sshortcircuit Posted June 24, 2013 Share Posted June 24, 2013 Fridge is blowing fuses on 12 volts dc. Can the experts advise me what the cold resistance should be? Element is 130w and ohms law comes out at 1.2 ohms Link to comment Share on other sites More sharing options...
hallii Posted June 25, 2013 Share Posted June 25, 2013 With the resistance so low the difference between hot and cold is going to be very small. The current will be over 10 amps so at least a 15 amp fuse should be used. A good meter will be needed to read the exact resistance and the calculated value may not match what you actually measure. The calculation assumes 12v but the voltage may be 13.8v with the engine running. The resistance would then be higher (1.4 Ohms) to give the 130w. Then there is the issue of voltage drop in the wiring. My guess (and guess it must be) is that the resistance will be somewhere around 1 or 2 Ohms. Anything lower might indicate a short and higher might be an open circuit. A current meter would be a good way to see what's going on, anything above 11 or 12 amps might indicate a problem. H Link to comment Share on other sites More sharing options...
lennyhb Posted June 25, 2013 Share Posted June 25, 2013 With the resistance of the element being so low the only way to read it accurately would be with a 4 wire resistance meter. All you can really do is disconnect the element and if the fuse does not blow with the engine running a good guess would be a duff element that is shorting. If the fuse still blows with the element disconnected it is probably a fault with the electronics on the fridge PCB. To prove the element you could connect it directly to a battery (with a 15a fuse in line) it will either blow the fuse or get warm. Link to comment Share on other sites More sharing options...
sshortcircuit Posted June 25, 2013 Author Share Posted June 25, 2013 Thanks for the replies which are very useful and prompted me to think a bit more. In the past I used to do electrical installation testing and forgot that I still had my tester which was used to measure accurately circuit resistance by passing 200mA through the circuit. Have now used this and have a reading of 0.94 ohms which looks ok. The supply from the split charge relay only provides current to the heater element through a relay on the pcb. A separate supply provides voltage for the fridge pcb. Will keep plodding away and see what my next inspiration will be, buy the element or use a separate inverter powered from the split charge to run fridge off 220 volts. Thanks again Link to comment Share on other sites More sharing options...
Brian Kirby Posted June 25, 2013 Share Posted June 25, 2013 Have you checked for volts at the heater element (or the supply terminal) with the engine running? Might be worthwhile before replacing element. Link to comment Share on other sites More sharing options...
Dave Newell Posted June 25, 2013 Share Posted June 25, 2013 The split charge relay does not provide any power to the fridge, its purpose is to pass current to the leisure battery when the engine is running, the fridge supply is via a separate relay, not the split charge unit. Having said that many hoby motorhomes use Schaudtt Elektroblock units which incorporate both split charge (with intelligent charging) and fridge supplies with engine running. Minor detail perhaps but possibly a significant one. D Link to comment Share on other sites More sharing options...
sshortcircuit Posted June 25, 2013 Author Share Posted June 25, 2013 Thanks again for input. Using a very poor quality tester with engine running I have 11.09 volts at fridge terminal block with a current of 9.27 amps so not to concerned. What I have subsequently found is the connections to the fuse for the "fridge supply relay" supply had a build up of ?? which I have scraped away to provide a clean connection. In addition I noted a heat build up at this point which would result in the fuse melting and not overloading. Hopefully tomorrow will get a quality fuse holder and quality fuses as I think the ones in use are rubbish. Will also remake the spade terminal crimp connections. Still want to do a bit more investigating as a relay on the pcb buzzed at the time of the fault suggesting a different fault possibly an intermittent fault on the dc supply to the pcb. It does not look like I am going to be changing the heater element. Its also pretty shocking the way in which the system has been wired as its just thrown in and cable stuck to the side with masking tape to keep them in place Link to comment Share on other sites More sharing options...
crinklystarfish Posted June 26, 2013 Share Posted June 26, 2013 Would have thought 11.09 a little low, the extra resistance described from poor quality connections / crud build up probably not helping. Is the cross section of the supply wires sufficient? No surprise at all regarding wiring runs, 'vans generally are at best adequately built. At least they had the good grace to tape it back with masking tape; that's an improvement on some installations. Link to comment Share on other sites More sharing options...
sshortcircuit Posted June 26, 2013 Author Share Posted June 26, 2013 Thanks crinkly. As I said the readings were taken with a poor quality meter so are an indication only. Will do a comparison today of meters just to see the difference. Wire looks ok and I am in the process of checking all connections. Link to comment Share on other sites More sharing options...
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