and then add an additional capacitor, 0.1 mfd or 0.47mfd, in addition to the ten mfd, and see what the results are. If the impedance during the low portion of the ripple voltage is not quite high, that will cause pulling down the output voltage.

It might be that the output capacitor of the stepup module is deffective, or possibly the output diode is shorted.
 
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