pcstru Posted November 24, 2015 Posted November 24, 2015 Does anyone know of (or supply) a kit which could be retrofitted to an existing laptop trolley which would a) give soft start and b) prevent shock from back EMF when the unit is unplugged?
difinity Posted November 24, 2015 Posted November 24, 2015 I thought back EMF was found when motors are concerned. I'm guessing that the shock is more likely to be a charge held in the capacitors of the power supply. Most of the big names offer soft start as an option but they look to be fairly integrated with the electrics inside the trolley, have you checked with the manufacturer of yours to see its now offered ? 1
southhamster Posted November 25, 2015 Posted November 25, 2015 Yes. By coincidence one of my schools has an old Bretford trolley and lost the safety device. Bretford helpfully suggested a timer unit with safety relay. Wasn't ridiculously expensive considering the timer will save some electricity too. PM me and I will send details. If it's not Bretford maybe there are parts on ebay. 1
pcstru Posted November 26, 2015 Author Posted November 26, 2015 I thought back EMF was found when motors are concerned. I'm guessing that the shock is more likely to be a charge held in the capacitors of the power supply. Properly it's CEMF (Counter Electro Magnetic Force) although Back EMF is commonly used term. It happens in motors when the motor is slowing and effectively becomes a generator but that is actually a slightly different thing. A transformer or any other inductor generates a magnetic field which, when power is cut collapses and returns current into the circuit. It's not generally a problem but with 30 transformers all being unplugged at once, it's effects can be shocking! Most of the big names offer soft start as an option but they look to be fairly integrated with the electrics inside the trolley, have you checked with the manufacturer of yours to see its now offered ? Ours are quite old but yes, I will check.
Oaktech Posted November 26, 2015 Posted November 26, 2015 I would speak to Bretford, or LapSafe - I've previously bought the charging innards of a LapSafe directly from them and fitted it to a cupboard.
southhamster Posted November 26, 2015 Posted November 26, 2015 The Bretford version is called a LAP EPM T We mounted the box outside the trolley but put a doorstop next to prevent it being smashed into the wall.
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