Broken Transformer?

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Broken Transformer?

Postby mrsayao » March 21st, 2012, 11:24 am

My barber uses a cordless hair clipper and his charger looks like it's dead. I took it apart and I'm getting a really miniscule amount of voltage (~100mV) off the secondary coil of the transformer that feeds power to the board. The charger's specs: 120V 60Hz 65W. The battery that it charges is NiCd 4.5V. How can I figure out a suitable replacement for the transformer if I don't know what the secondary coil voltage was? I may be able to ask one of the other barbers if I can take readings off of their charger, but I don't want to have to disturb their business. I also read that transformers rarely go bad so there may be something on the board that shorted out the coil. Any tips on testing transformers (to be sure it's really dead) and figuring out a replacement? Thanks in advance!
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Re: Broken Transformer?

Postby k-ww » March 21st, 2012, 1:02 pm

If you can get to the actual transformer secondary leads, use an ohmmeter - the resistance [with the unit unplugged] should be less than 30 ohms. Also measure the primary reistance at the power cord leads - it should be under 100 ohms. If either reads open circuit, that winding is open or in the case of the primary, there may be a fuse that is blown, or even the cord/plug may be bad. That can be checked by measuring from the transformer's winding back thru the cord to the plug.

The charger should have a rating on it for its output - if not, a constant current charging circuit can be made by using a 12VDC wall wart, and a series circuit of a 7805 regulator and a load resistor - if you want a charging current of 50MA, wire the + input of the 7805 to the wall wart + output, the negative of the battery to the negative of the wall wart, and connect a 100 ohm 1/2 W resistor between the output of the 7805 and the 7805 ground pin. then connect the 7805 ground pin / resistor junction to the positive of the battery. The 7805 / resistor combination acts as a constant current source to supply 50 ma thru the resistor [5V & 100 ohms] plus the operating curent of the 7805. Change the resistor value to change the charging current. You may need to heatsink the 7805. The cahring current for a constant charging setup for nicads should be 1/10C or 1/10 the ampere hour capacity of the battery [or less]. This circuit will work as long as the wall wart voltage minus the 5V drop of the 7805 [+ another volt or so] is above the voltage of the battery you want to charge.
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Re: Broken Transformer?

Postby mrsayao » March 21st, 2012, 1:17 pm

k-ww wrote:If you can get to the actual transformer secondary leads, use an ohmmeter - the resistance [with the unit unplugged] should be less than 30 ohms. Also measure the primary reistance at the power cord leads - it should be under 100 ohms. If either reads open circuit, that winding is open or in the case of the primary, there may be a fuse that is blown, or even the cord/plug may be bad. That can be checked by measuring from the transformer's winding back thru the cord to the plug.


Thanks k-ww, I'll check it out when I get home! Can I ask where you got those values? Is the fuse in the primary usually replaceable?
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Re: Broken Transformer?

Postby k-ww » March 21st, 2012, 1:21 pm

The values are guesstimates based on experience. check the transformer for a burnt smell and discoloration. If there is no fuse, the primary acts as one by burning out. Remember that line cords break at flex points - the plug and at the case [if the charger dosn't have the plug built in].

Any fuse is replaceable, but if it is blown, find out why first.
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Re: Broken Transformer?

Postby mrsayao » March 22nd, 2012, 12:46 am

k-ww wrote:If you can get to the actual transformer secondary leads, use an ohmmeter - the resistance [with the unit unplugged] should be less than 30 ohms. Also measure the primary reistance at the power cord leads - it should be under 100 ohms. If either reads open circuit, that winding is open or in the case of the primary, there may be a fuse that is blown, or even the cord/plug may be bad. That can be checked by measuring from the transformer's winding back thru the cord to the plug.


I took some readings and the secondary is showing ~0.3 ohms. I checked continuity and my meter detected a short. I checked the resistance from the prongs on the cord for the primary and I don't get a reading. I also checked continuity as well and it seems like it's open. I'm reluctant to crack the tape to check the resistance from the primary winding as I don't have any tape to seal it back up. :|
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Re: Broken Transformer?

Postby mrsayao » March 22nd, 2012, 10:36 am

k-ww wrote:The values are guesstimates based on experience. check the transformer for a burnt smell and discoloration. If there is no fuse, the primary acts as one by burning out. Remember that line cords break at flex points - the plug and at the case [if the charger dosn't have the plug built in].

Any fuse is replaceable, but if it is blown, find out why first.


I know someone who has the same charger. What I would need to do is take a few readings to figure out the replacement. Please let me know if I'm missing anything:

volts on primary (known 120vac)
volts on secondary
Current draw from secondary to board plugged in state
Current draw from secondary to board charging state

When I test current, does it matter which lead off the secondary I use?
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Re: Broken Transformer?

Postby semicolo » March 23rd, 2012, 12:08 pm

No it doesn't matter, current is the same through one loop and can be measured anywhere.
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Re: Broken Transformer?

Postby mrsayao » March 23rd, 2012, 2:25 pm

k-ww wrote:The values are guesstimates based on experience. check the transformer for a burnt smell and discoloration. If there is no fuse, the primary acts as one by burning out. Remember that line cords break at flex points - the plug and at the case [if the charger dosn't have the plug built in].

Any fuse is replaceable, but if it is blown, find out why first.


K-WW! I was able to borrow someone's charger to take readings. What I found is the secondary is 14.6V. Since the bottom of the charger says 65W the current draw is 4.5A when charging. I couldn't test accurately because I blew my meter (forgot to switch to the 10A test post) and didn't have an extra fuse on hand. Dumb mistake. You were right about the primary being dead! Any suggestions on where I can get a replacement? Would it be OK to get a 5A or would that pose a problem, how about a 4A instead? It would just charge slower right?

Thanks for pointing me in the right direction!
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Re: Broken Transformer?

Postby k-ww » March 23rd, 2012, 4:41 pm

Depends where you are located. In the US, I would check out Signal Transformer and/or their reps. They have an extensive line of transformers, with multiple primary [110/220V] and multi-tap secondarys on some product lines.

Is the 14.6V AC or DC?

When you get the fuse replaced, check the output voltage under load, and/or the battery size and rating of the clipper [voltage/ampere hours]

Did you double check that there was continuity from the plug to the transformer on both leads / that the primary was open, measured at the transformer directly? [measure twice, order replacement part once]

Other than that, make sure that it is cheaper to replace the transformer than buying a new charger.
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Re: Broken Transformer?

Postby mrsayao » March 23rd, 2012, 5:43 pm

k-ww wrote:Depends where you are located. In the US, I would check out Signal Transformer and/or their reps. They have an extensive line of transformers, with multiple primary [110/220V] and multi-tap secondarys on some product lines.

Is the 14.6V AC or DC?

When you get the fuse replaced, check the output voltage under load, and/or the battery size and rating of the clipper [voltage/ampere hours]

Did you double check that there was continuity from the plug to the transformer on both leads / that the primary was open, measured at the transformer directly? [measure twice, order replacement part once]

Other than that, make sure that it is cheaper to replace the transformer than buying a new charger.


I didn't remove the tape on the transformer, but I'm pretty sure there isn't a fuse. I assume the transformer is AC since wires from the primary come straight from the wall plug. I checked continuity at the plug wires, the primary was closed (on the dead transformer, it was open) I set my meter to AC when I read the voltage off the secondary, which was 14.6VA. Basic math tells me that A = 65W / 14.6VA which means I'm looking for a 4.5A transformer. Remember my meter blew when I read the current because I stupidly forgot to switch to the 10A test lead. 65W is on the sticker on the bottom of the charger. On Jameco (I live in San Francisco) I found a 12.6V 4A transformer http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=149771&langId=-1&storeId=10001&productId=149771. Would this suffice??? Having a hard time finding the exact voltage with the current I need (which I hear is common in the transformer world [higher voltage, lower current / lower voltage, higher current]. Thanks again.
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